Current Events

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Mon Nov 20th, 2017 - 12:30 noon
Seminar by Dr Volkher Scholz on
'Analytic Approaches to Tensor Networks for Critical Systems and Field Theories'

MPQ Large Lecture Hall (now moved to B0.32) at 12.30 noon.

Dr. Volkher Scholz, ETH Zurich

Analytic Approaches to Tensor Networks for Critical Systems and Field Theories

I will discuss analytic approaches to construct tensor network representations of quantum field theories, more specifically critical systems and conformal field theories in 1+1 dimensions. A key insight is that we should understand how well the tensor network can reproduce the correlation functions of the quantum field theory. Based on this measure of closeness, I will present rigorous results allowing for explicit error bounds which show that the multiscale renormalization Ansatz (MERA) does approximate conformal field theories.

In particular, I will discuss the case of free fermions, both on the lattice and in the continuum, as well as Wess-Zumino-Witten models.

[Based on joint work with Jutho Haegeman, Glen Evenbly, Jordan Cotler (lattice) and Brian Swingle and Michael Walter (lattice & continuum)]

Fri Oct 27th, 2017 - 11:00 am
Seminar by Prof. Michael Keyl on
'Controlling a d-Level Atom in a Cavity'

MPQ Large Lecture Hall (now moved to B0.32) at 11.00 am.

Prof. Michael Keyl, FU Berlin

Controlling a d-Level Atom in a Cavity

We study controllability of a d-level atom interacting with the electromagnetic field in a cavity. The system is modelled by an ordered graph Γ. The vertices of Γ describe the energy levels and the edges allowed transitions. To each edge of Γ we associate a harmonic oscillator representing one mode of the electromagnetic eld. The dynamics of the system (drift) is given by a natural generalization of the Jaynes-Cummings Hamiltonian.

If we add  suffcient control over the atom, the overall system (atom and electromagnetic field) becomes strongly controllable, i.e. each unitary on the system Hilbert space can be approximated with arbitrary precision in the strong topology by control unitaries. A key role in the proof is played by a topological *-algebra A(Γ) which is generated (roughly speaking) by the path of Γ. For that reason A(Γ) is called path algebra. It contains crucial structural information about the control problem, and it is therefore an important tool for the implementation of control tasks like preparing a particular state from the ground state.

This is demonstrated by a detailed discussion of different versions of 3-level systems.

[Based on joint work with Thomas Hofmann]

October 13th, 2017
Munich Quantum Day at TUM ZNN, Campus Garching, Am Coulombwall 4a

Symposion by Munich Quantum Centre (MQC)
Held at TUM ZNN, Campus Garching, Am Coulombwall 4a.

October 9th - 12th 2017 (to be continued February 2018)
Block Course by Prof. Michael Keyl (FU Berlin) on Mathematical Aspects of Quantum Field Theory (Part 1) together with Study Course 'Theoretical and Mathematical Physics' (TMP) at LMU and IMPRS 'Quantum Science and Technology' (QST) at MPQ, Theresienstrasse 39, room B101, Mon: 2-4pm; Tue, Wed, Thu 10-12am plus 2-4pm.

Block Course by Prof. Michael Keyl (FU Berlin) on Mathematical Aspects of Quantum Field Theory (Part 1) together with TMP and IMPRS-QST. Held at LMU Physics, Theresienstr. 39, Room B101, Mon: 2-4pm; Tue, Wed, Thu 10-12am plus 2-4pm.

In the beginning of the semester, we will study QFT in the Wightman framework. This includes in particular scalar and operator valued distributions, Wightman axioms, Wightman functions and the reconstruction theorem, and the Borchers-Uhlmann algebra with its representations. As an explicit example we will study the free scalar field, its Wick-ordered products, and self-interacting models in 1+1 dimensions. In the latter context questions of renormalization are also discussed.

The second week at the end of the semester will be devoted to perturbation theory. After a short look at the S-matrix, we will use the Epstein-Glaser formalism to construct the perturbation series term by term as a formal power series. This will be carried out in detail with \Phi^4 self interactions as an explicit example. In this context perturbative renormalizability will also be discussed.

For the script to part 1 follow this link.

3 ECTS points can be acquired by writing an at least 10 page essay on a topic related to the course. 

Topics

  1. Quantum fields and Wightman axioms
    1. Distributions
    2. Quantum fields
    3. Wightman distributions and the reconstruction theorem
  2. The free scalar field
    1. Representations of the Ponicaré group
    2. Relations to the Klein-Gordon equation
    3. Representations of the CCR
    4. the field and its properties
  3. Interacting fields or: why is this so difficult?
  4. Scattering theory (Winter 2018)
  5. Perturbative theory à la Epstein-Glaser (Winter 2018)

Literature

  • M. Reed and B. Simon, Methods of Modern Mathematical Physics, Vol 2. and 3.
  • N. N. Bolgolubov, A. A. Logunov, A. I. Osak, I. T. Todorov, General Principles of Quantum Field Theory, Kluwer (1990).
  • R. Haag, Local Quantum Physics, Springer (1996).

Further information on the TMS webpage.

 

Some Past Events

June 30th, 2017 - 11:15am
Seminar by Frederik vom Ende on
'Unitary Dilations of Discrete Quantum-Dynamical Semigroups'

MPQ Small Lecture Hall at 11.00am.

Frederik vom Ende

Unitary Dilations of Discrete Quantum-Dynamical Semigroups

The time evolution of physical systems is a main factor in order to understand their nature and properties. Operations and therefore time evolutions of open quantum systems are described by quantum maps which are linear, trace-preserving and completely positive.
Also for the discrete case, we want to understand why the condition of complete positivity is necessary and which structure it provides. Again we will see that every quantum channel has a Kraus decomposition and that it can be built from the basic operations of tensoring with a second system in a specified state, a unitary transformation, and the reduction to a subsystem.

Thus one can mathematically structure the solution of discrete quantum dynamical systems and even certain types of discrete quantum dynamical control systems.

[Work based on a masters thesis in maths at U Wuerzburg.]

 

 

June 23rd, 2017 - 11:00am
Seminar by Lukas Knips on
'How to Detect Entanglement - a Summary of Methods'

MPQ Small Lecture Hall at 11.00am.

Lukas Knips

How to Detect Entanglement – a Summary of Methods

Entanglement is a fascinating feature of quantum systems and one of the key resources for quantum information processing. In order to detect and quantify entanglement, and thus to attest the prepared system to be a useful resource, sophisticated methods are required.

I will review and compare some specialized and efficient entanglement criteria for multiqubit systems with standard tools such as the PPT criterion [1], which is easily applicable, but limited to small systems, and linear fidelity witnesses [2], which are helpful if prior knowledge about the state is present, but already need several measurements.

The toolbox, I will present, encompasses methods which, for example, detect entanglement after only two measurements [3], work without any prior knowledge about the state [4] or can be applied when one cannot even know the local reference frames [5].

[1] A. Peres, Phys. Rev. Lett. 77, 1413 (1996); M. Horodecki, P.
Horodecki, R. Horodecki, Phys. Lett. A 223, 1 (1996)

[2] O. Gühne, G. Tóth, Physics Reports 474, 1 (2009)

[3] L. Knips, C. Schwemmer, N. Klein, M. Wiesniak, H. Weinfurter, Phys. Rev. Lett. 117, 210504 (2016)

[4] W. Laskowski, D. Richart, C. Schwemmer, T. Paterek, H. Weinfurter, Phys. Rev. Lett. 108, 240501 (2012); W. Laskowski, C. Schwemmer, D. Richart, L. Knips, T. Paterek, H. Weinfurter, Phys. Rev. A 88, 022327 (2013)

[5] in preparation

June 9th, 2017 - 10:00am
Seminar by Nicola Pancotti on
'Almost Conserved Local Operators in MBL Systems '

MPQ Seminar Room of Theory Group in 2nd floor at 10.00am.

Nicola Pancotti

Long-time dynamics of non-integrable systems hold the key to fundamental questions (thermalization). Analytical tools can only apply to particular cases (integrable models, perturbative regimes). Numerical simulations, limited in time, have found evidence of different time scales.
A new numerical technique for constructing slowly evolving local operators was introduced by Kim et al. in Phys. Rev. E 92, 012128 (2015). Those operators have a small commutator with the Hamiltonian and they might give rise to long time scales.
In this work, we apply this technique to the many body localization problem. We show that this method can not only signal the difference between the ergodic and localized phases, but it is also sensitive to the presence of a subdiffusive phase between both.

May 26th, 2017
Munich Quantum Day at LMU Physics, Theresienstrasse 37, Room A 348/349, 12-4pm

Symposion by Munich Quantum Centre (MQC)
Held at LMU Physics, Theresienstr. 37 Room A348/349:

12.05am: Viatcheslav Mukhanov: „Quantum Mechanics in the Sky
12.35am: Frank Pollmann: „Dynamical Signatures of Spin Liquids“
1:10pm: Poster session with coffee and snacks

May 18th, 2017 - 5.00pm
IMPRS Career-Talk by Susanne Pielawa on
'From Theoretical Physics to Algorithms'

MPQ Herbert Walther Lecture Hall at 5.00pm.

Dr. Susanne Pielawa
(Google, Munich)

During their undergraduate and graduate studies, physicists acquire a broad set of transferable skills which make many career paths available, also outside of academia.

Dr. Susanne Pielawa studied Physics at the University of Ulm, did her PhD in Condensed Matter Theory at Harvard and then went on to a postdoc position, also in Condensed Matter Theory, at the Weizmann Institute of Science. Afterwards, she joined the Start-Up Yowza (in Tel Aviv) and developed algorithms for a 3D-model search engine. She is now a software engineer at Google Munich.

She will talk about what it is like to be a software engineer, and share impressions of her transition from theoretical physics to algorithm development and software engineering. She will also present career opportunities at Google, and talk about the company’s work culture.

May 5th, 2017 - 10:30am
Seminar by Prof. Witlef Wieczorek on
'Mechanical Devices for Quantum State Engineering and Sensing'

MPQ Herbert Walther Lecture Hall at 10.30am.

Prof. Witlef Wieczorek
(formerly QCCC, then U Vienna, now at U Gothenburg)

Impressive results have recently been achieved in controlling micro- and nanomechanical devices in the quantum regime. These achievements pave the way for exploring novel applications and tests of quantum mechanics employing mechanical devices. In my talk I will describe progress towards quantum control of optomechanical states. In particular, I will address the question on how does one optimally estimate the state of an optomechanical system.

Further, I will talk about a completely different approach of controlling mechanical systems that has been recently proposed by employing superconducting levitation. This experimental platform offers unrivaled low mechanical dissipation and coupling capability to superconducting circuits. Its envisioned applications range from studying of fundamental questions to novel sensing prospects. I will describe first ideas and experimental steps in this direction.

April 28th, 2017 - 10:30am
Seminar by Anna-Lena Hashagen on
'Randomised Benchmarking'

MPQ Herbert Walther Lecture Hall at 10.30am.

Anna-Lena Hashagen
(returning from Prof. Stephen Bartlett, U Sydney)

Randomised benchmarking is a widely used experimental technique to characterise the average error of quantum operations. Its robustness regarding state preparation and measurement errors as well as its efficient scaling has made it a standard and reliable choice.

I will give an overview and a short introduction to the randomised benchmarking protocol for characterising the average error of Clifford gates.

April 7th, 2017 - 10:30am
Seminar by Umut Kaya on 'Investigation of the Second Laws in the Catalytic Coherence Setup'

MPQ Seminar Room B0.22 at 10.30am.

Umut Kaya (formerly with Renato Renner at ETH Zurich)

Catalyst systems are just additional systems you include to your setup to widen the range of your allowable transformations on the main state. But if you naively use this ancillary system in an approximate manner, you come across a well-known phenomenon called thermal embezzling, which says you can reach any output state without any restrictions–thus no 2nd Law.

I will present some analytical and computational results showing that the Catalytic Coherence setup is limited by certain second-law like relations, therefore it does not suffer from this embezzling problem.

April 3rd - 5th, 2017
IAS Symposion on Quantum Control: Mathematical Aspects and Physical Applications

Symposion: Quantum Control Theory: Mathematical Aspects and Physical Applications
Mon-Wed/April 3rd-5th 2017 at TUM Institute of Advanced Study, IAS Garching, Lecture Hall

March 30th - April 1st, 2017
Symposion in Honour of the 70th Anniversary of
Max-Planck-Medaillist 2017, Herbert Spohn at TUM & LMU

Symposion: Macroscopic Limits of Quantum Systems
Thu/Fri (March 30th/31st), 2pm: TUM, IMETUM, Garching, Lecture Hall E.127

Sat (April 1st), 2pm: LMU Mathematics Department, Centre, Lecture Hall  A027

March 23rd, 2017 - 2:00pm
Seminar by Claudius Hubig on 'DMRG with Subspace Expansion on Symmetry-Protected Tensor Networks'

MPQ Theory Group Seminar Room 2nd floor at 2.00pm.

Claudius Hubig

The Density Matrix Renormalisation Group when applied to matrix- product states is the method of choice for ground-state search on one-dimensional systems and still highly competitive even in unfavourable circumstances, such as critical systems and higher dimensions.

In this talk, I will discuss two separate methods which can be used to
improve the computational efficiency of DMRG and related methods on matrix-product states and beyond. The first component is the
implementation of both abelian and non-abelian symmetries in an entirely general way suitable also for higher-rank tensors as encountered in, e.g., tree tensor network states. The second ingredient, the subspace expansion, allows for a fully single-site DMRG algorithm with favourable linear scaling in the local dimension of the tensor network. Even for common problems, this results in a considerable speed-up over the traditional two-site DMRG method or the density matrix perturbation approach for ground-state search at reduced algorithmic complexity.
Additionally, the subspace expansion can potentially be used in a large
set of other algorithms, such as the TDVP or the variational application
of a matrix-product operator onto a matrix-product state.

