Coherent phase transfer for real-world twin-field quantum key distribution

Cecilia Clivati,Simone Donadello,Andrew J Shields,Mirko Pittaluga,Zhiliang Yuan,Ivo Pietro Degiovanni,Salvatore Virzì,Filippo Levi,Alice Meda,Davide Calonico,Marco Lucamarini,Alberto Mura,Marco Genovese

doi:10.1038/s41467-021-27808-1

Cecilia Clivati, Simone Donadello + Show 11 more

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https://doi.org/10.1038/s41467-021-27808-1

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Quantum mechanics allows distribution of intrinsically secure encryption keys by optical means. Twin-field quantum key distribution is one of the most promising techniques for its implementation on long-distance fiber networks, but requires stabilizing the optical length of the communication channels between parties. In proof-of-principle experiments based on spooled fibers, this was achieved by interleaving the quantum communication with periodical stabilization frames. In this approach, longer duty cycles for the key streaming come at the cost of a looser control of channel length, and a successful key-transfer using this technique in real world remains a significant challenge. Using interferometry techniques derived from frequency metrology, we develop a solution for the simultaneous key streaming and channel length control, and demonstrate it on a 206 km field-deployed fiber with 65 dB loss. Our technique reduces the quantum-bit-error-rate contributed by channel length variations to <1%, representing an effective solution for real-world quantum communications.

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1, Salvatore Virzì 1, Marco Genovese[1,2], Filippo Levi[1, 5], Andrew J
We use a pair of ultrastable lasers with a linewidth of ~1 Hz and frequency νR = 194.4 THz (1542.14 nm) and νS = 194.25 THz (1543.33 nm), which are standard frequencies of the dense wavelength-division multiplexed (DWDM) grid
Cavity-stabilized lasers and frequency combs, the most stable of which are used as local oscillators in optical clocks and tick synchronously with narrow atomic resonances, are standard devices in the frequency metrology community and are highly-integrated and commercially available systems

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1, Salvatore Virzì 1, Marco Genovese[1,2], Filippo Levi[1, 5], Andrew J. In proof-of-principle experiments based on spooled fibers, this was achieved by interleaving the quantum communication with periodical stabilization frames In this approach, longer duty cycles for the key streaming come at the cost of a looser control of channel length, and a successful key-transfer using this technique in real world remains a significant challenge. In proof-of-principle experiments based on spooled fibers[31,32,33,34,35], this effect is mitigated by interleaving the QKD with classical transmission that provides information on the environmentally-induced noise and enables to periodically realign the phases of interfering pulses[31,32] (see Fig. 1b). We propose a solution derived from frequency metrology, where the transmission of coherent laser radiation over thousandkilometer distances is employed to compare distant atomic clocks at the highest accuracy[38,39,40,41,42,43,44] using ultrastable lasers and phasestabilized optical fibers[45]

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