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arXiv:1110.6771 · Physical Review Letters

High-Capacity Spatial Multimode Quantum Memories Based on Atomic Ensembles

Anna Grodecka-Grad, Emil Zeuthen, Anders Søndberg Sørensen

Published Sep 27, 2012• 51 citations
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Abstract

We study spatial multimode quantum memories based on light storage in extended ensembles of Λ-type atoms. We show that such quantum light-matter interfaces allow for highly efficient storage of many spatial modes. In particular, forward operating memories possess excellent scaling with the important physical parameters: quadratic scaling with the Fresnel number and even cubic with the optical depth of the atomic ensemble. Thus, the simultaneous use of both the longitudinal and transverse shape of the stored spin wave modes constitutes a valuable and so far overlooked resource for multimode quantum memories.

DOI
10.1103/physrevlett.109.133601

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We study spatial multimode quantum memories based on light storage in extended ensembles of Λ-type atoms. We show that such quantum light-matter interfaces allow for highly efficient storage of many spatial modes. In particular, forward operating memories possess excellent scaling with the important physical parameters: quadratic scaling with the Fresnel number and even cubic with the optical depth of the atomic ensemble. Thus, the simultaneous use of both the longitudinal and transverse shape of the stored spin wave modes constitutes a valuable and so far overlooked resource for multimode quantum memories.
We develop a three-dimensional theory for quantum memories based on light storage in ensembles of $\ensuremath{\Lambda}$-type atoms, where two long-lived atomic ground states are employed. We consider light storage in an ensemble of finite spatial extent and we show that within the paraxial approximation the Fresnel number of the atomic ensemble and the optical depth are the only important physical parameters determining the quality of the quantum memory. We analyze the influence of these parameters on the storage of light followed by either forward or backward read-out from the quantum memory. We show that for small Fresnel numbers the forward memory provides higher efficiencies, whereas for large Fresnel numbers the backward memory is advantageous. The optimal light modes to store in the memory are presented together with the corresponding spin waves and outcoming light modes. We show that for high optical depths such $\ensuremath{\Lambda}$-type atomic ensembles allow for highly efficient backward and forward memories even for small Fresnel numbers $F\ensuremath{\gtrsim}0.1$.
We study the coherent storage and retrieval of a very short multimode light pulse in an atomic ensemble. We consider a quantum memory process based on the conversion of a signal pulse into a long-lived spin coherence via light matter interaction in an on-resonant $\ensuremath{\Lambda}$ -type system. In order to study the writing and reading processes we analytically solve the partial differential equations describing the evolution of the field and of the atomic coherence in time as well as in space. We show how to optimize the process for writing as well as for reading. If the medium length is fixed, for each length, there is an optimal value of the pulse duration. We discuss the information capacity of this memory scheme and we estimate the number of transverse modes that can be stored as a quantum hologram.
An efficient multimode quantum memory is a crucial resource for long-distance quantum communication based on quantum repeaters. We propose a quantum memory based on spectral shaping of an inhomogeneously broadened optical transition into an atomic frequency comb (AFC). The spectral width of the AFC allows efficient storage of multiple temporal modes without the need to increase the absorption depth of the storage material, in contrast to previously known quantum memories. Efficient readout is possible thanks to rephasing of the atomic dipoles due to the AFC structure. Long-time storage and on-demand readout is achieved by use of spin states in a lambda-type configuration. We show that an AFC quantum memory realized in solids doped with rare-earth-metal ions could store hundreds of modes or more with close to unit efficiency, for material parameters achievable today.
In this paper, an optical memory based on an ensemble of cold Cesium atoms interacting with light in the electromagnetically induced transparency (EIT) regime is studied in view of spatially multimode quantum storage. The interfacing protocol involves a control field, generating EIT for the very weak field that carries the quantum signal to be stored and enabling its mapping into the collective hyperfine coherence of the cold atomic ensemble. For the demonstration of multimode storage, the signal pulse at the single photon level is prepared in a mode with (OAM), while the control field is in a TEM00 mode. After the writing process, the information is kept in the memory register. In the retrieval phase, the information is read back from atoms to light and we show that these pulses at the single photon level with OAM can be stored and retrieved with a preserved spatial phase and a good efficiency.