Filippo Levi

Quantum key distribution (QKD) ensures the sharing of secret cryptographic keys between distant entities (typically called Alice and Bob), whose intrinsic security is guaranteed by the laws of nature [1–3]. Besides pioneering experiments involving satellite transmission [4], the challenge is the integration of this technology in telecommunication fiber networks, in particular in long haul segments [5–11]. The longest achievable communication distance is limited by the channel loss which increases exponentially with the fiber length and noise in the deployed single photon detector. The secure QKD key rate decreases exponentially with the channel fiber length. Although the communication distance could be extended using quantum repeaters, the related research is still at a proof-of-principle level [12]. Presently the widely adopted solution is the exploitation of trusted nodes, whose security represents however a significant technical issue. An innovative approach that overcomes, at least partially, the need for trusted node is represented by the recently proposed QKD protocol dubbed twin-field QKD (TF-QKD) [13]. In TF-QKD, the information is encoded on dim laser pulses generated at distant Alice and Bob terminals and sent through optical fiber over half of the entire communication distance to the central node, Charlie, where they interfere. For this reason, the TF-QKD has weaker dependence on channel losses, essentially doubling the communication distance with respect to the conventional prepare-and-measure QKD solution. TF-QKDhas been proved secure against general attacks (see e.g. [14–18]), but its implementation is challenging as the optical pulses sent by Alice and Bob are required to be phase-coherent and preserve coherence when reaching Charlie after travelling the long fiber paths. While phase coherence can be achieved by phase-locking the two QKD lasers in Alice and Bob to a common reference laser transmitted through a service channel, uncorrelated phase changes due to the length and refractive index fluctuations in the long optical fibers still remain and will reduce the visibility of the interference measurement. In the TF-QKD proof-of-principle experiments [19–26], this effect was mitigated by interleaving the QKD frames with classical transmission frames that were used to periodically realign the phases of interfering pulses. Here we present an alternative solution derived from the metrological research community, more precisely from atomic clocks comparison technology. Specifically, transmission of coherent laser radiation over thousand-kilometer-long fibers is exploited for the comparison of distant atomic clocks at the highest accuracy [27–32]. In this case phase fluctuations in long fiber also need to be addressed, othewise they would substantially degrade the comparison results. Precise comparison among these atomic clocks are made possible by the use of ultra-stable lasers and the active cancellation of the noise introduced by connecting fibers. Here we demonstrate that this technique can be successfully adapted into a TF-QKD setup. More specifically, we designed and developed an apparatus suitable for actively cancelling phase fluctuations of both the lasers and of the connecting fibers in a TF-QKD setup. This is achieved by transmitting an additional sensing laser light at a slightly different wavelength in the same fiber as the QKD dim pulses. In Charlie, this sensing laser is used for the fiber optical length stabilisation. We show that this multiplexed solution can be properly tuned in order to avoid a sizeable impact on the number of background photons observed by the single-photon detectors in the QKD channels, allowing simultaneous key streaming and channels stabilization, ensuring longer duty-cycles of the QKD process and a tighter control of the optical phase on long-haul deployed fibers. Furthermore, we tested our solution in a real-world network where the net losses between Alice and Bob are as high as 65 dB, resulting here in a distance of 206 km, or equivalent at 325 km on a fiber haul at common nominal losses of 0.2 dB/km [33]. References [1] Bennett, C. H. & Brassard, G. Quantum cryptography: public key distribution and coin tossing. Theor. Comput. Sci. 560, 7–11 (2014). [2] Scarani, V. et al. The security of practical quantum key distribution. Rev. Mod. Phys. 81, 1301 (2009). [3] Kwong Lo, H., Curty, M. & Tamaki, K. Secure quantum key distribution. Nature Photonics 8, 595-604 (2014). 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