Polarization-Independent Photon Storage System with Variable Time Delay

Quantum optical memories are a key component to a variety of quantum information applications, from extending quantum communication channels to building high-efficiency single-photon sources to synchronizing multiple protocols. However, most current broad bandwidth photon storage systems operate with somewhat shorter storage times (on the order of 10 ns), or require cryogenic operation. Here we develop a system with multiplexed free-space storage cavities, able to store single photons with high efficiency over variable delays [N x 12.5 ns, 1 ≤ N ≤ 999], and over several nanometers bandwidth. The system can store multiple photons simultaneously and can potentially store qubits encoded in various degrees of freedom, e.g., spatial modes, time-bin, and polarization. For the latter, we have demonstrated a memory fidelity >90% for storage times up to 500 ns. A future goal for this experiment is to achieve storage of hyperentanglement. While previous hyperentangled photon storage systems only achieved 5% efficiency, we have currently demonstrated a free-space transmission above 50% for delay times up to 5 μs.