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.
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 gate light pulse containing only one photon enters an ultracold atomic gas. This gate pulse changes the transmission of a subsequent target light pulse through the gas. We observed a gain of 20, i.e. a single gate 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%.
It is a natural question whether the giant optical nonlinearity attainable with Rydberg atoms allows one to build a photon-photon quantum-logic gate. Several proposals suggest that the answer will be yes. As a crucial step toward this goal, we recently 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, the gate photon creates only little absorption and instead creates a π phase shift for the target light, which we detect interferometrically. The next goal is to build a photon-photon gate based on this π phase shift.
Max Planck Institute for Quantum Optics