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Broadband quantum memory is critical to enabling the operation of emerging photonic quantum technology at high speeds. Here we review a central challenge to achieving broadband quantum memory in atomic ensembles—what we call the ‘linewidth-bandwidth mismatch’ problem—and the relative merits of various memory protocols and hardware used for accomplishing this task. We also review the theory underlying atomic ensemble quantum memory and its extensions to optimizing memory efficiency and characterizing memory sensitivity. Finally, we examine the state-of-the-art performance of broadband atomic ensemble quantum memories with respect to three key metrics: efficiency, memory lifetime, and noise. 

We measure 95.6±0.3% storage efficiency of ultrafast photons in a collisionally broadened barium vapor quantum memory. We measure 31±1% total efficiency, limited by control field power, and a 0.515(6) ns lifetime, limited by motional dephasing.

 

We examine the sensitivity of Λ-type optical quantum memories to experimental fluctuations using a variance-based analysis. The results agree with physical interpretations of quantum memory protocols, and are important for practical implementations.

 

Quantum memories are of critical importance to the scalability of quantum information processing and quantum technologies in communication, measurement, and computation. Here we present numerical simulation of the storage of ultra-broadband photons in hot atomic barium vapor, which allows for quantum memory operation at telecom wavelengths. We numerically calculate the optimal control field profiles for the storage process both through direct Nedler-Mead simplex search and by singular value decomposition of the storage kernel, where the latter guarantees optimality. We provide a physical interpretation of our numerical results related in part to recent work on Autler-Townes-Splitting (ATS) based quantum memory, and show saturation of the protocol-independent bound on storage efficiency imposed by the optical depth for pulses of duration 200 fs to 17.5 ps. In conclusion we provide an outlook for implementing these results experimentally.