Quantum Key Distribution (QKD) is a strategy for securely generating cryptographic keys between two authenticated users. However, in real-world QKD applications, maintaining a consistent reference frame between users is crucial, typically requiring complex systems to control quantum properties of photons, as polarization or phase. To overcome these challenges, a free-running approach to increase the level of secrecy of the distilled key rate, with a fully passive and self-characterizing receiver, is presented. This approach exploits the phase invariance of the time-bin (TB) strategy for the key distribution together with the robustness of the polarization encoding for the state measurement. Furthermore, the presented receiving apparatus implements a self-characterization procedure, in order to reconstruct the experimental POVMs describing the real measurement procedure implemented by the utilized receiver, taking into account all the unavoidable imperfections. The results provided in this work confirm the maturity of RFI-based quantum key distribution protocols without the need of implementing actively compensating systems. This kind of receivers offer less expensive solutions in terms of costs and complexity for a variety of quantum network implementations in environmental noisy quantum networks.
Quantum Key Distribution (QKD) is a strategy for securely generating cryptographic keys between two authenticated users. However, in real-world QKD applications, maintaining a consistent reference frame between users is crucial, typically requiring complex systems to control quantum properties of photons, as polarization or phase. To overcome these challenges, a free-running approach to increase the level of secrecy of the distilled key rate, with a fully passive and self-characterizing receiver, is presented. This approach exploits the phase invariance of the time-bin (TB) strategy for the key distribution together with the robustness of the polarization encoding for the state measurement. Furthermore, the presented receiving apparatus implements a self-characterization procedure, in order to reconstruct the experimental POVMs describing the real measurement procedure implemented by the utilized receiver, taking into account all the unavoidable imperfections. The results provided in this work confirm the maturity of RFI-based quantum key distribution protocols without the need of implementing actively compensating systems. This kind of receivers offer less expensive solutions in terms of costs and complexity for a variety of quantum network implementations in environmental noisy quantum networks.
Quantum Key Distribution (QKD) promises to revolutionize the field of security in communication, with applications ranging from state secrets to personal data, making it a key player in the ongoing battle against cyber threats. Reference-Frame-Independent (RFI) QKD aims to simplify QKD implementations by allowing to reduce the requirements of alignment on a shared reference frame. This is done by performing two mutually unbiased measurements on the control states. In this work, we present a novel fully passive receiver for time-bin encoded RFI-QKD. Conversion of time-bin to polarization is employed to perform the required quantum measurement in a fully passive manner. Furthermore, to overcome experimental errors, we retrieved a complete description of our measurement apparatus by employing a recently introduced Quantum Detector Self-Characterization technique, without performing tomographic studies on the detection stage. In fact, the security analysis carried out in this work uses experimentally retrieved Positive Operator Valued Measurements, which consider our receiver defects, substituting the ideal expected operators and thus increasing the overall level of secrecy. Lastly, we conducted a proof-of-principle experiment that validated the feasibility of our method and its applicability to QKD applications.
In quantum key distribution (QKD), the users need to share the same reference frame. If their reference frames are inconsistent, the QKD system will not function properly. The most widely used method today is the active real-time calibration of both communication reference frames by using classical communication. In order to get rid of the real-time calibration operation of the reference frames, a QKD protocol independent of reference frame is proposed, called reference-frame-independent QKD (RFI-QKD). The RFI-QKD protocol is immune to the effects of slowly changing reference frame drift, requiring that only one set of bases should be aligned by Alice and Bob, and the remaining two sets of bases can slowly change in the channel. In the real QKD system, a set of basis vectors can always be found to maintain a stable alignment. However, some assumptions are made for the sources in most reported researches, i.e. with a trusted and fixed photon-number distribution (PND), which usually cannot be satisfied in practical implementations. Those unreasonable assumptions will inevitably compromise the security of practical QKD systems. To solve the problem, in this work, we present a passive light source monitoring (PLSM) scheme for RFI-QKD, which is accomplished by a passive monitoring module consisting of a beam splitter and two detectors on the source side. Through the PLSM module, we can have four monitoring events by using two local detectors and then precisely estimate the bounds of source distributions. Specifically, we take the three-intensity decoy-state-based RFI-QKD for example for illustraing the events. Compared with the original RFI-QKD, our PLSM method can passively monitor the PND and has many advantages, with light source fluctuations, finite-size effects and reference frame deflection angles taken into account.
