Xinlun Cai

Photonic integration in quantum communication holds significant potential for miniaturization and enabling commercial applications. Among various platforms, thin-film lithium niobate (TFLN) stands out due to its exceptional combination of high electro-optical efficiency, low propagation loss, and compact footprint. Here, we demonstrate a 2.5 GHz chip-to-chip fully integrated quantum key distribution (QKD) system based on a TFLN platform, which incorporates high-speed dual-polarization time-bin phase encoding and decoding functionalities. We achieve an extremely low quantum bit error rate of 0.53% and a secret key rate exceeding 10 Mbps over 25 km fiber spools. The design of cascaded Mach–Zehnder modulators effectively suppresses the patterning effect in high speed QKD. Notably, the TFLN chips used in both the transmitter and receiver share a similar architecture, highlighting the potential for creating a homogeneous transceiver. This work paves the way for high-speed, miniaturized QKD systems based on the lithium niobate integrated platform.

Integrated photonics provides a route both to miniaturize quantum key distribution (QKD) devices and to enhance their performance. A key element for discrete-variable QKD is single-photon detector. It is necessary to integrate such device onto a photonic chip to enable the realization of practical and scalable quantum networks. Here, we report a successful interfacing of Complementary Metal-Oxide-Semiconductor (CMOS)-compatible silicon nanophotonics with optical waveguide-integrated superconducting nanowire single-photon detector (SNSPD). We perform the first optimal Bell-state measurement (BSM) of time-bin encoded qubits generated from two independent lasers benefited from the reduced dead time of SNSPD ∼3.4 ns. The optimal BSM enables an increased key rate of measurement-device-independent QKD, which is immune to all attacks against the detection system and hence provides the basis for a QKD network with untrusted relays. Together with the time-multiplexed technique, we have enhanced the sifted key rate by almost one order of magnitude. Combined with integrated QKD transmitters, a scalable, chip-based and cost-effective QKD network should become realizable in the near future.