American researchers have managed to transfer quantum information carried in light particles over a 100km distance, four times the previous record.
The experiment by a team from the National Institute of Standards and Technology (NIST) is the first to prove that quantum information can be transferred over large distances in optical fibres. Previous experiments only achieved this in free space.
The team was only able to carry out the experiment thanks to some new, extremely sensitive photon detectors that they built.
"Only about one per cent of photons make it all the way through 100km of fibre," said NIST researcher Marty Stevens. "We never could have done this experiment without these new detectors, which can measure this incredibly weak signal."
Quantum teleportation involves the transfer, or remote reconstruction, of information encoded in quantum states of matter or light. It can be used in quantum communications and quantum computing and could enable unbreakable encryption in future.
The new record, described in the journal Optica, represents a major breakthrough. Previously, it has only been possible to transfer quantum data in optical fibres over very short distances with most of the information getting lost along the way.
The new technique, the researchers believe, could be used to develop the so-called quantum repeaters that would resend data periodically in order to extend the network’s reach.
Previously, researchers thought quantum repeaters might need to rely on atoms or other matter, instead of light - a difficult engineering challenge that would also slow down transmission.
In the NIST experiment, quantum states were used to indicate when in a sequence of time slots a single photon arrives. The teleportation method is novel in that four of NIST's photon detectors were positioned to filter out specific quantum states.
The detectors rely on superconducting nanowires made of molybdenum silicide and are able to detect more than 80 per cent of the arriving photons, revealing whether they are in the same or different time slots, each just one nanosecond long. The experiments were performed at wavelengths commonly used in telecommunications.