A Quantum Key Distribution Terminal with Torsion-Phase Stabilisation

Quantum key distribution turns the disturbance of quantum states into an alarm: any eavesdropping on the quantum channel raises the error rate, and the secret key rate falls as errors rise. This paper designs a decoy-state BB84 terminal for a 20 km fibre link and computes the value of stabilising the encoding interferometer against a torsion phase reference. Stabilisation that reduces the quantum bit error rate from 3.0 percent to 1.5 percent raises the asymptotic secret key rate by a factor of 1.27, to 47046 kbit per second at a 1 GHz clock. The calculation uses the standard decoy-state rate formula with the stated channel and detector parameters. The terminal is a design study; no key has been generated.