February 3rd, 2017 - 10:30am
Seminar by Dr. Manfred Liebmann on 'Quantum Physics by the Non-Associative Cayley Algebra'

MPQ Seminar Room B0.22 at 10.30am.

Dr. Manfred Liebmann (U Graz, formerly Maths. Dept. TUM)

In the lecture I will discuss several striking consequences of the following generalization that will give rise to a new perspective on structures in the standard model of particle physics:

The Hurwitz theorem states that a bilinear product on $R^n$ with the property x◦y = ||x|| . ||y|| can only exist in dimensions n = 1, 2, 4, 8. The associated algebras are the division algebras of the real numbers R, the complex numbers C, the quaternions H, and the non-associative algebra of the octonions O, also known as Cayley algebra.

It turns out that the structure of the Dirac equation is deeply related to the non-assoziative multiplication law of the Cayley algebra. The Dirac matrices can be identified with complex versions of the left action maps Lx(y) := x◦y. However not all left actions can be complex represented and this leads to a generalization of the Dirac equation that transcends the traditional complex Hilbert space framework of quantum physics.

January 30th, 2017 at MPQ, Grand Lecture Hall
Tensor Networks and Applications:
Video Recording of Prof. Cirac, MPQ

11am, Prof. Cirac, MPQ
Tensor Networks:
A Quantum Information Perspective to Many-Body Physics

Abstract: The theory of entanglement offers a new perspective to view many-body quantum systems. In particular, systems in thermal equilibrium and with local interactions contain very little entanglement, which allows us to describe them efficiently, circumventing the exponential growth of parameters with the system size. Tensor Networks offer such a description, where few simple tensors contain all the information about all physical properties. In this talk I will review some of the latest results on entanglement and tensor networks, and explain some of their connections to quantum computing, condensed matter, and high-energy physics.

January 9-12th, 2017
Symposion on Machine Learning Challenges in Complex Multiscale Physical Systems at IAS Garching

See programme, in particular for Mon afternoon and Tue morning.

Events 2016

December 19th, 2016 at MPQ, room B0.21
ExQM Christmas Seminar

19.12.2016 at 16:00

4pm, Dr. Thomas Schulte-Herbrüggen
Season’s Applications of Knot Theory

Abstract: Surprise.

November 18-19th, 2016
Symposion in Honour of the 75th Anniversary of Nobel Laureate Ted Hänsch at LMU
November 18th, 2016 - 4:30PM
Seminar by Dr. Juan Bermejo-Vega on Understanding Contextuality as a Quantum Computational Resource without Wigner Functions
MPQ Lecture Hall at 4.30pm.

Dr. Juan Bermejo-Vega (FU Berlin)

A central question in quantum computation is to identify the resources that are responsible for quantum speed-up. Quantum contextuality has been recently shown to be a resource for quantum computation with magic states for odd-prime dimensional qudits and two-dimensional systems with real wavefunctions.The phenomenon of state-independent contextuality poses a priori an obstruction to characterizing the case of regular qubits, the fundamental building block of quantum computation. Here, we establish contextuality of magic states as a necessary resource for a large class of quantum computation schemes on qubits. Our proof exploits novel simple arguments and, for the first time, does not rely on Wigner functions. Our new methods can be extended to a family of magic-state protocols on qudits of any local dimension and lead to a new most-general proof in the odd dimensional case.
Based on:
https://arxiv.org/abs/1610.08529
https://arxiv.org/abs/1610.07093 

October 27-28th, 2016
Munich Quantum Symposion 2016 at MPQ and WSI

The IMPRS „Quantum Science and Technology“ is inaugurated. ExQM has an e-poster session on Fri, Oct. 28th at 1:30 pm in WSI.

munich_quantum_symposium_16

October 21st, 2016 - 11AM
Seminar by Prof. Michael Keyl on Some Maths behind the 2016 Nobel Prize in Physics
MPQ B0.21 at 11.00am.

Prof. Michael Keyl

The toric code introduced by Kitaev is one of the most simple models of topological order and therefore a good candidate to discuss some mathematical topics related to the last Nobel Prize in Physics. Apart from topological phases this includes in particular the emergence of quasi particles with exotic statistics in low dimensions.The latter relates to the representation theory of the braid group and the algebraic theory of superselection sectors, which we will briefly discuss.

July 5th, 2016 - 7th
Sommerfeld Lectures by Prof. John Preskill
Prof. John Preskill from Caltech next week in LMU.

05.07.2016 at 17:15
Public Lecture: Quantum Computing and the Entanglement Frontier

Professor John Preskill, California Institute of Technology, USA more

Room B052 – Faculty of Physics – LMU – Theresienstr. 39 – 80333 München

06.07.2016 at 16:15
Theory Colloquium: Quantum Information and Spacetime

Professor John Preskill, California Institute of Technology, USA more

Room A348/349 – Faculty of Physics – LMU – Theresienstr. 37, 80333 München

07.07.2016 at 16:15
Fields and Strings Seminar: Holographic Quantum Codes

Professor John Preskill, California Institute of Technology, USA more

Room A348/349 – Faculty of Physics – LMU – Theresienstr. 37, 80333 München

Details on: http://www.asc.physik.lmu.de/activities/lectures/

June 29th, 2016 - 2.30pm
Lecture by Robert König on Fault-Tolerance in Quantum Information Processing
Robert König on „Fault-Tolerance in Quantum Information Processing“

June 29th 2016 at 2.30pm in Lecture Hall 3 in the TUM Maths Dept. (Hörsaal 3 (MI 00.06.011)).

Classical information theory provides quantitative answers to basic questions about communication and computation. In the presence of quantum effects, its basic tenets need to be reassessed as fundamentally novel information-processing primitives become possible. Their potential appears promising, but their realization hinges on our ability to construct mechanisms protecting information against unwanted noise.

In this talk, I will consider two problems associated with communication and computation. First, I will discuss the additivity problem for classical capacities. I will review some of the more recent results in this direction: for continuous-variable channels, quantum generalizations of certain geometric inequalities yield operationally relevant statements. Second, I will discuss the problem of performing gates on information encoded in an error-correcting code, and explain how this relates to automorphisms of the latter.

June 19th - 22nd, 2016
First International ExQM Workshop at Lake Chiemsee
First International ExQM Workshop at Lake Chiemsee from Sun June 19th till Wed June 22nd 2016.

overall programme: see ProgrammExQM2016rev.pdf

detailed programme: see BookletWorkshop2016new.pdf

June 1st, 2016 - 11.30am
Seminar by Julian Roos on From Geometry To Topology
Seminar by our ExQM colleague Julian Roos on „From Geometry To Topology“.

Wednesday June 1st 2016 at 11.30am at MPQ lecture hall.

I will give a pedagogical non-formal introduction to the notion of topology. The idea is to outline an intuitive pathway from geometrical to topological concepts by increasing the number of allowed transformations within the congruence classes of Euclidean geometry. I will then pick one or the other basic system from the class of topological phases of matter and discuss why the word topological is used to describe it.

Here is a little teaser by Martin Gardner :

topo_teaser_julian2016

Draw a continuous line across the closed network shown so that the line crosses each of the 16 segments of the network only once. The curved line shown in the attached image does not solve it, because it leaves one segment uncrossed. It is not difficult to prove that the puzzle cannot be solved on a plane surface. Can it be solved on the surface of a sphere? On the surface of a torus?

April 29th, 2016 - 11AM
Seminar by Stephan Welte on A cavity-mediated photon-photon gate
Friday Apr. 29th at 11am at MPQ seminar room B0.21 in

Stephan Welte on „A cavity-mediated photon-photon gate“.

Photons are promising candidates for applications in quantum information processing and quantum communication. However, the direct interaction between two photons is negligible in free space, which is a drawback when it comes to the implementation of quantum logic gates between them. A solution to this problem was offered by Duan and Kimble [1] who proposed that a strongly coupled atom in an optical cavity [2] could mediate an effective interaction between two photons. We experimentally demonstrate that an implementation of this proposal is indeed possible. To this end, the universal CNOT operation of the gate as well as its capability to entangle two separable input photons are characterized. We will discuss details of our experimental implementation and present intriguing implications of our gate for photonic quantum information processing.

[1] L.-M. Duan, H.J.Kimble, Phys. Rev. Lett. 92, 127902 (2004)

[2] A.Reiserer, G.Rempe, Rev. Mod. Phys. 87, 1379 (2015)

April 8th, 2016 - 11AM
Seminar by Jakob Wierzbowski on Symmetry control of layered 2D semiconducting materials in novel nanodevices
ExQM seminar this Friday Apr. 8th at 11am as a lab tour in Walter-Schottky-Institute (meeting point: entrance hall).

Follow-up on last week’s talk on „Symmetry control of layered 2D semiconducting materials in novel nanodevices“.

Jakob Wierzbowski gives a short lab tour showing related experiments… also welcoming those who could not come last Fri, of course.

April 1st, 2016 - 11AM
Seminar by Jakob Wierzbowski on Symmetry control of layered 2D semiconducting materials in novel nanodevices
Friday Apr. 1st 2016 at 11am in MPQ seminar room B0.21.

Jakob Wierzbowski on „Symmetry control of layered 2D semiconducting materials in novel nanodevices“.

The emergence of truly two-dimensional materials like graphene [1] extends the range of commercially available material systems for future electronic and optoelectronic devices. Especially, semiconducting transition metal dichalcogenides (STMDs) MoS2, MoSe2, WS2 and WSe2 play a key role in bridging the gap between bandgap-less graphene and insulating hBN. Importantly, TMDs exhibit a direct bandgap in the monolayer limit with optical transitions in the visible and near-infrared wavelength range [2]. Here, intrinsic electronic and optical signatures in few-layer crystals strongly depend on crystallographic symmetry properties.

Specifically, the inherently broken inversion symmetry in monolayer crystals naturally facilitates valleytronic applications [3]. Here, we present the electrical control of symmetry properties of few-layer MoS2 crystals embedded within electrically tunable Si-SiO2-STMD-Al2O3-metal microcapacitors with optical access as presented in Figure 1. By tuning the electric field, we induce strong static electric fields exceeding ±3.5 MV/cm resulting in significant DC Stark shifts of the interband emission (>15 meV) for mono- to pentalayer crystals. We extract an effective exciton polarizability of β=(0.58±0.25)×10−8 DmV−1; independent of the number of layers probed [4].

In polarization resolved photoluminescence measurements performed on mono- to trilayer MoS2, we observe pronounced electric field control of valley optical dichroism for bilayer crystals. Importantly, we are able to continuously tune the degree of circular polarization of the emission from η=20 % up to 58 % [5]. Selected data are presented in Figure 2. Moreover, for the bilayer crystal, we demonstrate intense electrical control of second-harmonic generation (SHG) originating in naturally inversion symmetric 2H stacked MoS2. Using ultrashort pulses of ~ 70 fs within a spectral window 840 – 1000 nm (1.24 – 1.47 eV), we observe broadband tunability of the SHG signal throughout the probed region with a ~ 60 fold conversion amplification at its optimum [6]. Our results demonstrate the potential for emergent spin- and valleytronic devices based on two-dimensional atomically thin crystals and efficient electrically driven broadband frequency doubling by external control of the symmetry properties of 2H MoS2.

[1] A. K. Geim, I. V. Grigorieva, Nature 499, 419-425 (2013)
[2] Mak et al., Phys. Rev. Lett. 105, 136805 (2010)
[3] Zeng et al., Nature Nano. 7, 490-493 (2012)
[4] J. Klein and J. Wierzbowski et al., Nano Lett. DOI: 10.1021/acs.nanolett.5b03954 (2016)
[5] J. Wierzbowski and J. Klein et al., in preparation (2016)
[6] J. Wierzbowski and J. Klein et al., in preparation (2016)

March 11th, 2016 - 10.30AM
Prof Michael Keyl on:
'Controlling a d-level atom in a cavity' and Mrs Margret Heinze on 'Quantum control for a Jaynes-Cummings-Hubbard model'
Friday Mar. 11th 2016 at 10.30am in MPQ seminar room B0.21.

Double feature of two short DPG talks:

Prof Michael Keyl will dwell on“Controlling a d-level atom in a cavity“

In this talk we discuss quantum control theory for a d-level atom in a cavity. The atom is described by a Graph Γ with energy levels as vertices and edges e as allowed transitions. For each such e the atom interacts (via a Jaynes-Cummings like interaction term) with a different mode of the cavity. We consider controllability of the overall system (i.e. atom and cavity) under the assumption that all atom-cavity interactions can be switched on and off individually and that the atom itself is fully controllable. Our main tools are symmetry based arguments recently introduced for the discussion of the two-level case [M. Keyl, R. Zeier, T. Schulte-Herbrüggen, NJP 16 (2014) 065010]. The basic idea is to divide the control Hamiltonians into two sets. One which is invariant under the action of an Abelian symmetry group G and a second set which breaks this symmetry. We will discuss how the group G and its action are related to the graph Γ and its fundamental groupoid, and how these structure can be used to prove full controllability – at least if Γ is acyclic. For Graphs containing cycles the situation is more difficult and the universal covering graph has to be used. We demonstrate this, using the fully connected graph on three vertices as an example.
and Mrs Margret Heinze continues on „Quantum control for a Jaynes-Cummings-Hubbard model „.

We examine the control of a quantum system consisting of several two-level atoms with each atom interacting with a different mode of an electromagnetic field.

More precisely, the system is a Jaynes-Cummings-Hubbard model where each cavity contains an atom and a bosonic excitation that can tunnel to the neighbouring cavities. The interaction strengths can be time dependently tuned in order to achieve controllability.

We discuss if it is possible that every pure state can be reached from a given reference state (pure-state controllability). This analysis is lifted to the level of operators where each unitary has to be approximated with arbitrarily small error by a time evolution operator for appropriate control functions and finite time (strong controllability).

