Decoy-state method has greatly filled in loopholes from multi-photon pulses in the field of quantum key distribution (QKD) and it has been employed in almost all practical QKD systems in recent years. However, security and performance of the practical decoy-state QKDs are still limited by inaccurate modulations such as random intensity fluctuation and correlated intensity fluctuation. In this work, an intensity tomography method is proposed as a countermeasure against these fluctuations. It has an ability to tightly bound the unknown density matrices of photon-number state and accurately calibrate the fluctuations by measuring the decoy states with a local single-photon detector. We verified the tomography method by experiments and simulations. The results indicated that our method can efficiently mitigate the flaws from these fluctuations to improve the protocol performance and practical security. Noting that these fluctuations are inevitable in practical QKDs, our method would pave the avenue for QKD's practical applications.
Decoy-state method has greatly filled in loopholes from multi-photon pulses in the field of quantum key distribution (QKD) and it has been employed in almost all practical QKD systems in recent years. However, security and performance of the practical decoy-state QKDs are still limited by inaccurate modulations such as random intensity fluctuation and correlated intensity fluctuation. In this work, an intensity tomography method is proposed as a countermeasure against these fluctuations. It has an ability to tightly bound the unknown density matrices of photon-number state and accurately calibrate the fluctuations by measuring the decoy states with a local single-photon detector. We verified the tomography method by experiments and simulations. The results indicated that our method can efficiently mitigate the flaws from these fluctuations to improve the protocol performance and practical security. Noting that these fluctuations are inevitable in practical QKDs, our method would pave the avenue for QKD's practical applications.
The decoy-state method substantially improves the performance of quantum key distribution (QKD) and perfectly solves crucial issues caused by multiphoton pulses. In recent years, the decoy-state method has occupied a key position in practicality, and almost all the QKD systems have employed the decoy-state method. However, the imperfections of traditional intensity modulators limit the performance of the decoy-state method and bring side-channels. In this work, a special intensity modulator and its accompanying modulation method are designed and experimentally verified for the secure, stable and high-performance decoy-state QKDs. The experimental result indicates that its stable and adjustable intensities, convenient two-level modulation, inherently high speed, and compact structure is extremely fit the future trends and will help the decoy-state method to be perfectly applied to QKD systems.
Abstract The decoy-state method substantially improves the performance of quantum key distribution (QKD) and perfectly solves crucial issues caused by multiphoton pulses. In recent years, the decoy-state method has occupied a key position in practicality, and almost all the QKD systems have employed the decoy-state method. However, the imperfections of traditional intensity modulators limit the performance of the decoy-state method and bring side channels. In this work, a special intensity modulator and its accompanying modulation method are designed and experimentally verified for the secure, stable, and high-performance decoy-state QKDs. The experimental result indicates that its stable and adjustable intensities, convenient two-level modulation, inherently high speed, and compact structure is extremely fit for future trends and will help the decoy-state method to be perfectly applied to QKD systems.
Intensity correlations between neighbouring pulses open a prevalent yet often overlooked security loophole in decoy-state quantum key distribution (QKD). As a solution, we present and experimentally demonstrate an intensity-correlation-tolerant QKD protocol that mitigates the negative effect that this phenomenon has on the secret key rate according to existing security analyses. Compared to previous approaches, our method significantly enhances the robustness against correlations, notably improving both the maximum transmission distances and the achievable secret key rates across different scenarios. By relaxing constraints on correlation parameters, our protocol enables practical devices to counter intensity correlations. We experimentally demonstrate this first practical solution that directly overcomes this security vulnerability, establish the feasibility and efficacy of our proposal, taking a major step towards loophole-free and high-performance QKD.
The decoy-state method for the quantum key distribution (QKD) is a great countermeasure against the fatal photon number splitting attack. At present, the decoy-state method has been widely employed in practical QKD systems. However, the patterning effect, namely the correlated intensity fluctuations in practical QKD systems, violates the important assumption of the decoy-state method and seriously threatens the protocol security. The previous works investigated the patterning effect between the adjacent pulses and proposed the countermeasures from the degree of post selection and modulators. In this work, we investigate the patterning effect with larger correlation range and present an algorithm to calibrate the driving voltages to mitigate the patterning effect. According to our methods, we experimentally observe the higher-order patterning effect and successfully mitigate it by our calibration algorithm. The experiment results indicate that our methods efficiently improve the protocol security and are expected to be widely employed in practical decoy-state QKD systems.
One of the most prominent techniques to enhance the performance of practical quantum key distribution (QKD) systems with laser sources is the decoy-state method. Current decoy-state QKD setups operate at gigahertz repetition rates, a regime where memory effects in the modulators and electronics that control them create correlations between the intensities of the emitted pulses. This translates into information leakage about the selected intensities, which cripples a crucial premise of the decoy-state method, thus invalidating the use of standard security analyzes. To overcome this problem, a security proof that exploits the Cauchy-Schwarz constraint has been introduced recently. Its main drawback is, however, that the achievable key rate is significantly lower than that of the ideal scenario without intensity correlations. Here, we improve this security proof technique by combining it with a fine-grained decoy-state analysis, which can deliver a tight estimation of the relevant parameters that determine the secret key rate. This results in a notable performance enhancement, being now the attainable distance double that of previous analyzes for certain parameter regimes. Also, we show that when the probability density function of the intensity fluctuations, conditioned on the current and previous intensity choices, is known, our approach provides a key rate very similar to the ideal scenario, which highlights the importance of an accurate experimental characterization of the correlations.
Abstract The performance of quantum key distribution (QKD) is severely limited by multiphoton emissions, due to the photon-number-splitting attack. The most efficient solution, the decoy-state method, requires that the phases of all transmitted pulses are independent and uniformly random. In practice, however, these phases are often correlated, especially in high-speed systems, which opens a security loophole. Here, we address this pressing problem by providing a security proof for decoy-state QKD with correlated phases that offers key rates close to the ideal scenario. Our work paves the way towards high-performance secure QKD with practical laser sources, and may have applications beyond QKD.