Highly-efficient quantum memory for polarization qubits in a spatially-multiplexed cold atomic ensemble

Quantum memory for flying optical qubits is a key enabler for a wide range of applications in quantum information. A critical figure of merit is the overall storage and retrieval efficiency. So far, despite the recent achievements of efficient memories for light pulses, the storage of qubits has suffered from limited efficiency. Here we report on a quantum memory for polarization qubits that combines an average conditional fidelity above 99% and efficiency around 68%, thereby demonstrating a reversible qubit mapping where more information is retrieved than lost. The qubits are encoded with weak coherent states at the single-photon level and the memory is based on electromagnetically-induced transparency in an elongated laser-cooled ensemble of cesium atoms, spatially multiplexed for dual-rail storage. This implementation preserves high optical depth on both rails, without compromise between multiplexing and storage efficiency. Our work provides an efficient node for future tests of quantum network functionalities and advanced photonic circuits.

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Quantum memory for polarization qubits in a multiplexed large OD atomic cloud. a A polarization qubit is encoded via a quarter (QWP) and a half wave plate (HWP) and converted into a dual-rail qubit with a beam displacer (BD). The orthogonally polarized beams, separated by 4 mm, are then mapped into an elongated ensemble of laser-cooled cesium atoms prepared in a 2D magneto-optical trap in a glass chamber. The spatial multiplexing is realized by focusing the two parallel paths into the 2.5-centimeter-long ensemble with a small crossing angle of 0.5° in order to preserve a large OD for each mode, an essential but challenging feature. A single control beam propagates with an angle of 1° relative to the signal modes in the plane of symmetry. b A large OD of 300 is obtained. The blue points correspond to the experimental data while the red solid line gives the theoretical fit. c Energy levels of the Cs D2 line involved in the EIT scheme. The atoms are prepared in F = 3 and populate all the Zeeman levels. Signal and control fields have the same circular polarization to avoid residual absorption. A comprehensive model is derived to take into account all the atomic levels, including the excited levels out of resonance. This model allows to understand the fundamental limits for storage and retrieval in such a setting, as described in the text. d Typical EIT spectrum as a function of the signal detuning when the control beam is kept on resonance. The red solid line corresponds to the full model