Photonic systems metrology for quantum communications hardware

Quantum key distribution (QKD) is a method of sharing encryption keys between parties
for the purposes of secure information transfer via public communication channels. QKD
systems achieve this by exchanging single photons encoded with the key bit information,
where the quantum properties of the optical signals ensures the secrecy of the final key.
This is superior to classical key distribution schemes and post-quantum cryptography, as
the protocols are not at risk from code-breaking algorithms and advanced computational
methods. However, the security of QKD protocols does rely upon the operation and
accurate modelling of the hardware used in these systems, to generate and detect the
single-photon level signal. To ensure the security of any shared key, the QKD devices must
be characterised, where measurements are performed to test and assess the properties of
the hardware. Many QKD protocols place requirements on the phase of the optical signals,
both for encoding and security purposes.
This work explores the design, construction and initial implementation testing of a
measurement system that can be used to characterise the phase properties of QKD transmitter
and receiver modules. The system is designed so that it can be used to perform
non-intrusive assessments of QKD hardware by sending and measuring optical signals, reducing
the risk of any disruption to the device and its performance, which is particularly
critical when assessing commercial devices. It is plug-and-play by design, so that it can
be easily applied to the assessment of many different devices. It is also portable, so that
it can be used to assess QKD hardware in situ.
This work constitutes a step towards providing phase characterisation measurements
for QKD hardware, which is necessary to ensure the security of the keys generated via
QKD protocols.