Highly efficient and broadband optical delay line toward a quantum memory

We demonstrate a high-efficiency, free space optical delay line utilizing a nested multipass cell architecture. This design supports extended optical paths with low loss, aided by custom broadband dielectric coatings that provide high reflectivity across a wide spectral bandwidth. The cell is characterized using polarization-entangled photon pairs, with signal photons routed through the delay line and idler photons used as a timing reference. Quantum state tomography performed on the entangled pairs reveals entanglement preservation with fidelity of 99.6(9)% following a single-transit delay of up to 687 ns, accompanied by a photon retrieval efficiency of 95.390(5)%. The delay is controllable and can be set between 1.8 and 687 ns in ∼12.6 ns increments. These results position this delay line as a strong candidate for all-optical quantum memories and synchronization modules for scalable quantum networks.