Quantum Dynamics

Max Planck Institute of Quantum Optics

Hans-Kopfermann-Str 1

85748 Garching


Research webpage


Research focus: quantum optics, ultracold atoms, photon quantum logic

We experimentally investigate ultracold atomic gases. In recent years our focus has been on studies of large optical nonlinearities at the single-photon level created by coupling photons to atomic Rydberg excitations.

Single photon transistor

Toggling an all-optical switch with a single photon is a nontrivial task because the nonlinearities of conventional nonlinear crystals are tiny at the single-photon level. We use a combination of electromagnetically induced transparency and Rydberg blockade in an ultracold atomic gas to overcome this limitation. In this way, we managed to realize a single-photon transistor. In this device, an incoming control light pulse containing only one photon enters an ultracold atomic gas. This control pulse changes the transmission of a subsequent target light pulse through the gas. We observed a gain of 20, i.e. a single control photon causes the number of transmitted target photons to change by 20. As a first application, we experimentally demonstrated the nondestructive optical detection of a single Rydberg excitation in the atomic gas with a fidelity of 86%.

Photon-photon quantum gate

A variety of proposals suggest that the giant optical nonlinearity attainable with Rydberg atoms should allow one to build a photon-photon quantum gate. As a crucial first step toward this goal, we demonstrated a Pi phase shift based on Rydberg interactions. We use a scheme which resembles the single-photon transistor but operates with light fields detuned from the atomic resonances. As a result, a single control photon creates only little absorption and instead creates a Pi phase shift for the target light, which we detect interferometrically. To build a quantum gate based on this Pi phase shift, we map the presence or absence of the control photon onto a polarization qubit. In this way, we recently demonstrated the first photon-photon quantum gate based on Rydberg interactions. We achieve postselected fidelities between 64% and 70%. The efficiency of the atomic system lies between 0.5% and 8% depending on the input polarizations. Our next goal is to improve the efficiency by placing the atomic ensemble inside an optical resonator with moderate finesse.

Selected publications

  • A photon–photon quantum gate based on Rydberg interactions, Nature Physics 15, 124 (2018).
  • Optical Pi phase shift created with a single-photon pulse, Science Advances 2, e1600036 (2016).
  • Single-Photon Transistor Using a Förster Resonance, Phys. Rev. Lett. 113, 053602 (2014).
  • Single-Photon Switch Based on Rydberg Blockade, Phys. Rev. Lett. 112, 073901 (2014).


Dark-time decay of the retrieval efficiency of light stored as a Rydberg excitation in a noninteracting ultracold gas

S. Schmidt-Eberle, T. Stolz, G. Rempe, S.Dürr.

Physical Review A 101, 013421 (2020).

Show Abstract

We study the dark-time decay of the retrieval efficiency for light stored in a Rydberg state in an ultracold gas of 87Rb atoms based on electromagnetically induced transparency (EIT). Using low atomic density to avoid dephasing caused by atom-atom interactions, we measure a 1/e time of 30µs for the 80S state in free expansion. One of the dominant limitations is the combination of photon recoil and thermal atomic motion at 0.2µK. If the 1064-nm dipole trap is left on, then the 1/e time is reduced to 13 µs, in agreement with a model taking differential light shifts and gravitational sag into account. To characterize how coherent the retrieved light is, we overlap it with reference light and measure the visibility V of the resulting interference pattern, obtaining V>90% for short dark time. Our experimental work is accompanied by a detailed model for the dark-time decay of the retrieval efficiency of light stored in atomic ensembles. The model is generally applicable for photon storage in Dicke states, such as in EIT with $\lamda$-type or ladder-type level schemes and in Duan-Lukin-Cirac-Zoller single-photon sources. The model includes a treatment of the dephasing caused by thermal atomic motion combined with net photon recoil, as well as the influence of trapping potentials. It takes into account that the signal light field is typically not a plane wave. The model maps the retrieval efficiency to single-atom properties and shows that the retrieval efficiency is related to the decay of fringe visibility in Ramsey spectroscopy and to the spatial first-order coherence function of the gas.

DOI: 10.1103/PhysRevA.101.013421

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