Hou-Man Chin

Continuous-variable quantum key distribution offers a practical way for doing secure key exchange by means of broadband modulators and coherent detectors operating in the telecom band. Recent advances in theory and practice have improved the security and eased the system implementation. These include composable security with a finite number of distributed Gaussian-modulated coherent states and the use of pilot/reference signals and a real local oscillator for sharing the phase reference across the communicating parties. Here we report the first prepare-and-measure continuous-variable quantum key distribution experiment that can produce composable keys in the finite-size regime with security against collective attacks. Through novel improvements in the existing security proofs and a fast, yet low-noise and highly stable system operation, we obtain a secret key rate $>$5 Mbps over a 20 km long fiber channel. Our demonstration verifies the security of practical continuous-variable quantum key distribution when used for encryption or other cryptographic tasks.

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.

Squeezed states of light promise significant advantages for enhancing the performance of continuous-variable quantum key distribution (CV-QKD) systems. These advantages include the ability to reach longer distances, tolerate higher levels of excess noise, and operate at lower information reconciliation efficiency. So far those advantages were only predicted in theory. In this work, we experimentally demonstrate a CV-QKD system over 40 km fibre using squeezed light achieving a secret key rate of 0.0318 bits per channel use, surpassing the equivalent coherent state system. Similar to state-of-the-art coherent state QKD systems our system employs digital signal processing for impairment compensation eliminating the need for complex locking mechanisms and enhancing its suitability for practical implementations.

Quantum key distribution (QKD) enables two remote parties to share encryption keys with security based on physical laws. Continuous variable (CV) QKD based on coherent states and coherent detection is a suitable scheme for integration into existing telecom networks. However, thus far, long-distance CV-QKD has only been demonstrated using a highly complex transmitted local oscillator scheme, opening security loopholes for eavesdroppers and limiting its potential applications. Here, we report a long-distance CV-QKD experiment with a locally generated local oscillator over a 100 km fiber channel. This record-breaking distance is enabled by controlling the phase-noise component of excess noise, using a machine-learning framework for carrier recovery and optimizing the modulation variance. We consider the full CV-QKD protocol implementation and demonstrate the generation of keys secure against collective attacks in asymptotic and finite-size regimes. Our results set an essential milestone for CV quantum access networks realization, where a high loss budget is required, and pave the way for large-scale deployment of secure QK.

This work experimentally investigates the impact of residual phase noise on CVQKD systems using phase profiles obtained through simulated Wiener phase processes and experimental measurements, and compares the experimental measurements to the theoretical calculation.

This work experimentally investigates a clock recovery algorithm’s performance for a gaussian modulated CV-QKD system operating over 20km of fibre using a frequency multiplexed classical signal.