Quantum Key Distribution (QKD) is a technology that allows sharing secret cryptographic keys between two distant users (Alice and Bob), whose intrinsic security is guaranteed by fundamental principles of quantum mechanics. QKD is a mature technology even if one of the main remaining challenges is the integration of different solutions in already deployed telecommunication fiber networks, in particular in long-haul segments. An approach able to cover long distances is the Twin-field QKD (TF-QKD) protocol; TF-QKD exploits interference of optical pulses in a central untrusted node (Charlie), allowing to double the communication distance with respect to the conventional prepare-and-measure solutions. Here we present a solution to one of the main issues of Twin-Field QKD, the phase stabilization within the optical path, demonstrating a strong advantage in performances of real word TF- QKD and testing our solution in a segment of the Italian Quantum Backbone. Furthermore, we analyze in detail the expected gain in terms of key rate exploiting our stabilization technique in the main TF-QKD-based protocols, even when they are declared insensitive to the phase noise.
Quantum key distribution (QKD) allows the sharing of secret cryptographic keys between two distant users, whose intrinsic security is guaranteed by the laws of nature. Nowadays, the most promising technique for the integration of QKD in already deployed long-haul telecommunication fiber networks is the Twin-field QKD (TF-QKD) protocol, but it requires that the communication channel is stable in terms of phase oscillations and it is free from background light, that reduces the transmission key-rate. Recently, we presented a solution to the phase stabilization problem, derived from atomic clocks comparison technology, demonstrating advantages in performances of real word TF-QKD. Here we quantify and characterize the background photons, analyzing in details their effects on the transmission and the practicalities to reduce their contribution to a negligible level.
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