All-Optical Photonic Quantum Memory

Photonic quantum memories will play an essential role in enabling several key quantum information protocols, including distributed quantum computing, efficient quantum sensing, and the synchronization of repeater nodes. One method of storing photonic qubits is to convert the energy into an excitation of an atomic ensemble or other matter-based system, but this approach has limitations. Namely, these matter-memories are usually narrow bandwidth, they often require costly overhead in the form of cryogenic cooling, and they typically have low retrieval efficiency into single-mode fiber. We have developed a photonic quantum memory that operates at room temperature in free space, allowing us to avoid all these drawbacks. Because our memory operates in free space, we have complete control over the path that our stored photons take, which lets us reliably out-couple stored photons into single-mode fiber. In addition, the bandwidth is limited only by the optics employed, leading to a record Time-Bandwidth product of 6×10⁶. Our system uses three multiplexed free-space delay lines with storage times of 12.5 ns, 125 ns and 1.25 µs, giving us a digital memory that can store photons in increments of 12.5 ns. We have demonstrated storage of photons for up to 12.5 µs, with high retrieval efficiencies into single-mode fiber and high (>99%) polarization-qubit preservation.