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To observe the loophole-free violation of the Clauser–Horne–Shimony–Holt (CHSH) inequality between distant two parties, i.e. the CHSH value S > 2 , the main limitation of distance stems from the loss in the transmission channel. The entanglement swapping relay (ESR) is a simple way to amplify the signal and enables us to evade the impact of the transmission loss. Here, we experimentally test the heralded nonlocality amplifier protocol based on the ESR. We observe that the obtained probability distribution is in excellent agreement with those expected by the numerical simulation with experimental parameters which are precisely characterized in a separate measurement. Moreover, we experimentally estimate the nonlocality of the heralded state after the transmission of 10 dB loss just before final detection. The estimated CHSH value is S = 2.113 > 2 , which indicates that our final state possesses nonlocality even with transmission loss and various experimental imperfections. Our result clarifies an important benchmark of the ESR protocol, and paves the way towards the long-distance realization of the loophole-free CHSH-violation as well as device-independent quantum key distribution.

Diamond nanowires are fabricated on a bulk, single crystalline diamond near an edge of aluminum coating using inductively coupled plasma reactive ion etching. Two different density areas are simultaneously appeared where the dense area has 9 times higher density than that of the sparse area while keeping the size of nanowires almost uniform in these areas. The nanowire sizes realized in the dense (sparse) area are 858 ± 22 nm (876 ± 25 nm) in height and 126 ± 6 nm (124 ± 7 nm) in diameter, which is suitable for applications in optical quantum information processing.

We have developed a high-performance single-photon source (SPS) operating at 1.5 µm wavelength. The source is an InAs/InP quantum dot with a horn-shaped nanostructure. A resonant excitation to the p-shell state helps achieve a single-photon efficiency of 5.8% after coupling into a single-mode fiber with a second-order correlation value of g(2)(0)~0.055. The performance of the source has been assessed by integrating it into a conventional quantum key distribution system. We have successfully transmitted secure keys over a 50 km commercial fiber, exceeding the previously reported range for an SPS operating below 1.3 µm.