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 explore 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 and a previously demonstrated machine learning 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 hardware implementation complexity which can seamlessly work on diverse transmission lines.
Highlights
Continuous-variable quantum key distribution (CV-QKD) enables information-theoretically secure key exchange between two parties using the continuous-variable properties of the quantised electromagnetic light field[1,2,3,4,5]
We note that widespread deployment of CV-QKD systems using standard telecoms lasers is more realistic than using ultra low linewidth
Using the unscented Kalman filter (UKF) it is always possible to extract a secret key using either transmitter laser, while even at a SNRpilot = 26 dB, the reference method could achieve at best e = 0.015, which is still too high for key generation phase and quadrature modulator
Summary
Continuous-variable quantum key distribution (CV-QKD) enables information-theoretically secure key exchange between two parties using the continuous-variable properties of the quantised electromagnetic light field[1,2,3,4,5]. The quantum information used for generating the secret key can be imprinted onto coherent states in the amplitude and phase quadratures of laser light using electro-optical modulators at the transmitter. Modern CV-QKD implementations generate the LO from a laser at the receiver, which is independent of the transmitter laser This simplifies the CV-QKD implementation and increases security, at the cost of requiring to recover the frequency and phase of the quantum signal. This process, commonly known as carrier recovery in telecommunication[7], is of utmost importance for the performance of CV-QKD implementations as an impairment cannot be distinguished from excess noise generated by an eavesdropper
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