In this letter, a W-band slot array antenna based on ridge gap waveguide (RGW) with low sidelobe and high-gain characteristics has been proposed. It adopts a series and parallel hybrid feeding network to realize low sidelobe levels in E-and H-plane, respectively. The tunable pins and large power ratio dividers are introduced in the distribution network, which improves the phase consistency and brings flexibility to amplitude-tapering. The proposed antenna overcomes the shortcomings of grating lobes caused by quasi-Taylor distribution of conventional slot array with backed cavities. The prototype of a 16 Â 18 slot array is fabricated and measured. The measured impedance bandwidth is from 81.2 to 86 GHz, and the peak gain is higher than 29.4 dBi with the sidelobe levels (SLLs) of À19.7 dB and À30 dB on E-and H-plane, respectively.

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Integrated photonics has the advantages of miniaturization, low cost, and CMOS compatibility, and it provides a stable, highly integrated, and practical platform for quantum key distribution (QKD). While photonic integration of optical components has greatly reduced the overall cost of QKD systems, single-photon detectors (SPDs) have become the most expensive part of a practical QKD system. In order to circumvent this obstacle and make full use of SPDs, we have designed and fabricated a QKD receiver chip for multiple users. Our chip is based on time-division multiplexing technique and makes use of a single set of SPDs to support up to four users QKD. Our proof-ofprinciple chip-based QKD system is capable of producing an average secret key rate of 13.68 kbps for four users with a quantum bit error rate (QBER) as low as 0.51% over a simulated distance of 20 km in fiber. Our result clearly demonstrates the feasibility of multiplexing SPDs for setting QKD channels with different users using photonic integrated chip and may find applications in the commercialization of quantum communication technology.

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