Four-state discrete modulation continuous variable quantum key distribution based on hardware synchronization

Abstract

In case of continuous-variable quantum key distribution (CV-QKD) systems, synchronization is a key technology that ensures both the transmitter and receiver obtain corresponding data synchronously. By designing an ingenious time sequence for the transmitter and receiver and using the peaking value acquisition technique along with time domain heterodyne detection, we experimentally realized a four-state discrete modulation CV-QKD with a repetition rate of 10 MHz transmitted over a distance of 25 km. With well-designed time sequence of hardware, Alice and Bob could obtain corresponding data automatically without using numerous software calculation methods.<br>The secure key rates were calculated using the method proposed by the Lütkenhaus group at the University of Waterloo in Canada. In the calculation, we first estimated the first and second moments using the measured quadratures of displaced thermal states, followed by calculating the secret key rate using the convex optimization method with the reconstruction of the moments. There was no need to assume a linear quantum transmission channel to estimate the excess noise. Finally, secure key rates of 0.0022–0.0091 bit/pulse were achieved, and the excess noise was between 0.016 and 0.103.<br>In this study, first, we introduced the prepare-and-measure scheme and entanglement-based scheme of the four-state discrete modulation protocol. The Wigner images of the four coherent states at Alice’s side and four displaced thermal states at Bob’s side were presented. Second, the designed time sequences of hardware synchronization were comprehensively introduced. Third, the CV-QKD experiment setup was introduced and the time sequence was verified. Finally, the calculation method of secure key rates using the first and second moments of quadratures was interpreted in detail. The phase space distribution of quadratures was also presented. The secret key rate was between 0.0022 and 0.0091 bits/pulse, and the equivalent excess noise was between 0.016 and 0.103. The average secret key bit rate was 24 Kbps. During the experiment, the first and second moments of the quantum state at the receiver end were found to fluctuate owing to the finite-size effect. This effect decreased the value of the secure key rate and limited the transmission distance of the CV-QKD system.<br>In conclusion, four-state discrete modulation CV-QKD based on hardware synchronization was designed and demonstrated. The proposed hardware synchronization method could effectively reduce the cost, size, and power consumption. In the future, the finite-size effect can be theoretically and experimentally investigated to improve the performance of system.