Frequency-matching quantum key distribution

Abstract

Quantum key distribution (QKD) enables information-theoretically secure communication against eavesdropping. However, phase instability remains a challenge across many QKD applications, particularly in schemes such as twin-field QKD and measurement-device-independent QKD. The most dominant source of phase fluctuation arises from the frequency offset between independent lasers. Here we propose a method to address this issue by employing a classical photodiode to compensate for the laser frequency difference. As an application of this method, we implement this technique in a mode-pairing QKD system, achieving an error rate approaching the theoretical limit and surpassing the linear key-rate bound over a fiber distance of 296.8 km. This approach provides a practical solution for frequency matching between independent lasers and can be extended to other fields requiring precise phase stabilization.

Figures (4)

• Figure 1: (a) Alice and Bob transmit quantum light through a quantum channel to an intermediate node, Charlie, for QKD. Concurrently, they employ a classical channel to transmit classical light for conventional communication, such as synchronization of remote clocks, and utilize frequency-matching light to facilitate real-time, high-precision measurement of the frequency difference between two independent laser sources. The frequency-matching light and the classical communication light are wavelength-division multiplexed for co-fiber transmission.(b) The photodiode's measurement records the beat-note interference between the two independent lasers from Alice and Bob. The extracted frequency difference is correlated with the detection events of the single-photon detectors, enabling precise determination of the phase difference induced by the frequency offset between the independent laser sources.
• Figure 2: Experimental Setup. Alice and Bob employ narrow-linewidth lasers as light sources, splitting the output into two beams. One beam is directed to Charlie for interference, where photodiodes facilitate frequency matching. The second beam undergoes quantum light modulation, including chopping and decoy state modulation via an intensity modulator and a Sagnac loop, comprising a circulator, beam splitter, phase modulator, and optical fibers. A phase modulator applies 16 distinct phase levels for phase randomization. The modulated quantum light pulses are transmitted to Charlie, where polarization feedback aligns the polarization of the two beams. Interference is then performed using a beam splitter. The resulting signals from both PDs and single-photon detectors are processed through a time-to-digital converter for post-processing, ultimately generating secure keys. BS: beam splitter; IM: intensity modulator; CIR: fiber optical circulator; PM: phase modulator; EVOA: electrical variable optical attenuator; EPC: electric polarization controller; PBS: polarizing beam splitter; PD: photodiode; SPD: single-photon detector.
• Figure 3: Error drift. Data point represents the sequence number of data, with each data point collected approximately every 30 seconds, as the size of every data file is fixed.
• Figure 4: X-basis error varied with maximum pairing length at 296.80 km. Here the handling data is $3.25\times 10^{11}$.