Quantum cryptography, or more precisely quantum key distribution (QKD) [1], provides a secure way to distribute cryptographic keys on fibre optic networks, the secrecy of which can be guaranteed by the laws of quantum mechanics. Most QKD systems that have been realized so far have encoded the bit information as a phase delay in an interferometer [2–10]. This is largely due to the availability of high-quality telecom phase modulator, as well as the fact that phase-encoded cubits are relatively resilient to de coherence in optical fibres [11]. Despite this, stabilising the interferometer path lengths to within the wavelength of the photons is a major technical problem, currently limiting the usefulness of many applications.