Quantum key distribution (QKD) constitutes a symmetric secret key negotiation protocol capable of maintaining information-theoretic security. Given the recent advances in QKD networks, they have evolved from academic research to some preliminary applications. A QKD network consists of two or more QKD nodes interconnected by optical fiber or free space links. The secret keys are negotiated between any pair of QKD nodes, and then they can be delivered to multiple users in various areas for ensuring long-term protection and forward secrecy. We commence by introducing the QKD basics, followed by reviewing the development of QKD networks and their implementation in practice. Subsequently, we describe the general QKD network architecture, its elements, as well as its interfaces and protocols. Next, we provide an in-depth overview of the associated physical layer and network layer solutions, followed by the standardization efforts as well as the application scenarios associated with QKD networks. Finally, we discuss the potential future research directions and provide design guidelines for QKD networks.

We study decoy-state measurement-device-independent quantum key distribution using heralded single-photon sources. This has the advantage that the observed error rate in the $X$ basis is at higher order and not so large. We calculate the key rate and transmission distance for two cases: one using only triggered events, and the other using both triggered and nontriggered events. We compare the key rates of various protocols and find that our scheme using triggered and nontriggered events can give a higher key rate and a longer secure distance. Moreover, we also show the different behavior of our scheme when using different heralded single-photon sources, i.e., in Poisson or thermal distributions. We demonstrate that the former can generate a higher secure key rate than the latter and can thus work more efficiently in practical quantum key distributions. DOI: http://dx.doi.org/10.1103/PhysRevA.88.052332 ©2013 American Physical Society

Quantum digital signature (QDS) can guarantee message integrity and non-repudiation with information-theoretical security, and it has attracted more attention recently. Since proposed by Andersson et al. [Phys. Rev. A 93, 032325 (2016) [CrossRef] ], a quantum digital signature protocol using an insecure channel has been realized with several different quantum key distribution (QKD) systems. Here we report an experimental QDS based on a BB84 QKD system. An asymmetric Faraday–Sagnac–Michelson interferometer structure has been designed in our system, which is intrinsically stable against channel disturbance. The innovatory structure supports the system to work at high speed and, in practice, the repetition rate is in gigahertz. A 0.044 bit/s signature rate has been attained with a 25 dB channel loss composed of a 25 km installed fiber with additional optical attenuation in a ${10}^{-10}$ security level. Thus, our QDS device is stable and highly efficient. This Letter provides a further step for the practical application of QDS.

Practical implementation of quantum key distributions (QKD) requires efficient, real-time feedback control to maintain system stability when facing disturbance from either external environment or imperfect internal components. Usually, a “scanning-and-transmitting” program is adopted to compensate physical parameter variations of devices, which can provide accurate compensation but may cost plenty of time in stopping and calibrating processes, resulting in reduced efficiency in key transmissions. Here, we propose to employ a well-known machine learning model, i.e., the long short-term memory network (LSTM), to predict those physical parameter variations in advance and actively perform real-time control on corresponding QKD devices. Experimentally, we take the phase-coding scheme as an example and run the LSTM-model-based QKD system for more than 10 days. Experimental results show that we can keep the same level of quantum-bit error rate as the traditional scanning-and-transmitting program by employing our machine learning method, but dramatically reducing the scanning time and resulting in significantly enhanced key transmission efficiency. Furthermore, our current machine learning model should also be applicable to other QKD systems using any coding scheme or QKD protocols and thus seems a very promising candidate in large-scale application of quantum communication networks in the near future.

Quantum digital signatures (QDSs) offer the information-theoretic security verified by quantum mechanics to send a message that is robust against forging and repudiation. We report a proof-of-principle demonstration of a passive decoy-state QDS system that can avoid leaking the modulation information of decoy states. To realize this, four local detection events are generated from the idler light of a parametric down-conversion process, and are then used as decoy states to estimate channel parameters more accurately. As a result, we successfully sign a more-than-one-bit message within 7 s at 100 km. Besides, we achieve a transmission distance of 200 km, which represents a further step toward the practical application of QDSs.

In this paper, we present schemes on implementing the single-photon-added coherent source in quantum key distributions. We apply this source in either the standard BB84 protocol or the recently proposed measurement-device-independent quantum key distribution, comparing their performance with the cases of using other existing sources, e.g., the weak coherent source or the heralded single-photon source, and giving out corresponding numerical simulations. Our simulation results show that in both protocols the single-photon-added coherent source can greatly exceed all other existing sources. Moreover, even when taking statistical fluctuation into account, a high key generation rate can still be achieved at long distance by applying this source.