Contributed Talks 4b: Experimental QKD (Chairs: Felix Bussieres and Hugo Zbinden)

Abstract: Given that most implementations of quantum cryptography systems require low light operations for security reasons, limiting the energy of incoming/outgoing optical signals is a central task. In this submission, we propose and demonstrate a novel and practical power limiter using the thermo-optical defocusing effect of an acrylic prism. The results show that a power limiting in the regime of mW or lower can be achieved, and at the same time possess desirable features like compactness, robustness, polarization and spectrum dimension independence, etc. Our work provides an effective way for limiting the incoming/outgoing optical energy, which is important for practical quantum cryptographic protocols. We believe it will attract much interest and possess the potential to become a standard tool for practical quantum applications.

Abstract: The secret key rate of a continuous-variable quantum key distribution (CV-QKD) system is limited by excess noise. A key issue typical to all modern CV-QKD systems implemented with a reference or pilot signal and an independent local oscillator is controlling the excess noise generated from the frequency and phase noise accrued by the transmitter and receiver. Therefore accurate phase estimation and compensation, so-called carrier recovery, is a critical subsystem of CV-QKD. Here, we present the implementation of a machine learning framework based on Bayesian inference, namely an unscented Kalman filter (UKF), for estimation of phase noise and compare it to a standard reference method. Experimental results obtained over a 20 km fibre-optic link indicate that the UKF can ensure very low excess noise even at low pilot powers. The measurements exhibited low variance and high stability in excess noise over a wide range of pilot signal to noise ratios. This may enable CV-QKD systems with low implementation complexity which can seamlessly work on diverse transmission lines.

Abstract: An integrated chip system for MDI-QKD is demonstrated. The MDI-QKD transmitter chips and server chip work on a key rate per pulse of 2.923 × 10^(-6) over a distance corresponding to 50-km optical fiber with 25% detection efficiency.

Abstract: An integrated chip platform for CV-QKD system based on time and polarization multiplexing is designed and demonstrated. A proof-of-principle test is conducted, which shows the measurement results for key components. The secure key rate by simulation can reach 4 kbit/s at 40 km distance per transmission band.