Quantum key distribution with setting-choice-independently correlated light sources

Abstract

Despite the enormous theoretical and experimental progress made so far in quantum key distribution (QKD), the security of most existing QKD implementations is not rigorously established yet. A critical obstacle is that almost all existing security proofs make ideal assumptions on the QKD devices. Problematically, such assumptions are hard to satisfy in the experiments, and therefore it is not obvious how to apply such security proofs to practical QKD systems. Fortunately, any imperfections and security-loopholes in the measurement devices can be perfectly closed by measurement-device-independent QKD (MDI-QKD), and thus we only need to consider how to secure the source devices. Among imperfections in the source devices, correlations between the sending pulses are one of the principal problems. In this paper, we consider a setting-choice-independent correlation (SCIC) framework in which the sending pulses can present arbitrary correlations but they are independent of the previous setting choices such as the bit, the basis and the intensity settings. Within the framework of SCIC, we consider the dominant fluctuations of the sending states, such as the relative phases and the intensities, and provide a self-contained information theoretic security proof for the loss-tolerant QKD protocol in the finite-key regime. We demonstrate the feasibility of secure quantum communication within a reasonable number of pulses sent, and thus we are convinced that our work constitutes a crucial step toward guaranteeing implementation security of QKD.

Simple security proof of twin-field type quantum key distribution protocol

“…That is, it is mainly the interference between the single-photon component generated by either Alice or Bob that leads to security. Finally, we note that since the structure of the security proof of Protocol 3 resembles that for the loss-tolerant QKD protocol, 49 its extension to the finite-key scenario could be readily done by using similar techniques like those employed in, [51][52][53] in combination with the decoy-state analysis employed in standard MDI QKD. 54 In summary, we have introduced a novel TF-type QKD protocol, together with a simple proof of its security, which can beat the fundamental bounds on the private capacity of point-to-point QKD over a lossy optical channel presented in.…”

Quantum key distribution with correlated sources

“…55 into a relationship in terms of numbers rather than probabilities. The procedure for this step is quite standard (17,18,26,47). For this, first, note that g U (x, y) and −g L (x, y) are concave functions with respect to 0 ≤ x ≤ 1 for any fixed 0 ≤ y ≤ 1.…”

“…However, all these works only consider restricted scenarios. In particular, the results in (25,26) and in (27) only consider setting choice-independent pulse correlations and intensity correlations between neighboring pulses, respectively. Therefore, none of them can deal with pulse correlations in terms of the secret key information nor with long-range correlations.…”

Methodology for replacing indirect measurements with direct measurements

“…The destructive swap test is a direct version of the swap test [8] which measures the overlap | ψ|ϕ | 2 between two quantum states |ψ and |ϕ . Initially proposed in [7], this method has been rediscovered by the machine learning approach [9], and it is now utilized in the application of NISQ devices [10][11][12]. Reference [13] has proposed to use the destructive swap test to measure | ψ|U |ψ | 2 for an arbitrary U by substituting |ϕ with U |ψ , and the protocol was extended to measure the quantity | ψ|P|ϕ | 2 , where P is a qubit-permutation operator, which can be employed to estimate nonlinear functionals of a quantum state ρ such as Tr(ρ n ) [14], with a low-depth circuit.…”