Tianyi Xing

Chip-based quantum key distribution (QKD) systems offer improved efficiency but may also introduce previously unrecognized security vulnerabilities. In this work, we identify and experimentally characterize cross-polarization-intensity (CPI) correlations in a real-world chip-based QKD system. Moreover, we introduce a security analysis that incorporates CPI correlations and apply it to evaluate the performance of an integrated high-speed QKD system. Our results emphasize the need for rigorous security assessments in chip-based QKD implementations.

One of the most significant vulnerabilities in the source unit of quantum key distribution~(QKD) is the correlation between quantum states after modulation, which shall be characterized and evaluated for its practical security performance. In this work, we propose a methodology to characterize the intensity correlation according to the single-photon detection results in the measurement unit without modifying the configuration of the QKD system. In contrast to the previous research that employs extra classical optical detector to measure the correlation, our method can directly analyse the detection data generated during the raw key exchange, enabling to characterize the feature of correlation in real-time system operation. The basic method is applied to a BB84 QKD system and the characterized correlation significantly decreases the secure key rate shown by the security proof. Furthermore, the method is extended and applied to characterize the correlation from the result of Bell-state measurement, which demonstrates its applicability to a running full-scheme MDI QKD system. This study provides an approach for standard certification of a QKD system.

The Quantum Zeno effect inhibits the evolution of quantum states through repeated yet weak measurements, thereby significantly enhancing the detection probability of interaction-free measurement (IFM). This fundamental mechanism facilitates high-efficiency counterfactual quantum communication that information delivery without particle transmission through the channel. Regrettably, the original protocol left the weak trace of particles in the channel; fortunately, an upgraded counterfactual communication protocol eliminates this issue by modifying the structure unit according to two-state vector formalism~(TSVF). However, no study has realized the application of the quantum Zeno effect in weak-trace-free counterfactual communication to achieve efficient information transmission. In this paper, we experimentally demonstrate weak-trace-free counterfactual communication via the quantum Zeno effect on a nanophotonic chip, achieving a transmission probability of 74.2 ± 1.6% for bit 0 and 85.1 ± 1.3% for bit 1. Furthermore, we successfully transmit our Quanta group's logo through counterfactual communication implemented on the chip.