Quantum key distribution (QKD) offers information-theoretic security verified by quantum mechanics to share keys between legitimate users. Most of the existing QKD systems employ active decoy states based on weak coherent sources (WCS). Meanwhile, parametric down-conversion (PDC) sources are seldom used due to several of their shortcomings. In the present work, to show the superiority of PDC sources, we have accomplished a proof-of-principle demonstration of a PDC source-based QKD with over 40 dB based on the one-way BB84 protocol. In this QKD system, a novel passive decoy-state scheme-secure to coherent attacks-is proposed, using several built-in decoy states for parameter estimation. This not only avoids intensity modulating errors, but also diminishes all possible information leakage from the intensity modulating process. The experimental results show a significantly enhanced performance compared with existing PDC source-based QKD systems. In addition, it exhibits some superiority even over active decoy-state QKD systems based on WCS.
Quantum key distribution (QKD) offers information-theoretic security verified by quantum mechanics to share keys between legitimate users. Most of the existing QKD systems employ active decoy states based on weak coherent sources (WCS). Meanwhile, parametric down-conversion (PDC) sources are seldom used due to several of their shortcomings. In the present work, to show the superiority of PDC sources, we have accomplished a proof-of-principle demonstration of a PDC source-based QKD with over 40 dB based on the one-way BB84 protocol. In this QKD system, a novel passive decoy-state scheme-secure to coherent attacks-is proposed, using several built-in decoy states for parameter estimation. This not only avoids intensity modulating errors, but also diminishes all possible information leakage from the intensity modulating process. The experimental results show a significantly enhanced performance compared with existing PDC source-based QKD systems. In addition, it exhibits some superiority even over active decoy-state QKD systems based on WCS.
The fully passive source is capable of passively generating decoy states and performing passive encoding simultaneously, avoiding the side-channel risks caused by active modulation operations at the source end, thus effectively enhance the security in quantum key distribution (QKD). Existing fully passive QKD protocol and experiments exploit phase-randomized coherent pulses. In this paper, we propose a fully passive QKD protocol using parametric down-conversion source. The decoy state generation and encoding operation can be carried out passively by parameter down-conversion progress. This protocol has several advantages. First, it can also eliminate all side channels in active modulators. Second, compared with fully passive QKD protocol with phase-randomized coherent pulses, our protocol can significantly increase the key rate and extend the communication distance. Meanwhile, in terms of the transmission rate, our protocol is also closer to that of actively modulated QKD and can achieve fully passive modulation with fewer resources. Moreover, combined with measurement-device-independent (MDI) QKD, this protocol can even potentially achieve robustness against side channels in both detectors and modulators.
We present a new scheme for realizing passive decoy-state quantum key distribution with the SARG04 protocol. It is based on parametric down-conversion sources, and we do not need to modulate the pumping light into different intensities. By applying local-mode-splitting detections on Alice’s side, legitimate users can obtain four different kinds of counting events on Bob’s side, due to intrinsic correlations between photon pairs from parametric down-conversion processes. In the new scheme, we do not need to modulate signal pulses into different intensities and can, thus, avoid introducing any intensity modulating uncertainty. As a result, quite precise estimations of single- or multi-photon-pulse contributions can be obtained, enabling better performance of the key generation rate and transmission distance compared with other existing SARG04 protocols, e.g., the four-intensity decoy-state method using either parametric down-conversion sources or weak coherent sources. Besides, through comparisons, we also find that the proposed new passive decoy-state SARG04 protocol can also show some superiority over the conventional three-intensity decoy-state BB84 protocol at long transmission distances under certain experimental conditions.
Quantum key distribution (QKD) offers information-theoretic security based on the fundamental laws of physics. However, device imperfections, such as those in active modulators, may introduce side-channel leakage, thus compromising practical security. Attempts to remove active modulation, including passive decoy intensity preparation and polarization encoding, have faced theoretical constraints and inadequate security verification, thus hindering the achievement of a fully passive QKD scheme. Recent research [W. Wang et al., Phys. Rev. Lett. 130, 220801 (2023).PRLTAO0031-900710.1103/PhysRevLett.130.220801; 2V. Zapatero et al., Quantum Sci. Technol. 8, 025014 (2023).2058-956510.1088/2058-9565/acbc46] has systematically analyzed the security of a fully passive modulation protocol. Based on this, we utilize the gain-switching technique in combination with the postselection scheme and perform a proof-of-principle demonstration of a fully passive quantum key distribution with polarization encoding at channel losses of 7.2 dB, 11.6 dB, and 16.7 dB. Our work demonstrates the feasibility of active-modulation-free QKD in polarization-encoded systems.
Reference-frame-independent quantum key distribution (RFI-QKD) has been proven to be intrinsically robust against slowly varying reference frames under realistic environment. In this paper, we propose a simplified scheme for the passive decoy-state RFI-QKD considering the afterpulsing effect. We investigated the system performance with the weak coherent source (WCS) and parametric down conversion (PDC) source, respectively. Numerical simulation results show that the simplified scheme can achieve comparable secure key rate with the active decoy-state scheme. Our scheme is advantageous to simplify the system implementation and reduces the time cost. Besides, it is proven that the afterpulsing effect plays dominant role in the system. Our work could provide a useful solution for RFI-QKD in practical situations.
The authors demonstrate that passive decoy-state quantum digital signature (QDS) using parametric down-conversion sources can be used effectively for encrypted communication over a distance of 200 km. With this method, the probability of leaking information to the eavesdropper is avoided, which improves security. The superiority of this system is due to its efficient passive decoy-state scheme and low-loss experimental system. The present work can also be extended to the recently proposed measurement-device-independent quantum digital signatures, and thus represents a significant step for QDSs on the path from the laboratory to practical applications.
The security of quantum key distribution (QKD) is based on the basic principles of quantum mechanics, and therefore has unconditional security in theory. In existing quantum key distribution systems, weakly coherent sources (WCSs) are often used as light sources due to a high probability of vacuum pulses in these sources, resulting in limited transmission distances. Besides, there inevitably exist equipment defects in actual QKD systems, such as certain defects in phase modulators and intensity modulators, which lead to distinguishability of quantum states in higher dimensions and result in side-channel vulnerabilities. An eavesdropper can carry out corresponding attacks, thereby threatening the actual security of QKD systems. To overcome the above limitations, we propose an improved protocol on quantum key distribution based on monitoring heralded single-photon sources. Due to the simultaneity of parametric down-conversion photon pairs, we can accurately predict the arrival of one photon by measuring the arrival time of another photon. Through this way, we can greatly reduce the probability of vacuum states in the signal light, and increase the longest transmission distance of the QKD system. Moreover, a light source monitoring module is inserted into the sender’s side. By randomly selecting a certain period of time through the source monitoring module to measure the Hong-Ou-Mandel interference between the signal light and the idle light , we can estimate the side-channel information leakage of the source and then obtain the key generation rate. Compared with the QKD protocol based on monitoring weak coherent sources, our present protocol can give a better performance in either the transmission distance or the key generation rate, especially when the interference error is large. In addition, in principle, our present protocol can also be extended to other quantum key distribution protocols, such as the measurement-device-independent protocols, to further improve the security and practicability of QKD systems. Therefore, our present work can provide valuable references for realizing the large-scale application of quantum communication networks in the near future.