The challenge of this infinite dimensional control problem is met, by firstly examining the symmetries of the system. A finite dimensional block diagonal decomposition is obtained for the control Hamiltonians that obey an abelian symmetry and due to a cut-off finite dimensional Lie analysis can be applied. By then adding a Hamiltonian that breaks the symmetry pure state and strong controllability are examined, c.f. [New Journal of Physics 16 (2014): 065010].

February 25th & 26th, 2016
ExQM Miniworkshop:
Mathematics of Quantum Systems and Control Engineering
ExQM seminar as part of an entire miniworkshop on mathematics of quantum system and control theory on Thu 25 and Fri 26th Feb.

ExQM Miniworkshop:

„Mathematical Aspects of Quantum Systems and Control Engineering“

Feb. 25-26th 2016 at Math. Inst. TUM Campus Garching and MPQ.

 

Thu 25. Feb. Motto: „Mostly Finite-Dimensional Systems“

(all in Sem. Room Maths Dept. MI 03.08.011 on floor 3)
13:30h Dr Gunther Dirr (Uni Würzburg) „Some New Results on Ensemble Control“

To begin with, we present necessary and sufficient accessibility/controllability criteria for finitely many parallel connected bilinear systems.

Then, extending these ideas to infinitely many systems, we arrive at so-called bilinear ensembles and discuss first results on ensemble controllability.

14:30h Dr Thomas Schulte-Herbrüggen (TUM) „Systems Theory and Control of Closed & Open Markovian Quantum Systems: A Unified Lie Symmetry Approach“

We give necessary and sufficient symmetry conditions for controllability and simulability in closed systems. For open Markovian systems, we discuss accessibility in terms of Lie semigroups and their Lindblad-Kossakowski generators. We elucidate the Lie-algebraic structure of the Lie wedges and their embedding system algebras.

15:30h Coffee and extensive discussion

ca. 18:30h Dinner in Garching
Fri 26.Feb: Motto: „Infinite-Dimensional Systems“
10:30h Prof. Ugo Boscain (CNRS Paris Saclay) „Controlling the Schrödinger Equation via Adiabatic Methods Using Conical Intersection of Eigenvalues“ (ExQM Seminar at MPQ Room B0.21)

In this talk I will discuss how to obtain a population transfer in a quantum mechanical system using adiabatic methods and the presence of conical intersections in the space of controls. The method is powerful. In the finite dimensional case it permits to prove that if the system is “conically connected” then it is Lie bracket generated. Also it permits to control systems presenting a dispersion of parameters (ensamble controllability). Conical eigenvalue intersections are not rare. For systems with a real Hamiltoniana and two controls they are structurally stable.

12:00h Lunch at IPP

13:30h Dr Mario Sigalotti (INRIA Paris Saclay) „Control of the Discrete-Spectrum Schroedinger Equation“ (Sem. Room Maths Dept. MI 03.08.011 on floor 3)

We show how to deduce approximate controllability of the control-affine Schroedinger equation from the controllability properties of its Galerkin approximations, in the case in which the uncontrolled Hamiltonian has discrete spectrum.

14:30h Prof. Michael Keyl (TUM) „Controlling Atoms in a Cavity“ (Sem. Room Maths Dept. MI 03.08.011 on floor 3)

We treat control of several two-level atoms interacting with one mode of the electromagnetic field in a cavity. This provides a useful model to study pertinent aspects of quantum control in infinite dimensions via the emergence of infinite-dimensional system algebras.

15:30h Coffee and extensive final discussion panel

February 19th, 2016 - 11AM
Seminar by Dr Christian Schwemmer on Efficient Tomography of Multiphoton States
ExQM seminar this Friday Feb. 19th at 11.00am in MPQ seminar room B0.21.

Dr Christian Schwemmer will be so kind as to give his fare-well talk on „Efficient Tomography of Multiphoton States“.
Multipartite entangled quantum states offer great opportunities with potential applications in quantuminformation processing. Therefore, practical tools fo r entanglement detection and characterization are needed. However, conventional state tomography suffers from an exponentially increasing measurement effort with the number of qubits. In contrast, pure or symmetric states like W-, Dicke- or GHZ-states enable tomographic analysis at reduced effort. Here, we apply these schemes to experimentally analyze six photon symmetric Dicke states. For data processing, a fitting algorithm based on convex optimization is used offering significant improvements in terms of speed and accuracy.

Furthermore, it will be shown that implying additional constraints in quantum-state estimation, such as non-negativity of a quantum state, can introduce significant systematic errors.

February 12th, 2016 - 11AM
Seminar by David L. Goodwin on Taking Optimal Control toward a Tensor Formalism
ExQM seminar this Friday Feb. 12th at 11.00am in MPQ seminar room B0.21 is to trigger exchange between many-body tensor network techniques and control techniques.

David L. Goodwin (U. Southampton, Ilya Kuprov’s group) will give a perspective talk on „Taking Optimal Control toward a Tensor Formalism“

While reviewing the current state of Gradient Assisted Pulse Engineering (GRAPE), questions will be asked on the applicability of a Tensor version of this successful numerical optimal control algorithm. This Tensor-GRAPE is envisaged to use elements DMRG (or MPO), which sits in the low temperature approximation, and the successes of the SPINACH software toolbox in reducing state spaces and having efficient optimal control for simulation of spin system, sitting in the high-temperature approximation.

This talk will ask questions from the point of view of one that works with GRAPE, seeking hints at answers and possible problems that the audience may identify. Subjects of interest include: the use of an augmented exponential to compute exact control derivatives within SPINACH; recent publications of the Tensor-Trains formalism of Savostyanov et.al. to simulate exact 1D spin chains (as occurring also in protein backbones), and t-DMRG.

January 22nd, 2016 - 11AM
Seminar by Prof. Norbert Schuch on Tensor network models for the study of correlated quantum systems
ExQM seminar for this Friday Jan. 22nd 2016 at 11.00am in MPQ seminar room B0.21.

Prof. Norbert Schuch (now MPQ Garching) will talk on „Tensor network models for the study of correlated quantum systems“

Tensor network models provide a means of understanding the behaviour of correlated quantum systems from a local perspective. In this talk, I will give an introduction to the framework of tensor network models, and discuss examples of how they can be used to study the physics of complex quantum many-body systems, with a focus on systems with physical symmetries and those which display topological order.

Events 2015

December 11th, 2015 - 11AM
Seminar by Nicola Pancotti on Many body gates: from small chains to networks
ExQM seminar for this Friday Dec. 11th 2015 at 11.00h in MPQ seminar room B0.21.

Nicola Pancotti will talk on „Many body gates: from small chains to networks“

During the last years a great effort has been made in order tocharacterize, implement and optimize entangling gates between two distant particles. Here we study the evolution of a quantum complex system and we search whether there exists an optimal time t* in which a perfect entangling gate G is implemented. The aim of this research is dual. On one side we propose a brand new numeric method acquired from the machine learning community which gives a good speed up compared to the previous ones. On the other side we look for new, unknown solutions, Jopt , that parametrize the Hamiltonian so that exp(−iH ( Jopt ) t* ) = G ⊗ S; S is an operator which leaves the rest of the system in an unknown (don’t care) state. We start from a very simple 3-spins chain, for which we already know the solution. We move then forward to a N-spins chain and finally we enlarge the line to obtain a network. We aim to find perfect topologies and optimal two body interactions capable of implementing fast and high fidelity gates such as entangling, Toffoli, CCZ, etc.

December 4th, 2015 - 1:30PM
Seminar by Dr Mari-Carmen Banuls on Numerical studies of many-body systems using tensor networks
ExQM seminar for this Friday Dec. 4th 2015 at 13.30h in MPQ small lecture hall

Dr Mari-Carmen Banuls we will give a lecture on „Numerical studies of many-body systems using tensor networks“

Tensor network states have proven very successful in describing ground states of quantum many body systems. The paradigmatic example is that of Matrix Product States (MPS), which underlie the celebrated DMRG method for the study of one dimensional systems. Using these methods it is also possible to simulate dynamics. And the ansatz can be also extended to describe operators, in particular, mixed states.

In the last years, the progress has been fast both in the theoretical understanding and the application of tensor networks to diverse problems. In this talk, I will present these methods from the practical point of view, focusing on their application to the numerical study of diverse quantum many-body problems, and illustrate their potential with some recent results.

November 27th, 2015 - 11:30AM
Lecture by Juan Bermejo-Vega on Contextuality as a resource for qubit quantum computation
On short-term notice, our ExQM/QCCC fellow Juan Bermejo-Vega (now at FU Berlin) we will give a lecture in the Cirac group on 27th Nov. 2015 11.30am

„Contextuality as a resource for qubit quantum computation“

November 20th, 2015 - 11:15AM
ExQM Seminar for this Friday Nov. 20th at 11.15am in MPQ seminar room B0.21.

With Dr. Ville Bergholm (U. Helsinki) we will give a short double feature on „A First Glance into Quantum Control Engineering“

We go from a sketch of the unified background (by Thomas) to Ville showing on-line examples on the computer from optimized quantum state transfer and gate (or map) synthesis in closed and open systems. These examples include spin chains, NV centres, and exciton transfer in light-harvesting FMO complexes.

We give an outlook on the limits of open-loop versus closed-loop control.

November 6th, 2015 - 11AM
Seminar by Julian Roos on Looking inside a lithium-ion battery electrode: A materials modelling study
ExQM Seminar for this Friday Nov. 6th at 11.00am in MPQ seminar room B0.22.

Julian Roos will talk on „Looking inside a lithium-ion battery electrode: A materials modelling study“

In 1990, Sony introduced the first commercially viable Lithium-ion battery to the market, sparking a revolution in consumer portable electronics. This breakthrough was mainly due to the incorporation of John Goodenough’s layered intercalation cathode LiCoO2 into the cell, raising the energy density of such batteries to a practical level for the first time. Today, cathode materials are still regarded the major bottleneck (batteries with LiCoO2 are still found in most devices) and the route to new generation lithium-ion batteries is linked to the search for superior materials and their optimization. This calls for a better understanding of solid state properties and the fundamental physical processes inside electrode materials on the atomic scale. In this talk I will highlight the use of several numerical simulation techniques (both classical and quantum mechanical types) in providing complimentary insights to an experimental study of the novel exotic lithium-rich cathode Li7Mn(BO3)3, illustrating how such computational materials modelling is indispensable in modern materials optimization.

November 2nd, 2015 - 3:15PM
Seminar by Prof. Enrique Solano on From Quantum Theatre to Scalable Quantum Simulators
ExQM Seminar for next Monday Nov. 2nd at 15.15pm in MPQ major lecture hall.

Prof. Enrique Solano (Univ. Bilbao) will talk on „From Quantum Theatre to Scalable Quantum Simulators“

We will introduce the field of quantum simulations from a wide aesthetic and scientific perspective. Along these lines, we will discuss the relevance of quantum simulations as a playground for our quantum theatre, as communicating vessels between unconnected fields, and as a scalable quantum technology. We will also provide pedagogical examples of quantum simulations in trapped ions and superconducting circuits, relating nonrelativistic and relativistic quantum dynamics, physical and unphysical quantum operations, as well as strong and ultrastrong light-matter interactions. Finally, we will discuss the advantages and disadvantages of current paradigms of quantum simulators, involving digital and analog concepts, and propose novel paths and concepts for assuring their scalability.

September 25th, 2015 - 11AM
Seminar by Dr Christian Gogolin on Equilibration, thermalization, and local stability of thermal states
ExQM Seminar for this Friday Sep. 25th at 11.00am in MPQ major lecture hall.

Dr Christian Gogolin (ICFO Barcelona) will talk on „Equilibration, thermalization, and local stability of thermal states“

In this talk it is shown how finite dimensional quantum systems in pure states, which evolve unitarily according to the Schrödinger equation, can exhibit thermodynamic behavior. More precisely, it will be discussed under which conditions local equilibration and thermalization can be ensured in such systems. I then discuss results on structural properties of thermal states of locally interacting quantum systems that in particular imply lower bounds on the critical temperatures below which such systems can exhibit phases with long range order. Finally, I touch on some work in progress concerning many-body localization.

September 17th, 2015 - 1:30PM
Seminar by Dr Thorsten Wahl on Tensor network states for the description of quantum many-body systems
ExQM Seminar for Sep. 17th 2015 at 13.30pm in MPQ Seminar Room B0.21.

Dr Thorsten Wahl (with Ignacio’s group) will give his farewell talk before moving to Oxford about „Tensor network states for the description of quantum many-body systems“

Tensor network states (TNS) are applied to one and two dimensional systems: All translationally invariant matrix product states (one dimensional TNS) possessing long-range localizable entanglement, which is a non-local hidden order, are characterized. Furthermore, the first examples of chiral topological projected entangled pair states (two dimensional TNS) are presented. Their topological properties can be traced back to symmetries of the tensors describing the states. They are ground states of local gapless Hamiltonians and long-range gapped Hamiltonians.

August, 28th - 10:30AM
Seminar by Prof. Robert König on Protected gates for topological quantum field theories
ExQM Seminar this Fri, Aug. 28th at 10.30am in MPQ Seminar Room B0.22.

Prof. Robert König (with Michael Wolf’s group) talk on „Protected gates for topological quantum field theories“

We give restrictions on locality-preserving unitary automorphisms U, which are protected gates, for 2-dimensional topologically ordered systems. For generic anyon models, we show that such unitaries only generate a finite group, and hence do not provide universality. For non-abelian models, we find that such automorphisms are very limited: for example, there is no non-trivial gate for Fibonacci anyons. More generally, systems with computationally universal braiding have no such gates. For Ising anyons, protected gates are elements of the Pauli group.These results are derived by relating such automorphisms to symmetries of the underlying anyon model: protected gates realize automorphisms of the Verlinde algebra. We additionally use the compatibility with basis changes to characterize the logical action.

