Stephan Welte

PhD project

Deterministic Photon-Photon interactions

In my PhD thesis, I work with single atoms trapped at the center of a high-finesse optical cavity. The atoms interact strongly with single photons reflected from the cavity. As was shown in previous experiments in the group of Prof. Rempe, a quantum gate between a single atom and a single photon can be implemented employing this experimental platform [1].

Artist´s view of the photon reflection process in the implementation of the photon-photon gate. The cavity contains a single atom (red sphere with yellow electron orbitals). Two photons are successively reflected from the cavity.

Based on this result, the implementation of a quantum gate between two photons mediated by
an atom was realized experimentally during my PhD thesis [2]. This result was a long
standing goal in optical quantum information processing since it paves the way towards scalable optical quantum computing due to its deterministic nature. The implementation of such a gate is achieved by reflecting two photons (control and target) from the atom-cavity system successively. The gate requires a deterministic interaction of an atom and a photon which is so strong that a single control photon can influence a target photon by changing the state of the atom as an intermediate step. Afterwards, the target photon interacts with the atom and is influenced by its state. The nonlinear atom-cavity system provides the necessary platform to achieve this strong interaction.

[1] A. Reiserer, N. Kalb, G. Rempe, & S. Ritter “A quantum gate between a flying optical photon and a single trapped atom”,  Nature, 508 237-240 (2014)


[2] B. Hacker, S. Welte, G. Rempe, S. Ritter, “A photon-photon quantum gate based on a single atom in an optical resonator”, Nature 536, 193-196 (2016)

[3] F. Meinert, C. Basler, A. Lambrecht, S. Welte, and H. Helm, “Quantitative analysis of the transient response of the refractive index to conditions of electromagnetically induced transparency”, Phys. Rev. A85, 013820 (2012)

[4] S. Welte, C. Basler, and H. Helm, “Studies of Berry’s phase and its sign in quantum superposition states of thermal 87Rb atoms”, Phys. Rev. A89, 023412 (2014)

2014: NIM workshop at Glocknerhaus (Austria), “Nondestructive detection of an optical photon” (talk)

2015: ExQM seminar (Munich), “Nondestructive detection of an optical photon” (talk)
2015: Young Atom Opticians conference (Zurich), “Optimisation of an atom-photon quantum gate” (poster)
2015: Resonator QED conference (Munich), “Optimisation of an atom-photon quantum gate” (poster)
2016: DPG spring meeting (Hannover), “A cavity-mediated photon-photon gate” (poster)
2016: ExQM seminar (Munich), “A photon-photon quantum gate” (talk)
2016: Gordon Conference on Quantum science (Stonehill College, USA), “A cavity-mediated photon-photon gate” (poster)

Further information

Short CV and project description

Video Portrait


Max-Planck-Institute of Quantum Optics
Hans-Kopfermann-Straße 1
85748 Garching bei München
phone: +49 89 3 29 05 – 398 // -371