Entanglement-Based QKD Could Secure Optical Fiber Networks

A technique to help pairs of light particles smoothly navigate optical fiber networks was performed over 10 km of Singtel’s fiber network. Researchers from the National University of Singapore (NUS) and Singtel are driving the project, which could strengthen cybersecurity for networks that carry data encoded in pulses of light.

A device developed in the NUS-Singtel cybersecurity R&D lab that creates particles of light that are connected by the quantum property of entanglement. Courtesy of the National University of Singapore.

The new technique deploys quantum key distribution (QKD), a technology that detects individual photons to create encryption keys for secure quantum communication. The QKD trials being carried out by the NUS-Singtel team use pairs of photons that are connected through entanglement.

Most QKD schemes require that the sender and receiver of a secret message exchange photons directly or trust the source of their keys. Using the team’s approach, it is possible to check the security of a key provided by a third-party supplier. The supplier would create a pair of photons, then split them up, sending one each to the two parties that want to communicate securely. The photons would be entangled, so that when the parties measured their photons, they would get matching results, either a 0 or 1. Doing this for many photons would leave each party with identical patterns of 0s and 1s, giving both parties a key to lock and unlock a message.

Typically, each photon traveling an optical network encounters a different “obstacle course” of spliced fiber segments and junction boxes. Along their paths, the photons also undergo dispersion and spread out. This affects the operators’ ability to track the photons.

The new approach from NUS and Singtel would keep the entangled photons in sync as they traveled different paths through the network. This is important, said the researchers, because the photons are identified by the gaps between their arrival times at the detector. “Timing information is what allows us to link pairs of detection events together. Preserving this correlation will help us to create encryption keys faster,” said researcher James Grieve.

Senior research fellow James Grieve of the Centre for Quantum Technologies at NUS and Amelia Tan, senior R&D researcher of Trustwave, Singtel’s cybersecurity subsidiary. Courtesy of the National University of Singapore.

The photon source is designed to create pairs of light particles with colors that are either side of the zero-dispersion wavelength of the optical fiber. Normally, in optical fibers, bluer light would arrive faster than redder light, spreading out the photons’ arrival times. Working around the zero-dispersion point makes it possible to match the speeds through the photons’ time-energy entanglement, and the timing is preserved.

Professor Alexander Ling said, “Before these results, it was not known if the multisegment nature of deployed fiber would enable high-precision dispersion cancellation, because the segments don’t generally have identical zero dispersion wavelengths.”

In showing that its scheme can work, the team reinforces expectations for the potential use of QKD over commercial fiber. The entangled photons could find other applications, as well, said the researchers. For example, the photons in each pair are created within femtoseconds of each other. Their coordinated arrival times could be used to synchronize clocks for time-critical operations such as financial trading.

The research was published in Applied Physics Letters (https://aip.scitation.org/doi/10.1063/1.5088830).