A bird’s eye view of an entangled photon-pair source built by researchers at the National University of Singapore and Singtel

A laser (grey and black box, bottom left) emits light that is converted into pairs of entangled photons suitable for generating a quantum key. Credit: National University of Singapore

Quantum physics

How to ensure a quantum key arrives on time

Adjustment to photons’ wavelengths could make their transmission through conventional fibre-optic cables more reliable.

Quantum encryption could become more efficient thanks to a method for better coordinating the photons at the heart of the process.

Streams of ‘entangled’ photons — pairs of intrinsically linked light particles — can generate tamper-proof cryptographic keys. To produce these keys, each of the photons in an entangled pair is sent through a fibre-optic cable to a separate operator. Each photon’s arrival is synchronized with its partner’s, allowing operators to match up the two members of a pair. But errors can occur because the photons’ arrival times become distorted by the particles spreading out as they travel through the cable.

James Grieve and his colleagues at the National University of Singapore managed to cancel out this effect by using photons at particular wavelengths: those that sit either side of an ideal wavelength at which no spreading occurs. This fix kept the entangled pairs in sync.

The team found that the technique improved photons’ arrival timing even when the particles travelled through a 10-kilometre-long commercial network made up of fibres with varying characteristics. The technique could be used to improve cryptography across distances on the scale of cities, and to synchronize clocks in financial trading, say the authors.