Fig. 1: Principle schemes of TF-QKD.
From: Coherent phase transfer for real-world twin-field quantum key distribution
a In ideal TF-QKD, Alice and Bob encode quantum states (QS) on local lasers, attenuated to the single-photon level and with equal frequencies νA = νB. The resulting signals are sent to Charlie, where they interfere on single-photon detectors (D0 and D1). b In practical implementations, a reference laser with frequency νR is sent to Alice and Bob through a service fiber, to phase-lock the QKD lasers and ensure νA = νB = νR. After information encoding, QKD lasers are sent to Charlie through the QKD fibers. The transmission of QS is periodically interrupted to send reference phase states encoded in higher-intensity photon pulses (ref.), that allow detection of changes in the propagation path induced by length and refractive index fluctuations of the fiber. These are counteracted by either adjusting the phase of the incoming lasers through an actuator (act.) or by taking into account the instantaneous phase misalignment between the QKD fibers in post-processing. c In our approach, an additional sensing laser with frequency νS travels the service fiber with the reference laser, and the QKD fibers together with the QKD lasers. It can be spectrally separated because νS falls in a different channel of the dense wavelength-division multiplexed (DWDM) grid. While QKD lasers interfere on D0 and D1, the classical signals at νS are phase-compared on a photodiode (PD) to detect the noise of both the service and QKD fibers. This allows tight control of the fiber noise and simultaneous key streaming.
