ALternative Architecture Combination for Optical Quantum storage – ALACOQ

The storage and retrieval of a quantum state of light is expected to play a fundamental role in long distance quantum communication networks. The project is placed in this prospect. The core of the project is the investigation of storage architectures in atomic ensembles at the few-photon signal level. The systems to be considered are inhomogeneously broadened solid materials, more specifically, rare earth ion doped crystals. Those systems are known to exhibit long lifetime quantum superposition states. Quantum information storage and retrieval relies on contradictory requirements. The same medium should be opaque at storage and transparent at readout. This apparent paradox is solved with the help of intense driving fields that, conveniently and at will, modify the medium properties. With such protocols, relying on EIT (Electromagnetically Induced Transparency) one faces the problem of detecting a very weak signal against the background induced by strong driving fields. However this operating mode has been the most popular one in homogeneously broadened materials such as laser cooled atomic clouds. In inhomogeneously broadened materials, echo procedures may offer a way to temporally separate the weak signal from the intense driving pulses. They take advantage of atomic coherence deterministic dephasing and rephasing. We propose to explore this route at the few-photon level, a task that has not been undertaken yet. In addition to phase-reversing the atomic superposition state carrying the quantum information, the strong driving pulse simultaneously affects the atomic level population distribution. This may damage the retrieval fidelity. It has then been proposed to substitute optical driving with a reversible static electric field control. One proceeds along several steps. First a narrow spectral group of absorbing atoms is selected. Then appropriate electric field gradient is applied which, by way of Stark effect, gives rise to the desired inhomogeneous width. To phase-reverse the atomic coherences one just reverses the static field. Since the inhomogeneously broadened absorption line is obtained from a narrow initial group of atoms, « controlled reversible inhomogeneous broadening » (CRIB) proceeds at the expense of the opacity. To minimize opacity deterioration one can combine CRIB with optical pulse driving. Combination of CRIB with optical driving pulses has been suggested already in a different prospect. We propose to experimentally show that storage time and bandwidth can be increased simultaneously without deterioration of opacity . In addition we shall focus on minimization of opacity reduction and on few photon operation. Finally we propose to improve our understanding of three-level atom driving on simple model systems such as metastable helium gas, and to systematically investigate alternative ion-matrix compounds both theoretically and experimentally.