Quantum entanglement is essential to the development of quantum computation, communications, and technology. The controlled SWAP test, widely used for state comparison, can be adapted to an efficient and useful test for entanglement of a pure state. Here we show that the test can evidence the presence of entanglement (and further, genuine n-qubit entanglement), can distinguish entanglement classes, and that the concurrence of a two-qubit state is related to the test’s output probabilities. We also propose a multipartite measure of entanglement that acts similarly for n-qubit states. The average number of copies of the test state required to detect entanglement decreases for larger systems, to four on average for many (n ≳ 8) qubits for maximally entangled states. For non-maximally entangled states, the number of copies required to detect entanglement increases with decreasing entanglement. Furthermore, the results are robust to second order when typical small errors are introduced to the state under investigation.

A recently published patent ( https://www.ipo.gov.uk/p-ipsum/Case/PublicationNumber/GB2590064 ) has claimed the development of a novel quantum key distribution protocol purporting to achieve long-range quantum security without trusted nodes and without use of quantum repeaters. Here we present a straightforward analysis of this claim, and reach the conclusion that it is largely unfounded.

Quantum key distribution (QKD) can ensure information security between two remote parties. To commercialise QKD technology successfully, it should be integrated with dense wavelength division multiplexing optical transport. However, various challenges limit the QKD's performance in terms of the quantum key rate, quantum bit error rates, and maximum achievable distance. In this study, the authors discuss some of the major practical limiting factors for QKD performance such as spontaneous Raman scattering, four‐wave mixing, and amplified spontaneous emission.

We show how to freeze distributed entanglement that has been created from the natural dynamics of spin chain systems. The technique that we propose simply requires single-qubit operations and isolates the entanglement in specific qubits at the ends of branches. Such frozen entanglement provides a useful resource, for example for teleportation or distributed quantum processing. The scheme can be applied to a wide range of systems—including actual spin systems and alternative qubit embodiments in strings of quantum dots, molecules, or atoms.

As noisy intermediate‐scale quantum (NISQ) devices grow in number of qubits, determining good or even adequate parameter configurations for a given application, or for device calibration, becomes a cumbersome task. An evolutionary algorithm is presented here which allows for the automatic tuning of the parameters of any arrangement of coupled qubits, to perform a given task with high fidelity. The algorithm's use is exemplified with the generation of schemes for the distribution of quantum states and the design of multi‐qubit gates. The algorithm is demonstrated to converge very rapidly, yielding unforeseeable designs of quantum devices that perform their required tasks with excellent fidelities. Given these promising results, practical scalability, and application versatility, the approach has the potential to become a powerful technique to aid the design and calibration of NISQ devices.

To analyse and evaluate the physical layer performance of a UK national quantum network (UKQNtel) utilising COW-QKD, in O band, integrated with 500 Gb/s encrypted data, in C band, over 121 km between BT Labs and the University of Cambridge, a theoretical model encompassing COW-QKD and Raman scattering noise is developed and fitted with real-world experimental data. Different physical mechanisms underpinning the link performance are identified and discussed. The model developed in this work also provides a preliminary technical guidance for future UKQNtel upgrade and expansion, as well as potentially useful insights for quantum-secured networks design prior to practical deployment.