Quantum encryption systems, which encode signals into a series of single photons, have had trouble sending them along optical fibers. Until now, it has been unfeasible to piggyback these photons onto existing telecommunications lines. Now using a technique for detecting dim light signals, physicists have enabled the transmission of a quantum key along 90 km of noisy optical fiber.
The scientists published their findings in the journal Physics Review X. Finally quantum cryptography may enter mainstream technology.
Quantum systems cannot be measured without being noticeably disrupted. People can encode encryption keys into a series of photons and share it, knowing that any eavesdropper will trip the system’s alarms. However, such systems haven’t been able to transmit keys along telecommunication lines, because other data traffic swamps the encoded signal. As a result, quantum cryptography has had only some niche applications, like connecting offices to nearby back-up sites using expensive dark fibers that carry no other signals.
Physicists have tried to solve the problem by sending photons along a shared fiber on a quantum channel, at a characteristic wavelength. The trouble is that the fiber scatters light from the normal data into that wavelength, polluting the quantum channel with stray photons. Andrew Shields, a researcher at the Toshiba Cambridge Research Laboratory, UK, and his colleagues have developed a detector that picks out photons from this channel only if they strike it at a precise instant, calculated on the basis of when the encoded photons were sent.
Designing such a detector was difficult. Standard detectors use semi-conducting devices that create an avalanche of electrical charges when struck by a single photon. It takes more than one nanosecond for the avalanche to grow large enough to stand out against the detector’s internal electrical hiss, which is much longer than the window of 100 picoseconds needed to filter out a single photon.
The detector activates for 100 picoseconds, every nanosecond. The detector measures the difference between the signal recorded during one operational cycle and the signal from the preceding cycle, when no matching photo was likely detected. This cancels out the background hum and has enabled the team to transmit a quantum key along a 90-kilometer fiber, which carried noisy data at 1 billion bits per second in both directions. The team’s goal is to try this technique along a real telecommunications line.
There is a hypothesis that quantum signals cannot be transmitted beyond the range of a large city of 100 kilometers, since scattering accumulates over distance. It would be impossible to filter them all out, even with a precisely timed detector.