Reference-frame-independent quantum key distribution (RFI-QKD) has been proposed as a method to avoid the requirement of real-time alignment of reference frames. However, the source flaw has not been considered in most of the previously reported works. In this paper, based on the analysis of Wang et al. [Phys. Rev. A 92, 042319 (2015)], we perform a proof-of-concept experiment to demonstrate the loss-tolerant RFI-QKD by constructing a time-bin phase-encoded system and quantifying its source flaws. Moreover, the finite-key analysis is taken into account while estimating the secure key rate. Our experimental demonstration makes a positive contribution to the realization of quantum cryptography in a complex and imperfect real-world environment.
Quantum key distribution (QKD) enables the establishment of shared keys between two distant users, Alice and Bob, based on the fundamental principles of quantum mechanics, and it has proven to possess information-theoretic security. In most of QKD systems, Alice and Bob require a shared reference frame, and real-time calibration of the reference frame increases system costs and reduces its performance. Fortunately, the reference-frame-independent QKD protocol has been proposed, overcoming reference-frame drift issues and receiving widespread attention. However, in practical QKD systems, the non-ideal characteristics of realistic devices introduce certain inconsistency between the theory and the practice. In real-world quantum key distribution systems, device imperfections can lead to security vulnerabilities, thereby reducing system security. For example, imperfections in the encoding apparatus at the source end may result in errors in the quantum states. The inherent defects in the detection part may cause after-pulse effects and dead-time effects, thus reducing the key rate. Therefore, in this work, we propose a practical state-preparation error tolerant reference-frame-independent quantum key distribution protocol by taking imperfections in both the source and the detectors into account. Moreover, a three-intensity decoy-state scheme for modeling analysis and numerical simulations is employed. In this protocol, we reduce the influence of state-preparation errors on the key rate by utilizing virtual state methods to precisely estimate the phase-error rate. Furthermore, by characterizing the effects of after-pulses and dead-time on the key rate, our protocol exhibits higher robustness and can effectively address issues related to detector imperfections. This approach can also be extended to other quantum key distribution protocols with higher security levels, such as measurement-device-independent quantum key distribution protocol and twin-field quantum key distribution, further mitigating the influence of device imperfections on practical implementation of QKD system. Therefore, our present work provide important reference value for putting the quantum key distributions into practical application.
Information-theoretically secure quantum access network (QAN) provides a multiple-user structure for large-scale key distribution. However, active reference-frame compensation and clock synchronization require extra devices and signals, increasing the complexity of the system and time/resource consumption. In this work, a fully passive received QAN comprising both free-space and fiber channels is demonstrated. Based on an improved reference-frame-independent quantum key distribution (RFI-QKD) protocol and qubit-synchronization method, Bob only passively receives quantum signals from all Alices with various wavelengths and laser repetition rates. Extra classical signals for reference frame calibration and clock synchronization are not required, so that the QAN can be asynchronous. Furthermore, RFI-QKD with a 7 km free-space channel in an urban environment is first fully demonstrated with a positive key rate, and the maximal distance over a 250 km fiber channel for four-state RFI-QKD is also achieved. The experimental results show that our fully passive received QAN is robust, cost-effective, and flexible, paving its application in the future quantum network.
Reference-frame-independent measurement-device-independent quantum key distribution (RFI-MDI-QKD) eliminates detector side-channel attacks and avoids reference-frame calibration. While its feasibility has been widely demonstrated, existing implementations typically assume fixed or slowly drifting reference-frame misalignment, conditions rarely satisfied outside the laboratory. In realistic environments, rapid and free-running reference-frame variations can severely degrade both the key rate and transmission distance of conventional RFI-MDI-QKD. Here we propose a free-running RFI-MDI-QKD protocol that maintains high-rate key generation under rapid reference-frame variations. By introducing a classification-distillation method that reclassifies total detection events, secure keys can be extracted without modifying the experimental setup. Our protocol achieves a key rate more than nine times higher than the best previous RFI-MDI-QKD scheme and tolerates channel losses exceeding 24 dB, where earlier approaches fail. These results enable practical quantum key distribution on mobile platforms, including satellite-to-ground links and airborne nodes.