Improving the secret key rate is one of the vital issues in practical applications of quantum key distribution (QKD). In this paper, we propose an experimental scheme of high-dimensional measurement-device-independent quantum key distribution (MDI-QKD) based on spatial basis aiming to increase the key rate. Two groups of discrete position basis and momentum basis are applied as conjugate spaces to generate quantum secret key. The position states are transmitted by single-mode fibers and represented by the index of the fiber, while the momentum state is a coherent superposition of the position states characterized by the phase gradient. The measurement of each momentum basis is realized by multi-slit diffraction. This experimental proposal can be implemented with standard optical elements. In addition to an enhanced key rate in a higher dimension, this high-dimensional MDI-QKD exhibits a comparable security level/performance compared to conventional polarization-based MDI-QKD.
Improving the secret key rate is one of the vital issues in practical applications of quantum key distribution (QKD). In this paper, we propose an experimental scheme of high-dimensional measurement-device-independent quantum key distribution (MDI-QKD) based on spatial basis aiming to increase the key rate. Two groups of discrete position basis and momentum basis are applied as conjugate spaces to generate quantum secret key. The position states are transmitted by single-mode fibers and represented by the index of the fiber, while the momentum state is a coherent superposition of the position states characterized by the phase gradient. The measurement of each momentum basis is realized by multi-slit diffraction. This experimental proposal can be implemented with standard optical elements. In addition to an enhanced key rate in a higher dimension, this high-dimensional MDI-QKD exhibits a comparable security level/performance compared to conventional polarization-based MDI-QKD.
Quantum key distribution (QKD) provides ultimate cryptographic security based on the laws of quantum mechanics. For point-to-point QKD protocols, the security of the generated key is compromised by detector side channel attacks. This problem can be solved with measurement-device-independent QKD (mdi-QKD). However, mdi-QKD has shown limited performances in terms of the secret key generation rate, due to postselection in the Bell measurements. We show that high-dimensional (Hi-D) encoding (qudits) improves the performance of current mdi-QKD implementations. The scheme is proven to be unconditionally secure even for weak coherent pulses with decoy states, while the secret key rate is derived in the single-photon case. Our analysis includes phase errors, imperfect sources, and dark counts to mimic real systems. Compared to the standard bidimensional case, we show an improvement in the key generation rate.
Measurement-device-independent quantum key distribution (MDI-QKD) can remove all side-channel attacks on detectors. In the context of the dramatic progress of discrete-variable MDI-QKD and twin-field QKD, owing to the critical challenge of continuous-variable (CV) Bell-state measurement (BSM) of two remote independent quantum states, experimental demonstration of CV-MDI-QKD over optical fiber has remained elusive. To solve this problem, a technology for CV-BSM of remote independent quantum states is developed that consists of optical phase locking, phase estimation, real-time phase feedback, and quadrature remapping in the present work. With this technology, CV-BSM is accurately implemented, and the first CV-MDI-QKD over optical fiber is demonstrated, to our knowledge. The achieved secret key rates are 0.43 (0.19) bits per pulse over a 5-km (10-km) optical fiber. Our work shows that it is feasible to build a CV-MDI-QKD system over optical fiber. Further, the results pave the way towards realization of a high secret key rate and low-cost metropolitan MDI-QKD network, and serve as a stepping stone to a CV quantum repeater.
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 eliminate all detector side channels and it is practical with current technology. Previous implementations of MDI-QKD all used two symmetric channels with similar losses. However, the secret key rate is severely limited when different channels have different losses. Here we report the results of the first high-rate MDI-QKD experiment over asymmetric channels. By using the recent 7-intensity optimization approach, we demonstrate>10×higher key rate than the previous best-known protocols for MDI-QKD in the situation of large channel asymmetry, and extend the secure transmission distance by more than 20-50 km in standard telecom fiber. The results have moved MDI-QKD towards widespread applications in practical network settings, where the channel losses are asymmetric and user nodes could be dynamically added or deleted.
The ideal measurement-device-independent quantum key distribution (MDI-QKD), which is immune to all detector side-channel attacks, is built on the Bell state measurement of two single-photon states. However, in practical MDI-QKD where phase-randomized weak coherent pulses (PR-WCPs) are used, the mismatch between states preparation and measurement in X basis leads to the bit error rate of more than 25%, which poses a challenge to the security of QKD. In this paper, we provide a security analysis of practical MDI-QKD based on polarization coding. We analyze the Hong-Ou-Mandel interference of PR-WCPs and Poisson-distributed photon number states (PNs), and find that these two sources are equivalent in MDI-QKD. The security analysis based on entanglement distillation is carried out on PNs, and the measurement results in Z basis and X basis are averaged to overcome the noncorrelation of the error rates in these two bases. Compared with GLLP scheme, a tighter key rate is obtained in this work, and the key rate deviates from the linear key rate bound in a short distance due to the finite proportion of multiphoton terms.
We assess the overall performance of our quantum key distribution (QKD) system implementing the measurement-device-independent (MDI) protocol using components with varying capabilities such as different single-photon detectors and qubit preparation hardware. We experimentally show that superconducting nanowire single-photon detectors allow QKD over a channel featuring 60 dB loss, and QKD with more than 600 bits of secret key per second (not considering finite key effects) over a 16 dB loss channel. This corresponds to 300 and 80 km of standard telecommunication fiber, respectively. We also demonstrate that the integration of our QKD system into FPGA-based hardware (instead of state-of-the-art arbitrary waveform generators) does not impact on its performance. Our investigation allows us to acquire an improved understanding of the trade-offs between complexity, cost and system performance, which is required for future customization of MDI-QKD. Given that our system can be operated outside the laboratory over deployed fiber, we conclude that MDI-QKD is a promising approach to information-theoretic secure key distribution.