PUBLICATIONS

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.

Quantum repeaters are pivotal in the physical layer of the quantum internet, and quantum repeaters capable of efficient entanglement distribution are necessary for its development. Quantum repeater schemes based on single-photon interference are promising because of their potential efficiency. However, schemes involving first-order interference with photon sources at distant nodes require stringent phase stability of the components, which pose challenges for long-distance implementation. In this paper, we present a quantum repeater scheme that leverages single-photon interference and reduces the difficulty of achieving phase stabilization. Additionally, under specific conditions, our scheme achieves a higher entanglement distribution rate between end nodes compared with the existing schemes. Thus, the proposed approach could lead to improved rates with technologies that are currently unavailable but possible in the future and will ultimately facilitate the development of multimode quantum repeaters.

In quantum communication with quantum repeaters, multiplexed quantum memory is expected to enhance communication rates. When using an atomic frequency comb (AFC) for on-demand storage, the frequency mode number is often limited by the optical power of the control pulses. Here, using a space-coupled waveguide electro-optic modulator, we increased the output power, allowing us to apply control pulses to multiple modes simultaneously. Further, through enhancement of an experimental setup that increases power density, we increased the number of modes. Consequently, we pioneered, to the best of our knowledge, on-demand storage using five modes of AFC. This technology is a significant achievement toward frequency-multiplexed on-demand quantum memory.