This is joint work with M. Beverland, O. Buerschaper, F. Pastawski, J. Preskill and S. Sijher.

May 18th, 2015 - 11:30AM
Seminar by Dr Fernando Pastawski on Holographic quantum error-correcting codes
On Monday May 18th 2015, 11:30 am in MPQ, big lecture hall, Dr Fernando Pastawski (Caltech, USA) will talk on Holographic quantum error-correcting codes: Toy models for the bulk/boundary correspondence

In this talk I will introduce a family of exactly solvable toy models of a holographic correspondence based on a novel construction of quantum error-correcting codes with a tensor network structure. The building block for these models are a special type of tensor with maximal entanglement along any bipartition, which gives rise to an exact isometry from bulk operators to boundary operators. The entire tensor network is a quantum error-correcting code, where the bulk and boundary degrees of freedom may be identified as logical and physical degrees of freedom respectively. These models capture key features of entanglement in the holographic correspondence; in particular, the Ryu-Takayanagi formula and the negativity of tripartite information are obeyed exactly in many cases. I will describe how bulk operators may be represented on the boundary regions mimicking the Rindler-wedge reconstruction.

May 8th, 2015 - 10:15AM
Seminar by Michael Fischer on On-Chip Superconducting Microwave Interferometers
This Fri, May 8th, at 10.15am (again in MPQ, seminar room B0.22).

Michael Fischer (group R. Gross) will talk on On-Chip Superconducting Microwave Interferometers

In recent years, important progress towards using superconducting circuits for quantum information processing (QIP) has been made. In circuit quantum electrodynamics, photons inside superconducting transmission lines and resonators interact with artificial atoms, called qubits. In our approach to QIP, the qubit information may be encoded in a dual-rail setup, consisting of two superconducting transmission lines. Similarly to all-optical quantum computing the qubit states are superpositions of a microwave photon travelling in either one of the transmission lines. In QIP, operations between multiple qubits are needed to perform quantum algorithms. In order to use the dual-rail setup, these so called gates need to be implemented for two dual-rail encoded qubits. One important two qubit gate, a controlled phase gate, can be built with an interferometer equipped with a photon number dependent phase shifter. In this talk, I will present theoretical calculations and simulations, as well as measurements of on-chip interferometers fit for the application in such phase gates.

April 17th, 2015 - 10:15AM
Seminar by Dr. Guido Bacciagaluppi on Did Bohr Understand EPR?
This Fri, Apr. 17th 2015, at 10.15am (again in MPQ, seminar room B0.22).

Dr. Guido Bacciagaluppi (Reader at U Aberdeen) will talk on Did Bohr Understand EPR?

Contrary to widespread belief, I argue that Niels Bohr’s arguments in his reply to Einstein Podolsky and Rosen in 1935 take fully into account the separation between the two particles. Specifically, I argue that there is no sleight of hand in the passage from Bohr’s discussion of a single particle passing through a slit and his subsequent discussion of the EPR example.

April 10th, 2015 - 10:15AM
Seminar by Dr Jukka Kiukas on Local asymptotic normality for the estimation of dynamical parameters of an open quantum system
Fri, Apr. 10th 2015, at 10.15am (again in MPQ, seminar room B0.22).

Dr Jukka Kiukas (U Nottingham, formerly with Reinhard Werner) on: Local asymptotic normality for the estimation of dynamical parameters of an open quantum system

Input-output formalism is a well-known framework for describing continual monitoring of a Markovian open quantum system via measurements made on its environment (typically a quantised radiation field). Mathematically, the environmental noise is described in terms of quantum stochastic Wiener processes on the field Fock space. We consider the problem of identifying and estimating unknown dynamical parameters (Hamiltonian and the quantum jump operators) from the output field state. For this purpose, we first use quantum Ito calculus to derive an information geometric structure on the set of parameters, arising from the quantum Fisher information of the output state. The geometry comes with an associated CCR-algebra, and we then show that local estimation reduces asymptotically (with long observation times) to a Gaussian estimation problem on that CCR-algebra.

March 13th, 2015
Seminar by Jakob Wierzbowski on Polarization control in few-layer MoS2 by electric-field-induced symmetry breaking
Mar. 13th 2015, (again in MPQ, seminar room B0.22),

Jakob Wierzbowski will talk on „Polarization control in few-layer MoS2 by electric-field-induced symmetry breaking“

detailed abstract

March 6th, 2015
Seminar by Moritz August on A Brief Introduction to Neural Networks
This Fri, Mar. 6th 2015, (again in MPQ, seminar room B0.22).

Moritz August will talk on A Brief Introduction to Neural Networks

During the last decade, Machine Learning has become one of the most innovative and challenging fields at the intersection of Computer Science and Math. It has already been successfully applied to a wide array of domains, examples being the natural sciences, robotics and advertisement. Among the various techniques developed in the field, (Artificial) Neural Networks have proven to be one of the most powerful methods today and have led to significant improvements in many applications. Under the label of „Deep Learning“ they also have gained attention in the public media and are the catalyst of the recent debate about the dangers of Artificial Intelligence.

In this talk, a brief introduction to the fundamentals of Neural Networks will be given. The talk will focus on their mathematical nature rather than on the neuroscientific interpretation and it will be explained what the term „Deep Learning“ actually refers to.“

February 27th, 2015
Seminar by Stephan Welte on Experiments with single atoms and photons
This Fri, Feb. 27th 2015, (again in MPQ, seminar room B0.22).

Stephan Welte will talk on Experiments with single atoms and photons

In the field if cavity quantum electrodynamics, the deterministic interaction of single photons with single atoms can be achieved. I will present our experimental setup that allows to study and exploit atom-photon interactions in the strong-coupling regime. This is achieved by optically trapping atoms at the center of a high finesse cavity. As an example for the rich set of applications possible with this system, I will present the nondestructive detection of optical photons which are reflected off the cavity.

[Reiserer et al. Nondestructive Detection of an Optical Photon, Science342, 1349 (2013)]
After my talk, a lab tour is planned. All ExQM members are cordially invited to participate.

February, 20th - 2015
Seminar by Claudius Hubig on Strictly Single-Site DMRG (DMRG3S) with Subspace Expansion
This Fri, Feb. 20th, (again in MPQ, seminar room B0.22).

Claudius Hubig will talk on „Strictly Single-Site DMRG (DMRG3S) with Subspace Expansion“

The talk will also include an intro into finding ground states by DMRG before going into research results of http://arxiv.org/abs/1501.05504v1

February 13th, 2015 - 10:15AM
Seminar by Dr Volkher Scholz on Operationally-Motivated Uncertainty Relations for Joint Measurability and the Error-Disturbance Tradeoff
Here are in fact three annoncements:

Next Fri, Feb. 13th at 10.15am in MPQ (seminar room B0.22) we have the pleasure to hear

Dr Volkher Scholz (ETH Zurich) talk on Operationally-Motivated Uncertainty Relations for Joint Measurability and the Error-Disturbance Tradeoff

We derive new Heisenberg-type uncertainty relations for both joint measurability and the error- disturbance tradeoff for arbitrary observables of finite-dimensional systems (I will shortly mention the extension to position/momentum). The relations are formulated in terms of a directly operational quantity, namely the probability of distinguishing the actual operation of a device from its hypothetical ideal, by any possible testing procedure whatsoever. Moreover, they may be directly applied in information processing settings, for example to infer that devices which can faithfully transmit information regarding one observable do not leak any information about conjugate observables to the environment.

joint work with Joe Renes and Stefan Huber, ETH Zurich

February 6th, 2015 - 10:15AM
Seminar by Lukas Knips on Multipartite Entanglement Detection with Minimal Effort
Next Fri, Feb. 6th at 10.15am in MPQ (seminar room B0.22)

Lukas Knips (ExQM student in Weinfurter’s group) talk on „Multipartite Entanglement Detection with Minimal Effort“

Certifying entanglement in a multipartite state is a demanding task. As a state of $N$ qubits is parametrized by $4^N-1$ real numbers, one may expect that the measurement complexity of generic entanglement detection is also exponential with $N$.

However, in special cases we can design indicators for genuine multipartite quantum entanglement using measurements in only two settings. I will describe the general method of deriving such criteria, which are based on a more general entanglement criterion using correlation measurements.

In the corresponding experiment we test two such non-linear witnesses, one constructed for four-qubit GHZ states, the other for Cluster states.

After introducing the theory and explaining our scheme for detecting genuine multipartite entanglement, I would like to show you around in our lab.

January 30th & 31st, 2015 - 9AM
Workshop: Quantum Computation, Quantum Information, and the Exact Sciences
„Quantum Computation, Quantum Information, and the Exact Sciences“ motivated by foundations of quantum mechanics.

Fri Jan. 30th at 9.00am till Sat Jan. 31st at 18.30pm.

The venues are at LMU in the centre, see plans: http://www.qcompinfo2015.philosophie.uni-muenchen.de/practical-info/index.html
Apart from the more philosophial oriented talks, there are also some closer to us, in particular by
— Brukner (from Vienna, Zeilinger)
— Schack
— Briegel

For details, please see:
http://www.qcompinfo2015.philosophie.uni-muenchen.de/program/index.html
and:
http://www.qcompinfo2015.philosophie.uni-muenchen.de/program/program_v10.pdf

January 20th, 2015 - 10AM
Lecture by Dr Marc Cheneau on An atomic Hong-Ou-Mandel experiment

Dr Marc Cheneau (CNRS Paris) will talk in the MPQ lecture hall at 10am on Tuesday, January 20th 2015.

„An atomic Hong-Ou-Mandel experiment“

The celebrated Hong, Ou and Mandel (HOM) effect is one of the simplest illustrations of two-particle interference, and is unique to the quantum realm. In the original experiment, two photons arriving simultaneously in the input channels of a beam-splitter were observed to always emerge together in one of the output channels. Here, we report on the realisation of a closely analogous experiment with atoms instead of photons. This opens the prospect of testing Bell’s inequalities involving mechanical observables of massive particles, such as momentum, using methods inspired by quantum optics, with an eye on theories of the quantum-to-classical transition. Our work also demonstrates a new way to produce and benchmark twin-atom pairs that may be of interest for quantum information processing and quantum simulation.

January 16th, 2015 - 10:15AM
Seminar by Anna-Lena Hashagen on her masters work in finance mathematics
Fri Jan. 16th 2015 at 10.15am (MPQ small lecture hall),we will have an ExQM Seminar:

Anna-Lena Hashagen will give a survey talk on her masters work in finance mathematics (should be a good New Year refreshment)

„The Flesaker-Hughston Model for the Term-Structure of Interest Rates“,

An interesting and still widely debated problem is the mathematical modelling of the term-structure of interest rates. Even though many attempts have been made to put forward an interest-rate model that fulfils all the desirable properties, these models usually have more than one shortcoming. Another major issue that is common to nearly all areas within financial mathematics is the urge of agreement with market practice. In order to minimise these shortcomings, Flesaker and Hughston have put forward a new methodology of interest-rate term-structure modelling called the Flesaker-Hughston model. This model class is very tractable and guarantees the positivity of interest rates. In the last few years, one very special model of theirs that has received particular interest is called the Flesaker-Hughston rational lognormal model. On top of the guaranteed positivity it resembles well-known market pricing formulas for popular interest-rate derivatives such as caps and floors as well as swaptions. This talk discusses the Flesaker-Hughston model class and places it within the environment of already existing models. We then thoroughly analyse the Flesaker-Hughston rational lognormal model with respect to the underlying dynamics of the instantaneous short-rate and the bond price, as well as the inherent boundaries that appear in this new framework.

Pricing formulas for caps and swaptions are derived by letting the martingale follow a diffusion process. Generalising this to an exponential L\'{e}vy process and using a method called the generalised Fourier transform, we also derive the price of a cap in this more realistic setting that includes jumps. The model is then calibrated to a full data set of risk-free zero-coupon bond prices in conjunction with either cap implied volatility, cap price, caplet price or caplet implied volatility mid-quotes on the US dollar three-month LIBOR rate.

Since the Flesaker-Hughston rational lognormal model gives closed-form expressions for caps, the calibration is extremely efficient. Using the real market data sets, the calibration analysis and the thorough analysis of the inherent model boundaries reveal that the instantaneous short-rate is bounded to such an extend that the Flesaker-Hughston rational lognormal model seems useful to price interest-rate options only in very specific cases — for those cases that lie within the boundaries. The guaranteed positivity of the instantaneous short-rate, i.e. the lower bound of zero, comes at a price of a restrictive upper
bound.

 

Events 2014

December 8th, 2014 - 1:30PM
Seminar by Dr Volckmar Nebendahl on Optimized Quantum Error Correction Codes for Experiments
Mon., Dec 8th at 13.30h in MPQ in seminar room B0.21 there will be a seminar by:

Dr Volckmar Nebendahl (Blatt group, Innsbruck) on „Optimized Quantum Error Correction Codes for Experiments“.
Details in http://arxiv.org/pdf/1411.1779v1.pdf.

October 8th, 2014 - 3:15PM
Seminar by Luca Arceci on Quantum solitons in the XXZ model with staggered external field
Wed., Oct 8th 2014 at 15.15h (3.15pm) in MPQ in seminar room B0.21 there will be a seminar by the ExQM theory applicant:

Luca Arceci from Univ. of Bologna on „Quantum solitons in the XXZ model with staggered external field“.

The 1-D 1/2-spin XXZ model with staggered external magnetic field, when restricting to low field, can be mapped into the quantum sine-Gordon model through bosonization: this assures the presence of soliton, antisoliton and breather excitations in it. In particular, the action of the staggered field opens a gap so that these physical objects are stable against energetic fluctuations.

