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

TLDR

Quantum memory for flying optical qubits is essential for quantum information science, yet qubit storage has historically suffered from limited efficiency despite advances in light‑pulse memories. The authors implement a dual‑rail electromagnetically‑induced‑transparency memory in a laser‑cooled cesium ensemble, encoding weak coherent single‑photon‑level qubits and spatially multiplexing them for storage. The memory achieves an average conditional fidelity above 99 % and a 68 % storage‑retrieval efficiency, preserves high optical depth on both rails without sacrificing multiplexing, and offers an efficient node for future quantum network and photonic‑circuit experiments.

Abstract

Quantum memory for flying optical qubits is a key enabler for a wide range of applications in quantum information science and technology. 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 an efficiency equal to (68$\pm$ 2)%, 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.