Space-division multiplexed phase compensation for quantum communication: concept and field demonstration

    Riku Maruyama, Daisuke Yoshida, Koji Nagano, Kouyou Kuramitani, Hideyo Tsurusawa, and Tomoyuki Horikiri


    • Repeater

    • Interface

    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.

    A multiplexed quantum repeater based on single-photon interference with mild stabilization

    Daisuke Yoshida and Tomoyuki Horikiri

    arXiv 2401.09578

    • Repeater

    Quantum repeaters are pivotal in the physical layer of the quantum Internet. For its future development, it is desirable to have quantum repeaters capable of facilitating robust and high-speed communication. In terms of efficiency, quantum repeater schemes based on single-photon interference are seen as promising. However, this method, involving first-order interference with light sources at distant nodes, requires stringent phase stability in the components. In this paper, we present a quantum repeater scheme that leverages single-photon interference, utilizing multimode quantum memories and multimode two-photon sources. Compared to conventional quantum repeater methods, our proposed scheme significantly reduces the phase stability requirements by several orders of magnitude. Additionally, under specific conditions, it is demonstrated that our scheme achieves a higher coincidence rate between end nodes compared to existing schemes.

    Sagnac interferometer-type nondegenerate polarizationentangled two-photon source with a Fresnel rhomb

    Naoto Aizawa, Kazuya Niizeki, Riku Sasaki, and Tomoyuki Horikiri

    Applied Optics, 62, 2273-2277 (2023).

    • Photon source

    Telecommunication wavelength-entangled photon sources (EPS) are indispensable systems for a fiber-based quantum network. We developed a Sagnac-type spontaneous parametric down conversion systemadopting a Fresnel rhomb as a wideband and reasonable retarder. This novelty, to the best of ourknowledge, enables the generation of a highly nondegenerate two-photon entanglement comprising thetelecommunication wavelength (1550 nm) and quantum memory wavelength (606 nm for Pr:YSO) with onlyone nonlinear crystal. Quantum state tomography was performed to evaluate the degree of entanglement,and the fidelity with a Bell state |Φ+⟩ with a maximum of 94.4% was obtained. Therefore, this paper showsthe potential of nondegenerate EPSs that are compatible with both telecommunication wavelength andquantum-memory wavelength to be installed in quantum repeater architecture.