In the present work, this model is studied both analytically and numerically. On the one hand, analytical calculations are made to solve exactly the model through Bethe ansatz: the solution for the XX + h staggered model is found by means of Jordan-Wigner transformation and Bethe ansatz separately, while eff orts are made to extend the latter approach to the XXZ + h staggered model (without finding its solution). On the other hand, grounding on results from the application of quantum fi eld theories on the quantum sine-Gordon model, the energies of these excitations are pinpointed through static DMRG (Density Matrix Renormalization Group) for diff erent values of the parameters in the hamiltonian. Breathers are found to be in the antiferromagnetic region only, while solitons and antisolitons are present both in the ferromagnetic and antiferromagnetic region. Their single-site z- magnetization expectation values are also computed to see how they appear in real space, and time-dependent DMRG is employed to realize quenches on the hamiltonian parameters to monitor their time-evolution.

The results obtained reveal the quantum nature of these objects and provide some information about their features. Further study of their properties could lead to the realization of a two-state qubit through a soliton-antisoliton pair.

June 20th, 2014 - 10:15AM
Seminar by Dr Oleg Szehr on Quantum Phases in Systems with Matrix-Product Ground States
Fri., June 20th at 10.15am in MPQ in seminar room B0.22, there will be a QCCC farewell seminar by:

Dr Oleg Szehr (who just moved to Cambridge, UK) on „On Quantum Phases in Systems with Matrix-Product Ground States“

We introduce Matrix Product states and their parent Hamiltonians and define the notion of a quantum phase in this framework. We provide a classification of phases of one-dimensional systems both with unique as well as degenerate ground states. We address the question of how robust the energy gap in the parent Hamiltonian model is to perturbations and provide conditions under which robustness is guaranteed.

Our methods rely on a close connection between translation-invariant Matrix product states and the Perron-Frobenius theory of certain associated quantum channels.

June 18th, 2014 - 3PM
Seminar by Matteo Rossi on Dynamics of Quantum Correlations for Two-Qubit Systems Interacting with Classical Noisy Environments
Wed., June 18th at 3.00pm 2014 in MPQ in seminar room B0.21, there will be a seminar by the ExQM applicant:

Matteo Rossi from Univ. of Parma on „Dynamics of Quantum Correlations for Two-Qubit Systems Interacting with Classical Noisy Environments“.

In this talk we consider single- and two-qubit systems coupled to classical stochastic fields and address both the decoherence and the non-Markovianity induced by the external fields.

Studying the interaction of a quantum system with its environment plays a fundamental role in the development of quantum technologies. Decoherence is detrimental for applications and it may be induced by classical or quantum noise, i.e. by the interaction with an environment described classically or quantum-mechanically. The classical description is often more realistic to describes environments with a very large number of degrees of freedom and it has also been shown that certain quantum environments may be described with equivalent classical models. We thus analyze in detail the dynamics of quantum correlations (entanglement and quantum discord) and evaluate the non-Markovianity of the induced dynamical quantum map for two-qubit systems interacting with classical stochastic fields, focusing on Gaussian processes.

May 16th, 2014 - 10AM
Seminar by Dr. Dmitry Savostyanov on Alternating Minimal Energy Methods for Linear Systems in Higher Dimensions
This week, we have distinguished guests from U Southampton and the Max-Planck for Maths in the Sciences, Leipzig. We are sharing ideas with the Schollwöck (LMU) and the Huckle (TUM) group in view of simulating large systems.

On Friday, May 16th 2014 at 10.00am (sharp) in MPQ on campus Garching Small Lecture Hall we will have a Post-QCCC/Pre-ExQM seminar by:

Dr. Dmitry Savostyanov, University of Southampton (Co-authors: Sergey Dolgov, MPI MiS Leipzig, and Ilya Kuprov, U Southampton) on „Alternating Minimal Energy Methods for Linear Systems in Higher Dimensions“

When high-dimensional problems are concerned, not many algorithms can break the curse of dimensionality and solve them efficiently and reliably. Among those, tensor product algorithms seem to be the most promising.

The first attempt to merge classical iterative algorithms and DMRG/MPS methods was made in a way, where the second Krylov vector is used to expand the search space on the optimisation step. The idea proved to be useful, but the implementation was based on the fair amount of physical intuition, and the algorithm is not completely justified.

We have recently proposed the AMEn algorithm for linear systems [3, 4], that also injects the gradient direction in the optimisation step, but in a way that allows to prove the global convergence of the resulted scheme. The scheme can be easily applied for the computation of the ground state—the differences to the algorithm of S. White [13] are emphasized in [5]. The AMEn scheme was recently applied for the computation of extreme eigenstates [7], using the block-TT format proposed in [2].

We aim to extend this framework and the analysis to other problems: eigenproblems, time-dependent problems, high-dimensional interpolation, and matrix functions; as well as to a wider list of high-dimensional problems.

This is a jointwork with Sergey Dolgov at the Max-Planck Institute for Mathematics in the Sciences, Leipzig, and Ilya Kuprov at the University of Southampton, UK.

selected refs.:

[2] S. V. Dolgov, B. N. Khoromskij, I. V. Oseledets, and D. V. Savostyanov. Computation of extreme eigenvalues in higher dimensions using block tensor train format.
Computer Phys. Comm., 185(4):1207–1216, 2014. doi:10.1016/j.cpc.2013.12.017.

[3] S.V. Dolgov and D.V. Savostyanov. Alternating minimal energy methods for linear systems in higher dimensions. Part I: SPD systems. arXiv preprint 1301.6068, 2013.
URL: http://arxiv.org/abs/1301.6068.

[4] S. V. Dolgov and D. V. Savostyanov. Alternating minimal energy methods for linear systems in higher dimensions. Part II: Faster algorithm and application to nonsymmetric systems. arXiv preprint 1304.1222, 2013. URL: http://arxiv.org/abs/
1304.1222.

[5] S. V. Dolgov and D. V. Savostyanov. Corrected one-site density matrix renormalization group and alternating minimal energy algorithm. In Proc. of ENUMATH 2013, accepted, 2014. URL: http://arxiv.org/abs/1312.6542.

[7] D. Kressner, M. Steinlechner, and A. Uschmajew. Low-rank tensor methods with subspace correction for symmetric eigenvalue problems. MATHICSE preprint 40.2013, EPFL, Lausanne, 2013.

[13] Steven R. White. Density matrix renormalization group algorithms with a single center site. Phys. Rev. B, 72(18):180403, 2005. doi:10.1103/PhysRevB.72.180403.
more details can be found in: SavostjanovMPQ14.pdf

April 7th, 2014 - 5PM
Seminar by Dr. Andreas Ruschhaupt on Shortcuts to Adiabaticity
Monday, April 7th 2014 at 5pm in MPQ on campus Garching Seminar Room B0.22 we will have a Post-QCCC/Pre-ExQM seminar by:

Dr. Andreas Ruschhaupt, Cork University, Ireland (formerly jun. Prof. with Reinhard Werner) on „Shortcuts to Adiabaticity“

Quantum adiabatic processes -that keep constant the populations in the instantaneous eigenbasis of a time-dependent Hamiltonian-are very useful to prepare and manipulate states, but take typically a long time. This is often problematic because decoherence and noise may spoil the desired final state, or because some applications require many repetitions.

„Shortcuts to adiabaticity“ are alternative fast processes which reproduce the same final populations, or even the same final state, as the adiabatic process in a finite, shorter time [1]. We present such „shortcuts to adiabaticity“ for the manipulation of the atomic motional state [2] as well as for the passage from one internal atomic state to another [3-5]. We especially study and compare the stability of different shortcut schemes concerning different types of perturbations like, for example systematic and noise errors [4, 6] or errors originating from unwanted transitions to other levels [7].

References:
[1] E. Torrontegui, S. Ibáñez, S. Martínez-Garaot, M. Modugno,
A. del Campo, D. Guéry-Odelin, A. Ruschhaupt, Xi Chen and J. G. Muga,
Adv. At. Mol. Opt. Phys. 62 (2013) 117
[2] Xi Chen, A. Ruschhaupt, S. Schmidt, A. del Campo,
D. Guéry-Odelin and J. G. Muga,
Phys. Rev. Lett. 104 (2010) 063002
[3] Xi Chen, I. Lizuain, A. Ruschhaupt, D. Guéry-Odelin and J. G. Muga,
Phys. Rev. Lett. 105 (2010) 123003
[4] A. Ruschhaupt, X. Chen, D. Alonso and J. G. Muga,
New J. Phys. 14 (2012) 093040
[5] S. Ibáñez, Xi Chen, E. Torrontegui, J. G. Muga and A. Ruschhaupt,
Phys. Rev. Lett. 109 (2012) 100403
[6] D. Daems, A. Ruschhaupt, D. Sugny and S. Guerin,
Phys. Rev. Lett. 111 (2013) 050404
[7] A. Kiely and A. Ruschhaupt, arXiv:1312.3210

 

Former PhD Programme QCCC (Quantum Computing, Control, and Communication):

Events 2013

December 3rd, 2013
Big Data and Responsability
QCCC invited to the 5th ENB Forum in the Grand Aula of LMU with three lectures by the guests Dr. Frank Schirrmacher, Prof. Dr. Dieter Kranzlmüller and Dr. Richard von Schirach on the topic „Big Data and Responsability“. See video recording by eUniversity at LMU.
November 12th, 2013
Exchange of Ideas on Quantum-Many-Body Systems at MPQ
In order to prepare for „life after QCCC“, we will have an informal mini-workshop
to exchange ideas on quantum many-body systems with Prof. Schollwöck, who could not come to Prien. More precisely, the goal is to exchange ideas of numerical methods and the demands for them in backing quantum simulations of multi-body systems.

We meet Tue Nov. 12th, at the MPQ with the schedule as follows:

11:30am coffee/lunch gathering at MPQ cafeteria.

Seminar Room of Cirac Group (2nd floor):
12:00 am Prof. Schollwöck: „Nature of the Spin-Liquid Ground State of the S=1/2 Heisenberg Model
on the Kagome Lattice“
12:45 am extended discussion: Identifying Cutting Edge Problems

1:00-1.30pm coffee break

Seminar Room B0.21 of MPQ (ground floor):
1.30pm Thomas Barthel: „Some tensor network state techniques and entanglement in condensed matter systems with application to fermionic systems“
2.00pm : Thomas Schulte-Herbrüggen : „Symmetry Principles in the Quantum Systems Theory of Many-Body Systems“

2.45-3.15 pm Coffee

3.15-3.45 pm „Robert Zeier: „Two Teasers: 1. Simulating Sparse Qubit Systems,
2. Further Results on Fermionic Systems“
3.45-4.15 pm Marie Carmen Banuls: „Tensor Network Methods Applied to Lattice Gauge Theories“
4.15-4.45 pm Prof. Thomas Huckle/Konrad Waldherr: „(1) General Overview on Recent Tensor Methods in Maths
plus (2) Numerical (Multi-)Linear Algebra in Quantum Tensor Networks (Results and Perspectives)“

5.00 pm general discussion

~5.45 pm end with option to have dinner in Garching

October 17th - 21st, 2013
QCCC Closing Symposium and Workshop
QCCC Closing Symposium and Workshop on Campus Garching / in Prien am Chiemsee
September 9th, 2013
Quantum Measurement Theory lecture by Rob Spekkens
Three lectures on „Quantum Measurement Theory“ by Rob Spekkens, Paul Busch and Teiko Heinosaari, supported by QCCC.
July 19th, 2013
Business Meeting
Business Meeting regarding the 4th QCCC Workshop in Prien.
July 5th, 2013
Seminar by Daniel Reitzner on Quantum Measurements and Joint Measurability
Talk on
Quantum measurements and joint measurability

Dr. Daniel Reitzner (TUM)

Measurements in quantum mechanics are, compared to classical measurements, somewhat non-intuitive and in particular can be incompatible; i.e. a pair of measurements on a single system can turn out to be impossible to perform at once. Although it is quite often stated that this is a basis for Heisenberg uncertainties; we will show the limitations and dangers of this description.

With a more modern and in a sense more general definition of quantum measurement via POVMs that we shall introduce as well, we will see, that simultaneous (and/or sequential) measurements are a tricky and still unresolved concept that has an impact on modern applications within quantum information community.

May 14th, 2013
Seminar by Thorsten Wahl Localizable Entanglement & Gaussian Fermionic PEPS
Talk on
Localizable Entanglement & Gaussian Fermionic PEPS

Thorsten Wahl (MPQ)

This talk consists of two parts. In the first one I will introduce the concept of Localizable Entanglement, which is important for the detection of topological quantum phase transitions and ideal quantum repeaters in the case where the Localizable Entanglement is constant over arbitrary long distances. Finally, I will provide a necessary and sufficient condition for the later case (also denoted as long-range Localizable Entanglement) for Matrix Product States.

The second part of my talk is devoted to the approximation of topological insulators by Gaussian fermionic PEPS which are the free Fermionic version of Projected Entangled Pair States. I will show under which conditions Gaussian fermionic PEPS are topologically non-trivial.

February 18th/19th, 2013
QCCC & TMP Block Course by Michael Keyl on Quantum Information Theory in Infinite Dimensions: An Operator-Algebra Approach
Block course on:

Quantum Information Theory in Infinite Dimensions: An Operator-Algebra Approach.

PD Prof. Michael Keyl (FU Berlin)

Most of quantum information theory is developed in the framework of finite dimensional Hilbert spaces, and therefore not directly applicable to systems like free and interacting non-relativistic particles, spin-systems in the thermodynamic limit or relativistic field models, where an infinite dimensional description is required. In some cases a more or less direct generalization is possible (e.g. by replacing finite sums with absolutely converging sequences) but this approach is very limited and misses many of the more interesting aspects of infinite dimensional systems. In other words mathematically and conceptually new tools are needed. In this context the theory of operator algebras provides a very powerful framework, which is particularly useful for the study of infinite degrees of freedom systems.

