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.

    Ultrabright narrow-band telecom two-photon source for long-distance quantum communication

    Kazuya Niizeki, Kohei Ikeda, Mingyang Zheng, Xiuping Xie, Kotaro Okamura, Nobuyuki Takei, Naoto Namekata, Shuichiro Inoue, Hideo Kosaka, and Tomoyuki Horikiri

    Applied Physics Express, 11, 042801 (2018).

    • Photon source

    We demonstrate an ultrabright narrow-band two-photon source at the 1.5 μm telecom wavelength for long-distance quantum communication. By utilizing a bow-tie cavity, we obtain a cavity enhancement factor of 4.06 × 104. Our measurement of the second-order correlation function G(2)(τ) reveals that the linewidth of 2.4 MHz has been hitherto unachieved in the 1.5 μm telecom band. This two-photon source is useful for obtaining a high absorption probability close to unity by quantum memories set inside quantum repeater nodes. Furthermore, to the best of our knowledge, the observed spectral brightness of 3.94 × 105 pairs/(s· MHz· mW) is also the highest reported over all wavelengths.