We investigate the use of quantum scissors, as candidates for non-deterministic amplifiers, in continuous-variable quantum key distribution. Such devices rely on single-photon sources for their operation and as such, they do not necessarily preserve the Guassianity of the channel. Using exact analytical modeling for the system components, we bound the secret key generation rate for a protocol that uses quantum scissors. We find that, for certain non-zero values of excess noise, such a protocol can reach longer distances than the counterpart with no amplification. This sheds light into the prospect of using quantum scissors as an ingredient in continuous-variable quantum repeaters.

A typical implementation of BB84 protocol for quantum communication uses four laser diodes for transmitting weak coherent pulses, which may not have the same characteristics. We have characterized these lasers for mismatch in various parameters such as spectral width, pulse width, spatial mode, peak wavelength, polarization and their arrival times at the receiver. This information is utilized to calculate possible information leakage through side channel attacks by evaluating mutual information between source and eavesdropper. Based on our experimental observations of cross correlation between parameter values for different laser diodes, we suggest methods to reduce information leakage to Eve.

This paper reports a practical quantum-safe metro network, integrating optically-switched QKD systems with high speed reconfigurablility to protect classical network traffic. Quantum signals are routed by millisecond optical switches and secure keys are shared between any two endpoints or network nodes via low-latency reconfigurable connections. Efficient quantum encryption topologies between different end-users are also presented. We show experimentally the feasibility of a rapidly reconfigured QKD transmission system between multiple users in the proposed network. Classical data and control signals coexist with the quantum signals in the same fibre. Proof-ofconcept experiments are conducted over effective transmission distances of 30km, 31.7km, 33.1km and 44.6km. Software controlled QKD transmission is established between four different transmitters (Alice) and one receiver (Bob) with a switching time of a few milliseconds. The quantum bit error rates (QBER) for the four paths are proportional to the channel losses with values between 2.6% and 4.1%. Optimization of the reconciliation and clock distribution architecture is predicted to result in a maximum key generation delay of 20s, far shorter than previously demonstrated.

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.

Advances in highly sensitive detection techniques for classical coherent communication systems have reduced the received signal power requirements to a few photons per bit. At this level one can take advantage of the quantum noise to create secure communication, using continuous variable quantum key distribution (CV-QKD). In this work therefore we embed CV-QKD signals within classical signals and transmit classical data and secure keys simultaneously over 25 km of optical fibre. This is achieved by using a novel coherent displacement state generator, which has the potential for being used in a wide range of quantum optical experiments. This approach removes the need for separate channels for quantum communication systems and allows reduced system bandwidth for a given communications specification. This demonstration therefore demonstrates a way of implementing direct quantum physical layer security within a conventional classical communications system, offering a major advance in term of practical and low cost implementation.