The purpose of this lecture series is to introduce into this theory and its applications in qantum physics. Apart from the corresponding mathematical foundations we will show how elementary concepts of quantum theory can be reformulated and how the differences between finite dimensions, infinite dimensions but finite degrees of freedom, and infinite degrees of freedom can be related to operator algebras and their representations. Furthermore we will study infinite spin systems, their entanglement properties and their connection to advanced operator algebraic topics, like type and cassification of von Neumann algebras.

February 14th, 2013
Seminar by Oleg Szehr on Spectral Convergence Bounds for Finite Classical and Quantum Markov Chains
Talk on
Spectral Convergence Bounds for Finite Classical and Quantum Markov Chains

Oleg Szehr (TUM)

In this talk I present a new framework that yields spectral bounds on norms of functions of transition maps for finite, homogeneous Markov chains. The techniques employed work for bounded semigroups, in particular for classical as well as for quantum Markov chains and they do not require additional assumptions like detailed balance, irreducibility or aperiodicity. I use the method in order to derive convergence bounds that improve significantly upon known spectral bounds. The core technical observation is that power-boundedness of transition maps of Markov chains enables a Wiener algebra functional calculus in order to upper bound any norm of any holomorphic function of the transition map.

February 8th, 2013
Seminar by Michael Keyl on Mathematical Physics with hbar
Talk on
Mathematical Physics with \hbar

PD Prof. Michael Keyl (FU Berlin)

In this talk I will review a number of research projects from different areas of quantum physics, including: Mean field flucutations of spin-systems and their relation to continuous variable quantum systems; quantum field theory in space-times with causality violations; and quantum control of bosonic and Fermionic systems.

February 8th, 2013
Inaugural Lecture by Michael Wolf on Short Stories from Quantum Information Theory
Lecture on
Short Stories from Quantum Information Theory

Prof. Michael M. Wolf (TUM)

The talk aims at providing a taste of Quantum Information Theory exemplified through two problems from different branches of the field.

In the first part we will encounter quantum correlations that are arbitrarily stronger than their classical counterparts. In physics this is related to the foundations of quantum theory, in mathematics to Grothendieck type inequalities within operator space theory, and in theoretical computer science to the reduction of communication complexity. The latter perspective suggests how – in the distant future – the scheduling of the colloquium mightbe made more efficient.

The second part will shed new light on the energy gap problem from condensed matter theory. Despite considerable effort and interest, there is basically neither a proof technique nor a numerical method known for solving this type of problem. We will argue that the roots of this difficulty may be deeper than expected by showing that there are cases for which there cannot be a proof (in the sense of Gödel) or an algorithm (in the sense of Turing).

January 9th, 2013
TUM Mathematics Colloquium by David Gross on Compressed Sensing and Matrix Completion: From Single Pixel Cameras to Quantum State Tomography
Talk on
Compressed Sensing and Matrix Completion: From Single Pixel Cameras to Quantum State Tomography

Prof. David Gross (Uni Freiburg)

Very time the release button of a digital camera is pressed, several megabytes of raw data are recorded. But the size of a typical jpeg output file is only 10% of that. What a waste! Can’t we design a process which records only the relevant 10% of the data to begin with? The recently developed theory of compressed sensing achieves this trick for sparse signals. I will give a short introduction to the ideas and the math behind compressed sensing.

A basis-independent notion of „sparsity“ for a matrix is its rank. One is thus naturally led to the „low-rank matrix recovery“ problem: can one reconstruct and unknown low-rank matrix from few linear measurements? The answer is affirmative. The arguably simplest proof to date is based on ideas from quantum information theory. In the second half of the presentation, I will talk about applications and proof techniques for the matrix theory, including the links to quantum.

 

Events 2012

December 7th, 2012
Seminar by David Reeb on Fault-Ignorant Algorithms
„Fault-Ignorant Quantum Search“

Abstract: We investigate the problem of quantum searching on a noisy quantum computer. Taking a „fault-ignorant“ approach, we design quantum algorithms that solve the task for various different noise strengths, possibly unknown beforehand. The rationale is to avoid costly overheads, such as traditional quantum error correction.

Proving lower bounds on algorithm runtimes, which may depend on the actual level of noise, we find that the quadratic speedup is lost (in our noise models). Nevertheless, for low noise levels, our algorithms outperform the best noiseless classical search algorithm. Finally, we provide a more general framework to formulate fault-ignorant algorithms.

November 30th, 2012
Seminar by Marc Cheneau on Spin Physics in Optical Lattices
In this lecture, we will present two recent experimental results obtained in the Bloch group that illustrate the potential of ultracold atomic gases to understand the emergence of many-body phenomena. In a first part, we will show that a gas of ultracold atoms in a Mott-insulating state represent a fairly good realization of the Heisenberg model, with the electronic hyperfine state playing the role of an effective spin. Using a new experimental technique, we were able to directly reveal the Heisenberg coupling by monitoring the real-time dynamics of a single spin impurity introduced in the system in a controlled way. Beyond this paradigmatic, but analytically solvable problem, we also explored a regime where real particle hopping becomes important and the spin impurity gets dressed by the surrounding bath to form a polaronic quasiparticle.

In the second part of the lecture, we will introduce a different setup that is a realization of a traverse-field Ising model with long-range interactions. This time we coupled the atoms to a laser beam driving a transition to a highly-excited electronic state, a so-called Rydberg state. The enormous van der Waals interaction between two atoms in such a state gives rise to strong spatial correlations over distances much larger than the interparticle distance. Here we could observe the spontaneous formation of well-defined geometric structures of a few Rydberg excitations and gather some evidence that the system had been excited to a highly-entangled many-body state.

October 19th, 2012
Seminar by Christian Sames on Controllable Single‐Atom Phase Shifter Working in the Single‐Photon Level
„Controllable Single-Atom Phase Shifter Working in the Single-Photon Level“

In our system we strongly couple a single atom to the light field of an optical resonator. I will give a brief introduction to what can be deduced from the phase of the intra-cavity field and how we can build a single atom phase shifter with this system.

September 7th, 2012
Seminar by Daniel Lercher on Gaussian superactivation requires squeezing
The talk is dedicated to the topic of superactivation of the quantum capacity of Gaussian channels. Smith et. al. have recently shown that for two such channels with zero capacity Superactivation (0 + 0 > 0) can be achieved. They provided explicit examples that make use of active optical devices like squeezers.

In this talk I’ll show that there is no superactivation for Gaussian channels that are generated by passive means.

August 31st, 2012
Seminar by Juan Bermejo on A Gottesman-Knill Theorem for all Finite Abelian Groups
In this seminar, I will introduce „normaliser circuits“, a family of quantum operations that play a relevant role in quantum algorithms:
prominent examples of normaliser gates are quantum Fourier transforms (QFT)—sometimes said to be „the source of various exponential quantum speedups“—, subroutines that generate highly entangled states and adaptive measurements.

Recently we have investigated the computational power of normaliser circuits and found that, in spite of their apparent quantumness, they can be efficiently simulated in a classical computer. Thus, a quantum computer operating within this set of gates can not offer exponential quantum speed-ups over classical computation, regardless e.g. the number of QFT it uses. Our result generalises a well-known theorem of Gottesman and Knill, valid for qubits, to systems that do not decompose as products of small subsystems.

Format:
I will introduce some elements of group theory needed to understand our theorem and the main tool we developed to prove it: a stabiliser formalism for high dimensions. The latter may be of independent interest in quantum error correction and fault tolerant quantum computing. I will also explain the relation of these results with Shor’s algorithm.

June 29th, 2012
Seminar by Thomas Schulte-Herbrüggen on Reachability under Noise Control
„Combining Coherent and Noise Control:“
‚How to Transfer between Arbitrary n-Qubit States by Coherent Control and Simplest Switchable Noise on a Single Qubit‘

We explore reachable sets of open $n$-qubit quantum systems the coherent parts of which are under full unitary control and that have just one qubit whose unital or non-unital noise amplitudes can be modulated in time such as to provide an additional degree of incoherent control. In particular, adding bang-bang control of amplitude damping noise (non-unital) allows the dynamic system to act transitively on the entire set of density operators. This means one can transform any initial quantum state into any desired target state. Adding switchable bit-flip noise (unital), on the other hand, suffices to explore all states majorised by the initial state. We have extended our optimal control algorithm (DYNAMO) by degrees of incoherent control so that these unprecedented reachable sets can systematically be exploited for experimental settings. Numerical results are compared to constructive analytical schemes.

June 28th, 2012
Extreme Optics in Semiconductors
Seminar by Prof. Mark Sherwin from Santa Barbara on „Extreme Optics in Semiconductors: When Quasiparticles Collide, and 1+1=11“.]
June 6th, 2012
Seminar by Dr Oleg Szehr on Perturbation Theory for Fixed Points of Quantum Channels
Titel: Perturbation Theory for Fixed Points of Quantum Channels

Abstract: It is clear that if the transition matrix of an irreducible quantum Markov-process has a sub dominant eigenvalue which is close to 1 then the quantum Markov-process is ill conditioned in the sense that there are stationary states which are sensitive to perturbations in the transition matrix. However, the converse of this statement has heretofore been unresolved. The purpose of this talk is to present upper and lower bounds on the condition number of the chain such that the bounding terms are determined by the closeness of the sub dominant eigenvalue to unity.

We obtain perturbation bounds which relate the sensitivity of the chain under perturbation to its rate of convergence to stationarity.

June 5th, 2012
Seminar by Dr David Gross
Seminar by Dr David Gross.
April 13th, 2012
Seminar by Dr Markus Grassl on Polynomial Invariants of Three-Qubit Systems
Seminar by Dr Markus Grassl on „Polynomial Invariants of Three-Qubit Systems“:

Polynomial invariants provide a tool to characterise quantum states with respect to local unitary transformations. Unfortunately, the situation becomes very complicated already for mixed states of three qubits due to combinatorial explosion.

After an introduction to the mathematical background and general tools, the talk will present preliminary results for mixed quantum states and Hamiltonians for three-qubit systems.

The talk is based on joint work in progress with Robert Zeier.

 

Events 2011

October 7th - 10th, 2011
3rd QCCC Workshop
3rd QCCC Workshop in Bernried
September, 4th - 9th, 2011
CoQuS Summer School
Invitation to the CoQuS Summer School.
September, 4th - 9th, 2011
Heraeus Summer School
July 22nd, 2011
Seminar by Michael-James Kastoryano on Convergence of Quantum Processes: going beyond the gap
I will give a brief introduction to the problem of estimating the convergence time a quantum channel to its fixed point(s).

In particular, I will show that bounding the gap of the channel is sometimes insufficient for bounding the convergence time.

I will review some of the tools which are available (and some which will soon be), and discuss some of the difficulties in extending classical mixing time methods to the quantum setting.

Finally, I will provide some applications of these methods introduced, and give an outlook on some open problems.

July 19th, 2011
ENB Forum

ENB Forum: From the Nobel Prize to Being Entrepreneur
Lectures by

  • Theodor W. Hänsch: Passion for Precision
  • Ronald Holzwarth: Firmerngründung mit Nobelpreistechnologie
June 27th, 2011
Seminar by Bogdan Pirvu on MPS-Simulations of Translationally Invariant Spin Chains with Periodic Boundary Conditions
We present several algorithms for the simulation of translationally invariant (TI) quantum spin chains with periodic boundary conditions (PBC). By using TI matrix product states (MPS) we can in general reduce the computational cost by a factor N, with N the number of spins in the chain.

First we present an algorithm for the approximation of ground states (GS) that is based on the computation of the gradient of the energy [1]. We achieve a scaling of the computational cost of O(D3n2) + O(D3mn), where D is the virtual bond dimension of the MPS and m and n are some parameters that will be explained in more detail in the talk. There is a tradeoff between the parameters n and m and we show how to find the optimal balance. The analysis of the numerical results confirms previous observations regarding the induced correlation length of MPS with finite D [2, 3]. Furthermore we observe a crossover between the finite-N scaling and finite-D scaling in the context of critical quantum spin chains similar to the one observed by Nishino [4] in the context of classical two dimensional systems.

Next we present an algorithm for the approximation of dispersion relations that uses as an ansatz MPS-based states with well defined momentum [5]. Here, we achieve a scaling of the computational cost of O(D6N2). Due to the large D scaling we are restricted to comparatively small D. Nonetheless we obtain very good approximations of one-particle excitations. The numerical results yield some insight into the interpretation of the quasiparticles that occur in the exact solution of the Quantum Ising Model with PBC.

June 7th, 2011
Seminar by Christian Gogolin on Thermalization in nature and on a quantum computer
Using the assumption that thermodynamic systems evolve towards Gibbs states, i.e. states with a well defined temperature, statistical mechanics and thermodynamics have been amazingly successful in explaining a wide range of physical phenomena. In stark contrast to this strong justification by corroboration of these theories, the question of whether and how the methods of statistical mechanics and thermodynamics can be justified microscopically was still wide open until recently.

With new mathematical tools from quantum information theory becoming available, there has been a renewed effort to settle this old question.

I will present and discuss a necessary and a sufficient condition for the emergence of Gibbs states from the unitary dynamics of quantum mechanics and show how these new insights into the process of equilibration and thermalization can be used to design a quantum algorithm that prepares thermal states on a quantum computer/simulator.

