Mohsen Razavi

The security of prepare-and-measure satellite-based quantum key distribution (QKD), under restricted eavesdropping scenarios, is addressed. We particularly consider cases where the eavesdropper, Eve, has limited access to the transmitted signal by Alice, and/or Bob’s receiver station. For instance, Eve can only receive an attenuated version of the transmitted signals. This results in settings where an uncharacterized bypass channel, inaccessible to Eve, can also carry signals to Bob. We obtain generic bounds on the key rate in the presence of bypass channels and apply them to continuous-variable QKD protocols with Gaussian encoding as well as to the family of BB84 protocols. We find regimes of operation in which the above restrictions on Eve can considerably improve system performance. Our work opens up new security frameworks for spaceborne quantum communications systems.

Most current proposals for entanglement distribution networks assume a connection-oriented approach, where resources along a path may be reserved before the start of the session. This strategy, however, does not match the common practice in the existing infrastructure for the Internet, which relies on connectionless packet switching. In our work, we study how a hop-by-hop teleportation can be used to perform entanglement distribution across a network without any prior resource reservation. Specifically, we investigate the attainable secret key generation rate between two users employing this protocol in a repeater chain setup. We analyze this scenario for deterministic quantum repeaters with and without encoding, where we consider a three-qubit repetition code for error detection in the former case. Typical models for the operational errors in these protocols are considered. Our results suggest that the usage of quantum error detection schemes will enable trust-free secret key distribution at distances of interest.

We propose an all-photonic protocol for distributing multipartite entangled states in quantum networks, extending the two-party quantum repeater scheme of Azuma et al. (2015) to the multipartite regime. By introducing a minimal change to the measurement pattern at user nodes, our method achieves GHZ state distribution among multiple users without the need for quantum memories.This approach maintains the core structure of the original protocol, demonstrating that scalable, memory-free entanglement distribution is achievable using only photonic resources and measurement-based operations.

We consider discrete phase randomisation (DPR) for several quantum key distribution (QKD) protocols. Full continuous phase randomisation of weak laser pulses (WCPs) would create an output state that is diagonal in Fock basis. This will simplify the security proof of QKD protocols that rely on WCPs or decoy states. In practice, however, such an ideal phase randomisation may not be achievable. Instead, we may actively choose a discrete number of global phase values for our WCPs. The security proof with DPR has been reported for several QKD protocols, which often requires numerical optimisation. In this work, we develop analytical bounds on the secret key generation rate for BB84 and measurement-device-independent (MDI) QKD protocols with DPR. These analytical bounds closely match the numerical results. We then extend our results to the newly proposed mode-pairing (MP) QKD protocols, which offer favourable rate-versus-distance scaling, with DPR. We show that the number of phase slices needed for MP-QKD to approach the ideal case is larger than that of MDI-QKD.

Quantum data centres (QDCs) are a promising way of scaling up quantum computers. In a single-processor quantum computer, the number of high-quality computational qubits is limited by cross talk and difficulties in addressing individual qubits when many qubits share the same housing. QDCs circumvent these challenges by linking together multiple small quantum processing units (QPUs) over short distances. With this architecture, the intra-QPU noise is kept small, but additional noise is introduced due to the latency of and imperfections in the inter-QPU links. Understanding the trade-offs between these different types of noise is essential for guiding future efforts in QDC manufacture and compilation. We develop and use a classical simulator to emulate the execution of different quantum circuits on an imperfect QDC. An individual inter-QPU CNOT gate is first-considered and then a selection of larger circuits are investigated. In both cases, we implement inter-QPU gates using cat-comm and three variants of TP-comm, which we call 1TP-comm, 2TP-comm and TP-safe, respectively. We find that 1TP-comm and cat-comm yield different output fidelities despite both schemes having the same number of gates, measurements and inter-QPU entanglements. We also determine the relative impacts of entanglement error, intra-QPU gate error and memory depolarisation.

Quantum networks use platforms like quantum repeaters to enable quantum communications at arbitrary distances. Early quantum networks are expected to be deployed on pre-existing infrastructure, sharing resources with classical networks, and thus can benefit from compatible design principles and behaviours. In particular, most of the research on quantum repeater networks based on entanglement distribution assume that some connection is established in order to reserve resources for the distribution attempt. However, the most prominent classical network, the Internet, is built upon the concept of connectionless communications, and therefore this approach might not be ideal for the early stages of deployment. In our work, we investigate the performance of connectionless protocols over quantum networks. To do so, we consider both unencoded (standard) but deterministic repeaters, and repeaters using a 3-quit repetition code. We analyse the achievable secret key rates for both of these setups in different regimes of errors. Our results suggest that error correction techniques such as the usage of encoded quantum repeaters will be crucial to the success of early quantum networks.

