PUBLICATIONS

Multiplexing is essential for improving entanglement distribution rates in quantum communication. Frequency multiplexing provides a promising and scalable path toward large-capacity quantum networks. Further progress requires increasing the number of frequency modes and developing broadband photon-pair sources and quantum memories that are spectrally compatible. Here, we report the integration of a cavity-enhanced spontaneous parametric downconversion source in the telecom C-band with a frequency-multiplexed atomic frequency-comb memory. The bow-tie cavity source was simultaneously resonant at 606 and 1550 nm, generating non-degenerate photon pairs exhibiting a clustered frequency-comb spectrum. The atomic frequency-comb memory, implemented in a Praseodymium-doped Yttrium Orthosilicate crystal, provided up to 83 frequency modes with 123 MHz spacing and enabled broadband storage of 606 nm signal photons. By filtering the main cluster, we obtained 32.7 ± 4.8 effective modes, as confirmed from coincidence measurements. Importantly, we observed strong nonclassical correlations after storage, with cross correlation values of g _s,i ⁽²⁾ = 8.1 ± 0.7. Our experimental results demonstrate the feasibility of integrating cavity-enhanced photon-pair sources with rare-earth-ion-doped solid-state memories. The integration reveals a high frequency multiplicity that is essential for scalable quantum networks.

Single-shot high-resolution spectroscopy at the single photon-level has emerged as a promising measurement technique, enabling novel observations and evaluations that were previously challenging. This technology is particularly effective for spectroscopic applications aimed at realizing frequency-multiplexed quantum repeaters. In this study, we propose a single-shot high-resolution single-photon spectroscopy system that integrates high-resolution frequency-to-spatial mode mapping using a virtually imaged phased-array (VIPA) and high-precision spatial mode detection using a single-photon avalanche diode (SPAD) array. We experimentally demonstrated the principle of this system using weak coherent pulses with a frequency mode interval of 120 MHz. This interval closely matches the minimum frequency mode spacing of the atomic frequency comb quantum memory with the Pr³⁺-ion-doped Y₂SiO₅ crystal. By applying the proposed system, we expect to maximize the multiplexing capability of frequency-multiplexed quantum repeater schemes utilizing such quantum memories.

Phase-sensitive quantum communication has received considerable attention to overcome the distance limitation of quantum communication. A fundamental problem in phase-sensitive quantum communication is to compensate for phase drift in an optical fiber channel. A combination of time-, wavelength-, and space-division multiplexing can improve the phase stability of the optical fiber. However, the existing phase compensations have used only time- and wavelength-division multiplexing. Here, we demonstrate space-division multiplexed phase compensation in the Osaka metropolitan networks. Our compensation scheme uses two neighboring fibers, one for quantum communication and the other for sensing and compensating the phase drift. Our field investigations confirm the correlation of the phase drift patterns between the two neighboring fibers. Thanks to the correlation, our space-division multiplexed phase compensation significantly reduces the phase drift and improves the quantum bit error rate. Our phase compensation is scalable to a large number of fibers and can be implemented with simple instruments. Our study on space-multiplex phase compensation will support the field deployment of phase-sensitive quantum communication.