June 6th, 2011
Seminar by Volkher B. Scholz on Information theoretic questions on von-Neumann algebras
We study information theoretic questions on von-Neumann algebras. There, one first has to discuss the notion of independent subsystems, which leads to a well-known open question in operator algebra theory, the embedding problem of Connes. I will clarify the equivalence, and proceed by describing some information theoretic tasks in the setting of von-Neumann algebras, as well as discussing the connection to quantum cryptography. 
May 31st, 2011
Guided Lab Tour
Guided Lab Tour
March 29th, 2011
Seminar by John Marriott
March 10th, 2011
Seminar by Christine Tobler on A Matlab toolbox for tensors in hierarchical Tucker format and its application to the solution of high-dimensional linear systems
The hierarchical Tucker format is a storage-efficient scheme to approximate and represent tensors of possibly high order. From a tensor network perspective, it represents a network of order-3 tensors without cycles. Thus, it is a generalization of the Tensor Train or Matrix Product States (MPS) format.

This talk introduces a Matlab toolbox, along with the underlying methodology and algorithms, providing a convenient way to work with this format. The toolbox not only allows for the efficient storage and manipulation of tensors but also offers a set of tools for the development of higher-level algorithms.

As an example for the use of the toolbox, an algorithm for solving high-dimensional linear systems, namely parameter-dependent elliptic PDEs, is shown. This is joint work with Daniel Kressner, ETH Zurich.

 

Events 2010

December 21st, 2010
Seminar by Boris Khoromskij
December 20th, 2010
Seminar by Yuval Sanders on Relative quantifiers of quantum informational resources
A full quantification of the entanglement of a quantum state is desirable if the degradation of entanglement in some quantum information processing task is to be minimized. A protocol for the accomplishment of a task can then be evaluated by the amount of entanglement consumed to enact that protocol. Such quantification is typically provided by a collection of entanglement monotones.

We propose a generalization of entanglement monotones that may provide greater flexibility in the quantification of entanglement. Rather than quantifying the entanglement of a state directly, we suggest a relative quantification: a direct comparison of one entangled state to another. We provide an example of such relative quantification for a quantum information resource known as frameness.

December 15th, 2010
Seminar by Xinhua Peng on Spin qubits for quantum simulation – Simulating physical phenomena on a quantum computer
Simulating quantum mechanical systems is a classical hard problem because the computational difficulties hinges on the exponential growth of the size of Hilbert space with the number of particles in the system.

In the context of quantum information processing, this difficulty becomes the main source of power: in some situations, information processors based in quantum mechanics can process information exponentially faster than classical systems. From the perspective of a physicist, one of the most interesting applications of this type of information processing is the simulation of quantum systems. We call a quantum information processor that simulates other quantum systems a quantum simulator.

Using a kind of nuclear magnetic resonance simulator, we implement the simulations of the Heisenberg spin models by the use of average Hamiltonian theory and observe the quantum phase transitions by using different measurements, e.g., entanglement, fidelity decay and geometric phase: the qualitative changes that the ground states of some quantum mechanical systems exhibit when some parameters in their Hamiltonians change through some critical points. In particular, we consider the effect of the many-body interactions. Depending on the type and strength of interactions, the ground states can be product states or they can be maximally entangled states representing different types of entanglement.

When the many-body interaction (such as the three-body interaction) takes part in the competition, new critical phenomena that cannot be detected by the traditional two-spin correlation functions will occur.

By quantifying different types of entanglement, or by using suitable entanglement witnesses, we successfully detect two types of quantum transitions. Besides this, using such a NMR quantum simulator, we can also simulate the static properties and dynamics of chemical systems, such as the ground-state energy of Hydrogen molecule.

December 15th, 2010
Seminar by Martin Plenio on Describing system environment interactions in the non-perturbative regime
Prof. Martin Plenio (Universitaet Ulm):

Describing system environment interactions in the non-perturbative regime

Recent experiments have provided strong evidence for the existence of quantum coherence in the early stages of photosynthesis. Subsequent theory work shows that the optimal operating regime lies in the regime where the system-environment interaction is strong so that the system is neither fully quantum coherent nor fully classical, but rather half way in between. In this regime perturbative treatments of the system environment interaction are not valid. Here I discuss the above issue and then present a novel approach to the numerical and analytical study of spin systems in strong contact with environments made up of harmonic oscillators.

This talk is based on
M.B. Plenio and S.F. Huelga
– Dephasing assisted transport: Quantum networks and biomolecules –
New J. Phys. 10, 113019 (2008) and E-print arXiv:0807.4902 [quant-ph]

F. Caruso, A.W. Chin, A. Datta, S.F. Huelga and M.B. Plenio
– Highly efficient energy excitation transfer in light-harvesting complexes: The fundamental role of noise-assisted transport –
J. Chem. Phys. 131, 105106 (2009) and E-print arXiv:0901.4454 [quant-ph]

J. Prior, A.W. Chin, S.F. Huelga and M.B. Plenio
– Efficient simulation of strong system-environment interactions –
Phys. Rev. Lett. 105, 050404 (2010) and E-print arXiv:1003.5503 [quant-ph]

A.W. Chin, A. Rivas, S.F. Huelga and M.B. Plenio
– Exact mapping between system-reservoir quantum models and semi-infinite discrete chains using orthogonal polynomials –
J. Math. Phys. 51, 092109 (2010) and E-print arXiv:1006.4507 [quant-ph]

October 22nd, 2010
Seminar by Haidong Yuan on Optimal Control of Quantum Systems
Optimal Control of Quantum Systems

We consider the optimal control problem of transferring population between states of a quantum system where the coupling proceeds only via intermediate states that are subject to decay. We pose the question whether it is generally possible to carry out this transfer.

For a single intermediate decaying state, we recover the Stimulated Raman Adiabatic Passage (STIRAP) process for which we present analytic solutions in the finite time case. The solutions yield perfect state transfer only in the limit of infinite time.

We also present analytic solutions for the case of transfer that has to proceed via two consecutive intermediate decaying states. We show that in this case, for finite power the optimal control does not approach perfect state transfer even in the infinite time limit. We generalize our findings to characterize the topologies of paths that can be achieved by coherent control.

September 24th, 2010
Seminar by Corey O'Meara on Geometric Properties of Special Classes of Completely Positive Linear Maps
Completely positive linear mappings between C*-algebras were originally developed in the 1950’s as a special case of positive linear operators between matrix algebras. Within the last two decades, mathematical physicists have determined that completely positive maps play a crucial role in quantum information theory as structures which model information transfer between quantum systems.

This talk will serve as an introduction to two classes of completely positive maps: the Schur maps which arise from the Schur matrix product and maps which are equal to their adjoint.

After focusing on results concerning the geometry of these two sets of CP maps, we introduce a general framework that unifies certain classes of CP maps in terms of C*-subalgebras of Mn.

August 20th, 2010
Seminar by Patrick Rebentrost on The Role of Quantum Coherence in Photosynthetic Energy Transfer
Seminar by Patrick Rebentrost

On the Role of Quantum Coherence in Photosynthetic Energy Transfer

Recent experiments have provided evidence for long-lived electronic coherence in photosynthetic light-harvesting complexes at room temperature. This talk presents some of the work performed in theAspuru-Guzik group on the Fenna-Matthews-Olson complex. This includes the basic concept of environment-assisted excitonic transport and a quantification of the role of coherence by its contribution to the transport efficiency.

We find that, depending on the spatial correlations in the phonon environment, there is about a 10% contribution of coherent dynamics to the exciton transfer efficiency. In addition, we investigate a time-convolutionless non-Markovian master equation approach and show our quantum chemistry inspired way of incorporating atomistic detail of the protein environment into the exciton dynamics.

July 22nd, 2010
Workshop
Workshop on Optomechanics (programme)
July 16th, 2010
Seminar by Frank Wilhelm on Quantum optics on a chip - photon counters and NOON states
Seminar by Frank Wilhelm

Quantum optics on a chip – photon counters and NOON states

Circuit quantum electrodynamics is a maturing field in which the physics of quantum optical setups is realized in cryogenic electric circuits, profiting from large achievable coupling strengths. Elements like cavities, artificial atoms, mirrors, and beamsplitters have been successfully demonstrated. The missing element is a single-photon counter as microwave photons are usually amplified instead of counted, and as most of these amplifiers are noisy.

I will present the Josephson Photomultiplier, a simple device that allows single photon counting at high efficiency and bandwidth. Quantum optics with multiple modes has highlightes NOON states – states in which N photons are in a superposition of two arms of an interferometer for quantum-enhanced metrology. I am going to show how these can be created deterministically in circuit QED.

The success of such an experiment is difficult to determine as the reconstruction of a two-mode density matrix at large photon number is forbiddingly cumbersome. We are going to show that it is much more efficient to test for a hypothesis state and then estimate the overlap between the hypothetical state and the physical state using nonlinear programming.

May 27th, 2010
Seminar by Prof. M. Christandl on The Uncertainty Principle in the Presence of Quantum Memory
Seminar by Prof. M. Christandl

The uncertainty principle lies at the heart of quantum theory, illuminating a dramatic difference with classical mechanics. The principle bounds the uncertainties of the outcomes of any two observables on a system in terms of the expectation value of their commutator. It implies that an observer cannot predict the outcomes of two incompatible measurements to arbitrary precision.

However, this implication is only valid if the observer does not possess a quantum memory, an unrealistic assumption in light of recent technological advances. In this work we strengthen the uncertainty principle to one that applies even if the observer has a quantum memory. We provide a lower bound on the uncertainty of the outcomes of two measurements which depends on the entanglement between the system and the quantum memory.

We expect our uncertainty principle to have widespread use in quantum information theory, and describe in detail its application to quantum cryptography. The talk is based on joint work with Mario Berta, Roger Colbeck, Joe Renes and Renato Renner (http://arxiv.org/abs/0909.0950).

May 21st, 2010
Seminar by Dr. Robert Zeier on Dynamical Quantum Systems: Controllability, Symmetries, and Representation Theory
Seminar by Dr. Robert Zeier

„Dynamical Quantum Systems: Controllability, Symmetries, and Representation Theory“

We analyze the controllability of dynamical quantum systems. One can decide controllability by computing the Lie closure [1] which is sometimes cumbersome. These topics can be discussed likewise for translationally invariant lattices [2]. Building on previous work [3,4], we propose an additional method which utilizes the symmetry properties of the considered system.

We obtain as a necessary condition for controllability that the system should not have any symmetries and act therefore irreducibly. But this condition is not sufficient as there exist irreducible subalgebras of the maximal possible system Lie algebra. We classify the irreducible subalgebras and their inclusion relations relying on results of Dynkin [5]. Using optimized computer programs we can tabulate irreducible subalgebras up to dimension 215 (i.e. 15 qubits) complementing results of McKay and Patera [6].

For concrete dynamical quantum systems many irreducible subalgebras can be ruled out as obstructions for full controllability and we present algorithms to this end. Our results provide an insight into the question when spin, bosonic, and fermionic systems can simulate each other. We will give a short introduction to the relevant representation theory of Lie algebras.

[1] Jurdjevic/Sussmann, J. Diff. Eq. 12, 313 (1972)
[2] Kraus/Wolf/Cirac, Phys. Rev. A 75, 022303 (2007)
[3] Sander/Schulte-Herbrüggen, http://arxiv.org/abs/0904.4654
[4] Polack/Suchowski/Tannor, Phys. Rev. A 79, 053403 (2009)
[5] Borel/Siebenthal, Comment. Math. Helv. 23, 200 (1949);
Dynkin, Trudy Mosov. Mat. Obsh. 1, 39 (1952),
Amer. Math. Soc. Transl. (2) 6, 245 (1957);
Dynkin, Mat. Sbornik (N.S.) 30(72), 349 (1952),
Amer. Math. Soc. Transl. (2) 6, 111 (1957)
[6] McKay/Patera, Tables of Dimensions, Indices, and Branching Rules
for Representations of Simple Lie Algebras (1981)

March 1st, 2010
Squeezing the Matrix: efficient spin dynamics simulation algorithms
Seminar by Dr. Ilya Kuprov on „Squeezing the Matrix: efficient spin dynamics simulation algorithms“.
February 18th, 2010
The Energy Minimization Problem in the Control of Dissipative Spin-1/2 Particles
Seminar by Prof. Bernard Bonnard on „The Energy Minimization Problem in the Control of Dissipative Spin-1/2 Particles“.
January 14th, 2010
Seminar by Prof. Jr-Shin Li on Ensemble Controllability and by Justin Ruths on A Pseudospectral Method for Optimal Control of Open Quantum Systems
Seminar by Prof. Jr-Shin Li and by Justin Ruths

In this paper, we present a unified computational method based on pseudospectral approximations for the design of optimal pulse sequences in open quantum systems. The proposed method transforms the problem of optimal pulse design, which is formulated as a continuous-time optimal control problem, to a finite dimensional constrained nonlinear programming problem.

This resulting optimization problem can then be solved using existing numerical optimization suites. We apply the Legendre pseudospectral method to a series of optimal control problems on open quantum systems that arise in Nuclear Magnetic Resonance (NMR) spectroscopy in liquids. These problems have been well studied in previous literature and analytical optimal controls have been found.

We find an excellent agreement between the maximum transfer efficiency produced by our computational method and the analytical expressions. Moreover, our method permits us to extend the analysis and address practical concerns, including smoothing discontinuous controls as well as deriving minimum-energy and time-optimal controls. The method is not restricted to the systems studied in this article and is applicable to optimal manipulation of both closed and open quantum systems.

January 15th, 2010
Seminar by Corey O'Meara on Schur Maps and Self-Dual Quantum Channels
Seminar by Corey O’Meara

The relationship between characteristics of quantum channels and the geometry of their respective sets can provide a useful insight to some of their underlying properties. First we will discuss a special class of random unitary channels, namely, the Schur maps. We then use the generalization of these maps to motivate the study of what we call Self-Dual quantum channels. Some preliminary geometric properties of the set of such maps are investigated and compared to the geometry of the set of Schur maps. Finally, some preliminary algebraic results are discussed which involve the eigenvalues of Self-Dual quantum channels.

