The implementation of polarisation encoded quantum key distribution in fibre.

Abstract

Quantum Key Distribution (QKD) employs the laws of quantum mechanics for the purpose of cryptography. Two parties, commonly called Alice and Bob, are able to share a random key which is used to encrypt a message. Any eavesdropper trying to intercept their key will have to make measurements, thereby disturbing the system. This can be detected by Alice and Bob and they will then discard their key. Polarisation encoded QKD protocols use the polarisation of single photons as qubits to generate a cryptographic key. This can be implemented using a fibre optic link between Alice and Bob but the polarisation of light is altered when passed through a fibre due to birefringence caused by asymmetries in the fibre. This causes refractive differences for orthogonal components of the state of polarisation of light, so the polarisation is rotated as the photon is transmitted through the fibre. If the fibre is fixed, the change of polarisation will be unique and constant. This can be compensated by rotating each photon appropriately to its original state. Under typical environmental conditions, such as temperature changes and vibrations, the birefringence effects vary and should be compensated in real time. Therefore, an active polarisation controller is needed in order to maintain the state of polarisation of each qubit. An investigation was done to first track how the state of polarisation changes over time in a natural environment. Both wavelength-division multiplexing and time-division multiplexing were investigated as testing methods for the compensation system. A time-division multiplexed system was developed to compensate the changes in polarisation. Since QKD protocols such as BB84 and B92 utilise two non-orthogonal bases, two polarisation controllers are usually used for compensation. However, by using a search algorithm, one polarisation controller was able to isolate the plane on the Poincaré sphere that passes through both bases, thus compensating non-orthogonal states with one device.