While measurement-device-independent (MDI) quantum key distribution (QKD) allows two trusted parties to establish a shared secret key from a distance without needing to trust a central detection node, their quantum sources must be well characterized, with side channels at the source posing the greatest loophole to the protocol's security. In this paper, we identify a time-dependent side channel in a common polarization-based QKD source that employs a Faraday mirror for phase stabilization. We apply a recently developed numerical proof technique [Phys. Rev. A 99, 062332 (2019)] to quantify the sensitivity of the secret key rate to the quantum optical model for the side channel, and to develop strategies to mitigate the information leakage. In particular, we find that the MDI three-state and BB84 protocols, while yielding the same key rate under ideal conditions, have diverging results in the presence of a side channel, with BB84 proving more advantageous. While we consider only a representative case example, we expect the strategies developed and key rate analysis method to be broadly applicable to other leaky sources.
While measurement-device-independent (MDI) quantum key distribution (QKD) allows two trusted parties to establish a shared secret key from a distance without needing to trust a central detection node, their quantum sources must be well characterized, with side channels at the source posing the greatest loophole to the protocol's security. In this paper, we identify a time-dependent side channel in a common polarization-based QKD source that employs a Faraday mirror for phase stabilization. We apply a recently developed numerical proof technique [Phys. Rev. A 99, 062332 (2019)] to quantify the sensitivity of the secret key rate to the quantum optical model for the side channel, and to develop strategies to mitigate the information leakage. In particular, we find that the MDI three-state and BB84 protocols, while yielding the same key rate under ideal conditions, have diverging results in the presence of a side channel, with BB84 proving more advantageous. While we consider only a representative case example, we expect the strategies developed and key rate analysis method to be broadly applicable to other leaky sources.
The importance of secure and high-rate information transmission cannot be understated in today's society. Unfortunately, a security crisis is impending due to the development of quantum computing technologies which can break current encryption schemes. A promising solution is quantum key distribution (QKD), which offers information-theoretic security. However, practical implementations of QKD do not conform to the idealized security assumptions that are made in QKD security proofs and therefore form security loopholes. The discovery of the measurement-device-independent QKD (MDI QKD) protocol presented a rapid leap in QKD research, as it effectively closes all side channels at the detection end of QKD setups, while maintaining experimental feasibility. Naturally, a major remaining obstacle in QKD research is the vulnerability of practical sources. As the clock rate of QKD systems advances into the GHz range, specific imperfections at the source arise. The goal of this PhD thesis is to conduct characterizations of source imperfections (especially those arising from GHz modulation), quantify their impact on security, and find methods to mitigate them. Specifically, this thesis presents the following studies. A) Identification and analysis of a novel side channel in sources using a Faraday mirror for temperature-stabilized polarization modulation, leading to time-dependent passive leakage of quantum states. We demonstrate that precise side-channel characterization (rather than the pessimistic approximations commonly used in the literature) can improve the estimated secret key rate by multiple orders of magnitude. B) Identification and analysis of a new class of source side channels (hidden multidimensional modulation side channels), using high-speed time-varying encoding as an illustrative case example (which we characterize using experimental and simulation-based methods). We demonstrate that hidden degrees of freedom, such as time, may be coupled to the encoding process, enabling unambiguous state discrimination attacks. In our illustrative case example, this causes the maximum secure transmission distance to be reduced by over 50%. C) Experimental and simulation-based characterization of phase correlations at GHz repetition rates when using electro-optic phase modulation for phase/polarization encoding. Our findings indicate that pattern-dependent phase deviations are on the order of 0.01pi. D) Design and experimental demonstration of a path-selection modulation source -- a source using no active phase modulation components (to mitigate the side channels arising from such components). Phase randomization is achieved through gain-switching and polarization encoding is achieved by randomly selecting between multiple paths, each path corresponding to one of the desired encoded states. Overall, by identifying and characterizing multiple source side channels, this thesis underscores the importance of avoiding unwarranted assumptions regarding the security of QKD sources. Hidden imperfections can introduce significant security vulnerabilities, and these must be continually identified and analyzed. Furthermore, this thesis highlights the value of advancing key rate analysis techniques that can accommodate a broad range of implementation flaws while producing tight security bounds, thereby enabling precise characterization of the information leakage arising from such imperfections. Finally, this thesis emphasizes that active modulators, in general, constitute a major source of potential side channels, highlighting the importance of exploring modulator-free sources. The path-selection modulation scheme presented here provides a practically feasible example of such a source.