We study the implementation of quantum-key-distribution (QKD) systems over quantum-repeater infrastructures. We particularly consider quantum repeaters with encoding and compare them with probabilistic quantum repeaters. To that end, we propose two decoder structures for encoded repeaters that not only improve system performance but also make the implementation aspects easier by removing two-qubit gates from the QKD decoder. By developing several scalable numerical and analytical techniques, we then identify the resilience of the setup to various sources of error in gates, measurement modules, and initialization of the setup. We apply our techniques to three- and five-qubit repetition codes and obtain the normalized secret key generation rate per memory per second for encoded and probabilistic quantum repeaters. We quantify the regimes of operation, where one class of repeater outperforms the other, and find that there are feasible regimes of operation where encoded repeaters—based on simple three-qubit repetition codes—could offer practical advantages.

Twin-field (TF) quantum key distribution (QKD) can overcome fundamental secret-key-rate bounds on point-to-point QKD links, allowing us to reach longer distances than ever before. Since its introduction, several practical TF-QKD variants have been proposed, and some of them have already been implemented experimentally. All of them assume that the users can emit weak coherent pulses with a continuous random phase, either to generate the key, or to prove its security. In practice, this assumption is often not satisfied, which could open up security loopholes in their implementations. Here, we propose and prove the security of a TF-QKD variant that relies exclusively on discrete phase randomisation, which is easier to achieve in practice. Remarkably, our results show that it can also provide higher secret-key rates than other variants.

Quantum memories are used to improve the ate-versus-distance behavior in measurement-device-independent quantum key distribution (MDI-QKD) systems. The required specifications in terms of reading and writing times for such memories are obtained. It is shown that the faster the access times are, the higher the repetition rates and the lower the required coherence times would be. Additionally this protocol offers an immense security by removing side-attack channels over protocols such as the standard decoy-state BB84 protocol. A comparison of this protocol with the original MDI-QKD is given in terms of secret key generation rate under practical assumptions. Various sources of imperfection such as reading and writing efficiencies of the memories, channel and detector efficiencies and dark count rates are considered. The crossover distance is determined after which the present protocol outperforms the MDI-QKD and a typical quantum repeater network.

Measurement-independent quantum key distribution (MDI-QKD) over probabilistic quantum repeaters (QRs) is addressed. We calculate, under practical assumptions, the secret key generation rate, as the main figure of merit, to estimate the performance of such protocols. First, we consider an MDI-QKD phase encoding scheme having a coherent state as the source on the one side and an imperfect single-photon source, with a nonzero double photon probability, on the other side. We compare this system with the case when both parties have an imperfect single-photon source. For this system we calculate the key rate versus the distance. Then, we combine MDI-QKD and QRs protocols, by introducing quantum memories (QMs) in the original MDI-QKD scheme. We, first, generate entangled states between the QMs using the protocol proposed by Sangouard et. al. in [Phys. Rev. A 76, 050301 (2007)]. We calculate the key rate of such a protocol versus the distance up to two nesting levels. We consider various sources of imperfection in both protocols, such as dark counts in detectors, and inefficiencies in the channel, photodetectors and memories.

We develop a new formalism to investigate the effects of gravity and curvature on quantum communication protocols, in particular focusing on quantum key distribution setups. We are able to show that the effects can play a role, in particular when the aim is to establish entanglement to be used as a resource for quantum information processing between two distant sources in the gravitational field. Our results indicate that the spacetime acts as a channel and we provide a systematic way to understand how such channel affects communication. We study the impact on current and future planned quantum commuincation networks.

Three network architectures, compatible with passive optical networks, for future hybrid quantum-classical networks are proposed and compared. These setups rely on three different schemes for quantum key distribution (QKD): BB84, entanglement-based QKD, and measurement-device-independent QKD (MDI-QKD). It turns out that, while for small-to-moderate-size networks BB84 supports the highest secret key generation rate, it may fail to support large numbers of users. Its cost implications are also expected to be higher than other setups. For large networks, MDI-QKD offers the highest key rate if fast single-photon detectors are employed. Entanglement-based networks offer the longest security distance among the three setups. MDI-QKD is, however, the only architecture resilient to detection loopholes and possibly the most favorable with its less demanding end-user technology.Entanglement-based and MDI-QKD setups can both be combined with quantum repeater systems to allow for long-distance QKD with no trust constraints on the service provider.