  Quantum Key Distribution (QKD) using polarisation encoding can be hard to
implement over deployed telecom fibres because the routing geometry and the
birefringence of the fibre link can alter the polarisation states of the
propagating photons. These alterations cause a basis mismatch, leading to an
increased Quantum Bit Error Rate (QBER). In this work we demonstrate a
technique for dynamically compensating fibre-induced state alteration in a QKD
system over deployed fibre. This compensation scheme includes a feedback loop
that minimizes the QBER using a stochastic optimization algorithm.
