Entanglement distribution between quantum repeater nodes with an absorptive type memory

    Daisuke Yoshida, Kazuya Niizeki, Shuhei Tamura, and Tomoyuki Horikiri

    International Journal of Quantum Information,18, 2050026 (2020).

    • Memory

    Quantum repeaters, which are indispensable for long-distance quantum communication, are necessary for extending the entanglement from short distance to long distance; however, high-rate entanglement distribution, even between adjacent repeater nodes, has not been realized. In a recent work by [C. Jones et al., New J. Phys. 18 (2016) 083015], the entanglement distribution rate between adjacent repeater nodes was calculated for a plurality of quantum dots, nitrogen-vacancy centers in diamond, and trapped ions adopted as quantum memories inside the repeater nodes. Considering practical use, arranging a plurality of quantum memories becomes so difficult with the state-of-the art technology. It is desirable that high-rate entanglement distribution is realized with as few memory crystals as possible. Here, we propose new entanglement distribution scheme with one quantum memory based on the atomic frequency comb which enables temporal multimode operation with one crystal. The adopted absorptive-type quantum memory degrades the difficulty of multimode operation compared with the previously investigated quantum memories directly generating spin-photon entanglement. It is shown that this scheme improves the distribution rate by nearly two orders of magnitude compared with the result in [C. Jones et al., New J. Phys. 18(2016)083015] and the experimental implementation is close by utilizing state-of-the-art technology.

    Two-photon comb with wavelength conversion and 20-km distribution for quantum communication

    Kazuya Niizeki, Daisuke Yoshida, Ko Ito, Ippei Nakamura, Nobuyuki Takei, Kotaro Okamura, Ming-Yang Zheng, Xiu-Ping Xie and Tomoyuki Horikiri

    Communications Physics, 3, 138 (2020).

    • Photon source

    • Interface

    Quantum computing and quantum communication, have been greatly developed in recent years and expected to contribute to quantum internet technologies, including cloud quantum computing and unconditionally secure communication. However, long-distance quantum communication is challenging mainly because of optical fiber losses; quantum repeaters are indispensable for fiber-based transmission because unknown quantum states cannot be amplified with certainty. In this study, we demonstrate a versatile entanglement source in the telecom band for fiber-based quantum internet, which has a narrow linewidth of sub-MHz range, entanglement fidelity of more than 95%, and Bell-state generation even with frequency multimode. Furthermore, after a total distribution length of 20-km in fiber, two-photon correlation is observed with an easily identifiable normalized correlation coefficient, despite the limited bandwidth of the wavelength converter. The presented implementation promises an efficient method for entanglement distribution that is compatible with quantum memory and frequency-multiplexed long-distance quantum communication applications.