Teng-Yun Chen

Quantum key distribution is a cornerstone of quantum technology, offering information-theoretical secure keys for remote parties. With many quantum communication networks established globally, the mode-pairing protocol stands out for its efficacy over inter-city distances using simple setups, emerging as a promising solution. In this study, we employ the mode-pairing scheme into existing inter-city fiber links, conducting field tests across distances ranging from tens to about a hundred kilometers. Our system achieves a key rate of $1.217$ kbit/s in a $195.85$ km symmetric link and $3.089$ kbit/s in a $127.92$ km asymmetric link without global phase locking. The results demonstrate that the mode-pairing protocol can achieve key rates comparable to those of a single quantum link between two trusted nodes on the Beijing-Shanghai backbone line, effectively reducing the need for half of the trusted nodes. These field tests confirm the mode-pairing scheme's adaptability, efficiency, and practicality, positioning it as a highly suitable protocol for quantum networks.

Channel loss is one of the most severe limitation to extend the transmission distance of quan- tum key distribution in practice. The twin-field quantum key distribution can achieve a much longer transmission distance with improving the key rate from the linear scale of channel loss in the traditional decoy-state method to the square root scale of the channel transmittance. Here we demonstrate the real-optical-fibre experimental results of twin-field quantum key distribution through the sending-or-not-sending protocol, which is fault tolerant to large misalignment error. The phase locking technology developed in the frequency transfer field is adopted to ensure Alice’s and Bob’s source wavelengths are locked to each other. Phase reference pulses are used to monitor the phase difference between the channel. Further with a high performance single photon detector, we obtain the positive key rates for different distances, specifically, the obtained secure key rate at 150 km is higher than that of the measurement device independent QKD.

Quantum key distribution (QKD) holds the potential to establish secure keys over long distances. The distance of point-to-point QKD secure key distribution is primarily impeded by the transmission loss inherent to the channel. In the quest to realize a large-scale quantum network, increasing the QKD distance under current technology is of great research interest. Here we adopt the 3-intensity sending-or-not-sending twin-field QKD (TF-QKD) protocol with the actively-odd-parity-pairing method. The experiment demonstrates the feasibility of secure QKD over a 1002 km fibre channel considering the finite size effect. The secure key rate is $3.11 10^{-12}$ per pulse at this distance. Furthermore, by optimizing parameters for shorter fiber distances, we conducted performance tests on key distribution for fiber lengths ranging from 202 km to 505 km. Notably, the secure key rate for the 202 km, the normal distance between major cities, reached 111.74 kbps.

In this presentation, I will introduce two of our recent works: experimental realization of measurement-device-independent (MDI) quantum key distribution (QKD) [arXiv:1209.6178] and unambiguous-state-discrimination (USD) attack on a decoy-state QKD system without phase randomization [arXiv:1304.2541]. On one hand, the MDI-QKD is able to shield all practical attacks realized so far. We experimentally demonstrate the MDI-QKD protocol by implementing high-speed and low-noise up-conversion single photon detectors. The security of MDI-QKD relies on a trusted source scenario, where the decoy-state method is assumed. On the other hand, phase randomization is commonly ignored from the decoy-state method. We demonstrate a USD attack on a decoy-state QKD system when the phase randomization is ignored.