The work was done by researchers in the US, based at the Los Alamos National Laboratory (LANL) in New Mexico and the National Institute of Standards and Technology (NIST) in Boulder, Colorado (New Journal of Physics 8 193). The previous record of 122 km was set by researchers at Toshiba’s Cambridge Research Laboratory in April last year.

Quantum key distribution (QKD) allows two users (often posited as ‘Alice’ and ‘Bob’) to share a random ‘key’, which they can then use to encrypt and transmit information securely. This information is sent in the form of photons and an eavesdropper (‘Eve’) is unable to spy on the communication without disturbing the transmission and revealing her actions. The technique offers a ‘holy grail’ of completely secure communications guaranteed by the laws of quantum physics.

“We have used ultra-low-noise transition-edge sensors to create a distributed key that is secure against standard attacks over 184.6 km,” Danna Rosenberg, one of the researchers at LANL told “Demonstrating longer distances is important in terms of the separation of base stations, which may be needed to boost signals and enable users who are far away to communicate securely with each other.”

According to the researchers, the key technology that enabled the researchers to break the record was NIST’s Transition-Edge Sensor (TES). The TES, also used in astrophysics to detect faint light from stars, detects 65% of received photons, compared to 20% achieved by conventional commercial photodiodes. This means the researchers could detect single photons at a high efficiency and with a zero dark count (signal that appears even when there is no light incident on detector). “When the TES detectors are used for quantum key distribution, it results in more secret bits at longer ranges than conventional detectors,” said Rosenberg.

However, in QKD distribution, success is not only measured by transmission distance, but also security. The LANL/NIST 184.6 km record was set at a higher average number of photons per pulse than the previous record of 122 km. This leads to an increased probability of a laser pulse containing more than one photon; when this happens an eavesdropper hypothetically has a better chance of intercepting a duplicate photon in a pulse without being detected. This is known as a photon-number-splitting (PNS) attack.

Nevertheless, the LANL/NIST team achieved 148.7 km at the same average photon number as the Cambridge group. In addition, the LANL/NIST team generated a key that is completely secure against such PNS attacks over 67.5 km, beating the previous record of 50.6 km.

The research group’s next target is to exploit the recent development of decoy state QKD, which involves varying the intensity of the transmitted photons to create ‘decoys’ that will reveal any attempt by Eve to intercept the transmission and protect the key from PNS attacks. “Our next series of experiments will explore decoy-level quantum key distribution and the increases in security and range enabled by the use of decoy levels combined with the transition-edge sensors,” added Rosenberg.