Quantum Dynamics

Max Planck Institute of Quantum Optics

Hans-Kopfermann-Str 1

85748 Garching


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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).

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