Implementation of the RRDPS QKD protocol using time-domain Fourier transform via space-time duality

Quantum Key Distribution (QKD) is a secure communication method that provides protection against adversaries with unlimited power. The Round Robin Differential Phase Shift (RRDPS) QKD protocol has distinctive features that eliminate the need for signal disturbance monitoring, unlike other commonly used QKD protocols. Some experiments have shown that the RRDPS protocol is superior to the decoy-state BB84 protocol, particularly under high error rates.

In the RRDPS protocol, the pulse train containing phase information is divided into two parts. One part is randomly delayed to interfere with the other, and the interference signal is used to generate the key. A conventional method utilizes optical switches to achieve variable-delay interferometry. However, optical switches have a typical response time of microseconds, which restricts the key rate.

To address this issue, we have developed a new and simple optical circuit that uses Fourier transform and phase modulation to generate a variable delay to the pulse. This method eliminates the slow optical switches and boosts the transmission rate of the RRDPS protocol.

Furthermore, we use the space-time duality technique to implement the Fourier transform in the time domain, rather than the space domain. This technique offers a simple and cost-effective solution for electrical control of pulse delay, paving the way for practical applications.