M. Lucamarini

Marco Lucamarini, Zhiliang Yuan, James F. Dynes, Andrew J. Shields
Since its inception in 2012, Measurement-Device-Independent (MDI) Quantum Key Distribution (QKD) has grown into a well-established technique that features several remarkable properties: it overcomes all the implementation loopholes related to using sensitive detection apparatuses; it partially removes the restriction of a trusted-node architecture in a quantum network, using a single untrusted node (Charlie) to connect distant users (Alice and Bob); it improves the signal to noise ratio of point-to-point QKD, thus slightly extending its transmission range in the asymptotic scenario. The practicality of MDI-QKD has been convincingly proven in various experiments. Nevertheless, its overall key rate remains quite modest due to a coincidence detection Charlie has to perform on the two photons sent by Alice and Bob. Moreover, MDI-QKD key rate scales only linearly with the channel transmission 𝜂, similarly to the direct-link QKD, while the presence of an intermediate node could lead, in principle, to a better scaling. Theoretical papers have shown that MDI-QKD endowed with a quantum memory has the potential to approach the scaling law of a single repeater, 𝑂(√𝜂), but the realization of this proposal is still elusive. Recently, a far more efficient form of MDI-QKD named “Twin-Field” (TF) QKD has been introduced. This new technique inherits all the security-related benefits of MDI-QKD but, as an extra, it also features the single-repeater scaling law 𝑂(√𝜂) without employing a quantum memory, using only currently available technology. This makes it possible to overcome at long distance the Repeaterless Secret Key Capacity (RSKC), a bound that is unsurpassable with point-to-point QKD only. Overcoming this bound is the benchmark of a quantum repeater. Therefore, experiments performing TF-QKD beyond the RSKC limit, are considered the first realizations of an “effective quantum repeater”. In this talk, I will introduce the TF type of MDI-QKD protocols and rationalise the recent literature in this rapidly evolving research area. Starting from observations on the requirements of practical implementation, I will move towards security proofs and experimental results that compose the current landscape of TF quantum cryptography.