The researchers developed a detector that picks out photons only if they strike it at a precise instant, calculated on the basis of when the encoded photons were sent.
The team’s ‘self-differentiating’ detector activates for 100 picoseconds, every nanosecond. The weak charge triggered by a photon strike in this short interval would not normally stand out, but the detector measures the difference between the signal recorded during one operational cycle and the signal from the preceding cycle — when no matching photon was likely to be detected. This cancels out the background hum. Using this device, the team has transmitted a quantum key along a 90-kilometre fiber, which also carried noisy data at 1 billion bits per second in both directions — a rate typical of a telecommunications fiber."