Lewis Wooltorton

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.

Nonlocal tests on multipartite quantum correlations can certify randomness in a device-independent (DI) way. Such correlations admit a rich structure, making the task of choosing an appropriate witness, known as a Bell inequality, difficult. For example, extremal Bell inequalities are tight witnesses of nonlocality, however achieving their maximum violation places constraints on the underlying quantum system, which are often incompatible with optimal randomness generation. As a result we find a trade-off between maximum randomness and Bell violation. Understanding this trade-off for more than two parties has not been explored, and would inform the best way to generate DI randomness in this setting. Moreover, suitable techniques that enable maximum randomness certification for arbitrarily many parties are missing. Here, we study the maximum amount of randomness that can be certified by correlations exhibiting a violation of the Mermin-Ardehali-Belinskii-Klyshko (MABK) inequality. We find that maximum quantum violation and maximum randomness are incompatible for any even number of parties, with incompatibility diminishing as the number of parties grow, and conjecture the precise trade-off. We also show that maximum MABK violation is not necessary for maximum randomness for odd numbers of parties. To obtain our results, we derive new families of Bell inequalities certifying maximum randomness from a new technique for randomness certification, which we call "expanding Bell inequalities". Our technique allows one to take a bipartite Bell expression, known as the seed, and transform it into a multipartite Bell inequality tailored for randomness certification, showing how intuition learned in the bipartite case can find use in more complex scenarios.

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.