Measurement-device-independent quantum key distribution (MDI-QKD) is the only known QKD scheme that can completely overcome the problem of detection side-channel attacks. Yet, despite its practical importance, there is no standard approach towards proving the security of MDI-QKD. Here, we present a simple numerical method that can efficiently compute almost-tight security bounds for any discretely modulated MDI-QKD protocol. To demonstrate the broad utility of our method, we use it to analyze the security of coherent-state MDI-QKD, decoy-state MDI-QKD with leaky sources, and a variant of twin-field QKD called phase-matching QKD. In all of the numerical simulations (using realistic detection models) we find that our method gives significantly higher secret key rates than those obtained with current security proof techniques. Interestingly, we also find that phase-matching QKD using only two coherent test states is enough to overcome the fundamental rate-distance limit of QKD. Taken together, these findings suggest that our security proof method enables a versatile, fast, and possibly optimal approach towards the security validation of practical MDI-QKD systems.
Quantum key distribution (QKD) enables two legitimate parties to share a secret key even in the presence of an eavesdropper. The unconditional security of QKD is based on the fundamental laws of quantum physics. Original security proofs of QKD are based on a few assumptions, e.g., perfect single photon sources and perfect single-photon detectors. However, practical implementations of QKD systems do not fully comply with such assumptions due to technical limitations. The gap between theory and implementations leads to security loopholes in most QKD systems, and several attacks have been launched on sophisticated QKD systems. Particularly, the detectors have been found to be the most vulnerable part of QKD. Much effort has been put to build side-channel-free QKD systems. Solutions such as security patches and device-independent QKD have been proposed. However, the former are normally ad-hoc, and cannot close unidentified loopholes. The latter, while having the advantages of removing all assumptions on devices, is impractical to implement today. Measurement-device-independent QKD (MDI-QKD) turns out to be a promising solution to the security problem of QKD. In MDI-QKD, all security loopholes, including those yet-to-be discovered, have been removed from the detectors, the most critical part in QKD. In this thesis, we investigate issues related to the practical implementation and security of MDI-QKD. We first present a demonstration of polarization-encoding MDI-QKD. Taking finite key effect into account, we achieve a secret key rate of 0.005 bit per second (bps) over 10 km spooled telecom fiber, and a 1600-bit key is distributed. This work, together with other demonstrations, shows the practicality of MDI-QKD. Next we investigate a critical assumption of MDI-QKD: perfect state preparation. We apply the loss-tolerant QKD protocol and adapt it to MDI-QKD to quantify information leakage due to imperfect state preparation. We then present an experimental demonstration of MDI-QKD over 10 km and 40 km of spooled fiber, which for the first time considers the impact of inaccurate polarization state preparation on the secret key rate. This would not have been possible under previous security proofs, given the same amount of state preparation flaws.
Measurement-device-independent quantum key distribution (MDI-QKD) can remove all detection side-channels from quantum communication systems. The security proofs require, however, that certain assumptions on the sources are satisfied. This includes, for instance, the requirement that there is no information leakage from the transmitters of the senders, which unfortunately is very difficult to guarantee in practice. In this paper we relax this unrealistic assumption by presenting a general formalism to prove the security of MDI-QKD with leaky sources. With this formalism, we analyze the finite-key security of two prominent MDI-QKD schemes-a symmetric three-intensity decoy-state MDI-QKD protocol and a four-intensity decoy-state MDI-QKD protocol-and determine their robustness against information leakage from both the intensity modulator and the phase modulator of the transmitters. Our work shows that MDI-QKD is feasible within a reasonable time frame of signal transmission given that the sources are sufficiently isolated. Thus, it provides an essential reference for experimentalists to ensure the security of implementations of MDI-QKD in the presence of information leakage.
A recently proposed fully passive quantum key distribution (QKD) removes all source modulator side channels. In this work, we combine fully passive sources with measurement-device-independent (MDI) QKD to simultaneously remove side channels from source modulators and detectors. We show a numerical simulation of passive MDI QKD, and we obtain an acceptable key rate while achieving much better implementation security, as well as ease of implementation, compared with a recently proposed fully passive twin-field QKD, paving the way towards more secure and practical QKD systems. We prove that a fully passive protocol is compatible with MDI QKD and we propose a novel idea that can improve the sifting efficiency.