Rydberg Quantum Photonics
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 π 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 π phase shift for the target light, which we detect interferometrically. To build a quantum gate based on this π 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. This will allow it to operate the gate at lower atomic density, where dephasing is less of an issue, as we showed in a recent experimental study.
If you are interested in joining the group, please consult the Open Positions page, and contact Dr. Stephan Dürr or Prof. Gerhard Rempe.