December 11th, 2010
Seminar by Volkher Scholz on Anderson Localization in Disordered Quantum Walks
Seminar by Volkher Scholz

Anderson Localization in Disordered Quantum Walks
(**Volkher Scholz**, Albert Werner, and Andre Ahlbrecht)

We study a Spin-$\frac{1}{2}$-particle moving in a one dimensional lattice subjected to disorder induced by a random space dependent coin. The discrete time evolution is given by a family of random unitary quantum walk operators, where the shift operation is assumed to be non-random. Each coin is an independent identically distributed random variable with values in the group of two dimensional unitary matrices. We find that if the probability distribution of the coins is absolutely continuous with respect to the Haar measure, then the system exhibits localization. That is, every initially localized particle remains on average and up to exponential corrections in a finite region of space for all times.

 

Events 2009

December 4th, 2009
Seminar by Shai Machnes
Seminar by Shai Machnes on „Optimal control for superfast cooling of trapped ions, using a Matlab toolbox for quantum information calculations
November 20th, 2009
Seminar by Prof. Mikio Nakahara on Topological Quantum Computing with p-Wave Superfluid
Seminar by Prof. Mikio Nakahara

It is shown that Majorana fermions trapped in three p-wave superfluid vortices form a qubit in a topological quantum computing (TQC). Several similar ideas have already been proposed: Ivanov [Phys. Rev. Lett. 86, 268 (2001)] and Zhang et al [Phys. Rev. Lett. 99, 220502 (2007)] suggested schemes in which a qubit is implemented with two and four Majorana fermions, respectively, where a qubit operation is performed by braiding the world lines of these Majorana fermions. Naturally the set of quantum gates thus obtained is a discrete subset of the relevant unitary group.

We propose a new scheme, where three Majorana fermions form a qubit. We show that continuous qubit operations are made possible by braiding the Majorana fermions complemented with dynamical phase change. Moreover, it is possible to introduce entanglement between two such qubits by geometrical manipulation of some vortices involved.

October 10th - 14th, 2009
2nd QCCC Workshop in Bad Tölz
2nd QCCC Workshop in Bad Tölz
September 18th, 2009
Seminar by Fernando Pastawski on How Long Can Passive Quantum Memories Withstand Depolarizing Noise?
Seminar by Fernando Pastawski

How Long Can Passive Quantum Memories Withstand Depolarizing Noise?

Abstract: Existing fault tolerance theorems state that robust quantum computation and in particular, quantum memories may be achieved by growing the number of dedicated resources. Such theorems assume the availability of fresh ancillas (qubits in a predefined state) and the possibility of periodically applying recovery operations. Experimentally however, these requirements have shown to be hard to meet. In an attempt to provide a simpler path, many body Hamiltonians have been proposed with the hope that they could through their dynamics alone provide long protection times to quantum information. I will explain recent results which show that under a depolarizing noise model, protection times may not exceed O(log N) and such scaling is achievable by many body non-local Hamiltonians. I will go on to mention existing proposals for protecting Hamiltonians and describe some limitations we have found for the information lifetime under comparatively weak Hamiltonian perturbations.

September 11th, 2009
Seminar by Sebastian Nauerth on the Exploration of Side Channels in our BB84 Freespace Quantum Key Distribution System
Seminar by Sebastian Nauerth

Exploration of Side Channels in our BB84 Freespace Quantum Key Distribution System

The security of quantum key distribution, (QKD) is based on physical laws rather than assumptions about computational complexity: An adversary will necessarily disturb the communication by his quantum measurement. However, real implementations will be sensitive to side-channel attacks, i.e. to information losses due to distinguishabilities in other degrees of freedom, which an adversary can measure without causing errors.

We are running an implementation of the BB84 protocol installed on top of two university buildings in downtown Munich. Using attenuated laser pulses in combination with decoy states we are able to establish a secret key over a distance of 500 m. Our system is fully remote controlled and allows for continuous and fast QKD. I will report on the characterization of this QKD system with respect to side channels of the transmitter and the receiver and also show some attacks.

September 8th, 2009
Seminar by Pierre de Fouquieres on Limitations of Quantum Optimal Control
Limitations of Quantum Optimal Control

Abstract: The use of local, typically gradient based, optimisation algorithms has proven to be particularly effective in achieving control objectives for quantum mechanical systems. Some authors have sought a theoretical justification for this empirical observation of the numerical techniques‘ behavior. A set of papers, falling under the banner of „optimal control landscapes“ claim to offer such a justification, in the form of proofs that such optimisation always achieves the control objective (ignoring numerical limitations). I will present a number of problems inherent in said „landscape“ analysis.

July 30th, 2009
Seminar by Louis H. Kauffman on Topological Quantum Information Theory
Seminar by Louis H. Kauffman

In this talk we discuss relationships between topology and quantum computation.

Since the discovery of Peter Shor’s quantum algorithm for the prime factorization of natural numbers, there has been intense interest in the discovery of new quantum algorithms and in the construction of quantum computers. It is possible that topology will enter in a deep way in the construction of quantum computers based on phenomena such as the quantum Hall effect, where braiding of quasiparticles describes unitary transformations rich enough to produce the quantum computations.

This talk will describe the mathematics of such braiding and its relationship with algorithms to compute topological invariants such as the Jones polynomial.

Just so, relationships with braiding go beyond the quantum Hall effect and are of interest for constructing quantum gates and quantum algorithms. The talk will discuss these directions and our present project in collaboration with the research group of Prof. Glaser (on this campus) to instantiate quantum algorithms for the Jones polynomial using NMR (Nuclear Magnetic Resonance Spectroscopy).

The talk will be self-contained both in terms of mathematics and physics.

July 23rd, 2009
Seminar by Howard Carmichael
Seminar by Howard Carmichael on „Open Quantum Systems: Cavity QED with Dissipation“
April 30th, 2009
Seminar by Mercedes Rosas on The Reduced Kronecker Coefficients of the Symmetric Group
The reduced Kronecker coefficients of the symmetric group Mercedes Rosas, Universidad de Sevilla, Spain. (in collaboration with Emmanuel Briand, U. de Sevilla, and Rosa Orellana, Dartmouth College).

The understanding of the Kronecker coefficients of the symmetric group (the multiplicities of decomposition into irreducible the tensor products of two irreducible representations of the symmetric group) is a longstanding open problem. Recently, its study has appeared naturally in some seemingly unrelated areas.

For instance, Matthias Christandl has showed that the problem of the nonvanishing of Kronecker coefficients is equivalent to the problem of compatibility of local spectra, and Ketan Mulmuley has set the problem of proving that the positivity of a Kronecker coefÞcients can be decided in polynomial time at the heart of his Geometric Complexity Theory.

In view of the difficulty of studying of the Kronecker coefÞcients, it is legitimate to consider some closely related, and maybe simpler objects, the reduced Kronecker coefficients, defined as limits of certain stationary sequences of Kronecker coefficients. We attempt to show that the study of the reduced Kronecker coefficients could sheld light on the Kronecker coefficients.

We will introduce the reduced Kronecker coefficients, and describe some of their known properties. Then, we will describe a useful formula to compute Kronecker coefficients from the reduced ones, and, among other results, present a sharp bound for a family of Kronecker products to stabilize.

April 22nd, 2009
Seminar by Wu Xing
Seminar by Wu Xing on „Experiments on a Superconducting Atom Chip“.
March 26th, 2009 (MPQ seminar)
Seminar by Dmitry Yudin
March 13th, 2009
Seminar by Matthias Christandl on Post-selection technique for quantum channels with applications to quantum cryptography
Seminar by Matthias Christandl

Post-selection technique for quantum channels with applications to quantum cryptography

We propose a general method for studying properties of quantum channels acting on an n-partite system, whose action is invariant under permutations of the subsystems. Our main result is that, in order to prove that a certain property holds for any arbitrary input, it is sufficient to consider the special case where the input is a particular de Finetti-type state, i.e., a state which consists of n identical and independent copies of an (unknown) state on a single subsystem. A similar statement holds for more general channels which are covariant with respect to the action of an arbitrary finite or locally compact group.

Our technique can be applied to the analysis of information-theoretic problems. For example, in quantum cryptography, we get a simple proof for the fact that security of a discrete-variable quantum key distribution protocol against collective attacks implies security of the protocol against the most general attacks. The resulting security bounds are tighter than previously known bounds obtained by proofs relying on the exponential de Finetti theorem [Renner, Nature Physics 3,645(2007)]. This is joint work with Robert Koenig and Renato Renner http://arxiv.org/abs/0809.3019

March 11th, 2009
Seminar by Matthew B. Hastings
Seminar by Matthew B. Hastings on „A Counterexample to Additivity of Minimum Output Entropy“ (How to kill a most famous conjecture in quantum information – alternative title, not by the author).
January 16th, 2009
Seminar by Angela Meyer on Localisable Entanglement of 1D Cluster States
Title: Localisable entanglement of 1D cluster states

We consider quantum chains in cluster states under the influence of a variable magnetic field. After reviewing the derivation of the ground state we compute the localisable entanglement of the two outermost qubits, after local measurements have been made on the inner ones, for different chain lengths. The result is mostly as intuitively expected: the entanglement decreases monotonously with the field strength.

January 9th, 2009
Seminar by Dominique Sugny

 

Events 2008

November 19th, 2008
Seminar by Ulrich Schollwöck on Disentangling Many-Body Quantum Systems and Large-Scale Linear Algebra
Seminar by Ulrich Schollwöck

Title: DISENTANGLING MANY-BODY QUANTUM SYSTEMS AND LARGE-SCALE LINEAR ALGEBRA

In this talk I want to show how many important questions of quantum many-body physics (solid state physics, quantum optics) naturally lead to a highly efficient description of quantum states by sets of matrices whose manipulation involves large-scale linear algebra of sparse matrices. I will illustrate the various challenges by current physical problems from solid state physics and quantum optics and would like to try to give a flavour why theoretical physicists would be interested in insights from computer science and numerical mathematics to tackle such problems.

November 14th, 2008
Seminar by Andreas Eberlein on The Charge-Density Wave Behaviour in the t-J-Holstein Model
Seminar by Andreas Eberlein

Titel: „Charge-density wave behaviour in the t-J-Holstein Model“

Zusammenfassung:
„We study the charge-density wave behaviour in the one-dimensional t-J-Holstein Model. Using the Projector-based Renormalization Method (PRM), we investigate the influence of a small exchange interaction on the metal-insulator transition known from the spinless Holstein Model. In this talk, I will review the work on my diploma thesis and will present some results.“

October 23rd, 2008
Seminar by Guifre Vidal
Seminar by Guifre Vidal on Entanglement Renormalization.
September 8 - 12th, 2008
Summer School: Complex Quantum Systems
Summer School „Complex Quantum Systems“ hosted as a CoQuS-QCCC get-together by the CoQuS Phd programme of excellence at the University of Vienna.
August 28th, 2008
Seminar by Peter Pemberton-Ross
Seminar by Peter Pemberton-Ross on Controllability of Spin Systems.
August 13th, 2008
Seminar by Anne Nielsen on State Preparation by Means of Optical Measurements
Title: State Preparation by Means of Optical Measurements

Speaker: Anne Nielsen

The ability to control and manipulate the state of quantum systems is important in order to use such systems for technological purposes and fundamental studies of quantum mechanics. Subjecting a system to different Hamiltonians leads to different unitary time evolutions, but the state collapse accompanying quantum mechanical measurements opens several additional possibilities to change the state of a quantum system in a desired way, and measurements thus constitute a powerful state preparation tool. In the talk we investigate the influence of measurements on the dynamics of quantum systems and provide examples of various state preparation protocols that are based on optical measurements.

August 5th, 2008
Seminar by Daniel Burgarth on Quantum Control of Coupled Spin Systems: Algebraic and Open System Approach
Seminar by Daniel Burgarth

Title: Quantum Control of Coupled Spin Systems: Algebraic and Open System Approach

Speaker: Daniel Burgarth, Oxford

We compare two independent methods of controlling qubits which are coupled by an always-on Hamiltonian. In either case, it is possible to perform algorithms on large arrays by acting on a small subset. While the algebraic method has the advantage of requiring minimal resources, the open system approach provides an explicit way how to achieve control. We give examples of systems which are controllable only by the open system approach and show new results on spin chains as universal quantum interfaces.

July 21st, 2008
Guest lecture by Louis H. Kauffman on Quantum Computing and Quantum Topology
Guest lecture by Louis H. Kauffman

Title: Quantum Computing and Quantum Topology

Speaker: Louis H. Kauffman, UIC

Abstract:
This talk will discuss the construction of sets of universal gates for quantum computing and quantum information theory and their relationship with topological computing, quantum algorithms for computing quantum link invariants such as the Jones polynomial and questions about the relationship between quantum entanglement and topological entanglement. We will discuss the creation of universal gates (in the presence of local unitary transformations) by using solutions to the Yang-Baxter equation and we will discuss the use of braided recoupling theory (q-deforemed spin networks) to create unitary representations of the braid group rich enough to support quantum information theory and quantum computing. In particular we give quantum algorithms for computing the colored Jones polynomials and the Witten-Reshetikhin-Turaev invariants.

July 12th, 2008
Elite-Cup 2008 (ENB soccer tournament)
Elite-Cup 2008 (ENB soccer tournament) in Neufahrn near Freising
June 4th, 2008
QCCC Miniworkshop
QCCC Miniworkshop on „Mathematical Control Theory and Quantum Applications“ (with TopMath)“
June 2nd, 2008
Talk by Amr Fahmy
Talk by Amr Fahmy on „Thermal State Quantum Computing“
May 5 - 10th, 2008
Guest professor Marek Zukowski
March 31 - April 4th, 2008
Guest professor Reinhard Werner

 

 Events 2007

October 27 - 30th, 2007
QCCC Workshop

QCCC Workshop on „Foundations and Future Prospects of QIP“
For details see the workshop page.