In fact just this kind of photon-based quantum factoring has been accomplished before, but the ability to put the heart of the machine on a standard chip is promising for future applications of the idea.

BBC: Quantum computer slips onto chips

Such a photon counter is essential if quantum cryptography is to work, because it will allow what are known as quantum repeaters to be built.

ECONOMIST: Quantum information technology

Dr Shields's photon detector, however, permits cryptographers to use a phenomenon called quantum entanglement to make a repeater that does not destroy quantum states.

ECONOMIST: Quantum information technology

But to exploit the fullest security allowed by the peculiar rules of quantum mechanics, each "bit" of information in the key should be encoded onto exactly one photon, sent through air or down a fibre.

BBC: Quantum key distribution with single photons