Quantum 'alphabet' easier to read with laser boost

Jacob Aron, reporter

(Image: M. Bellini/National Inst. of Optics)

Secret agents take note: a new technique for measuring the shapes of individual photons could improve encrypted communications by dramatically boosting the light-particle "alphabet".

The method would expand our ability to make computers more powerful or send covert messages. And as with any good spy gadget, the key is to use lasers.

A photon doesn't have shape in the traditional sense. Instead its shape is based on the probability of detecting the photon at a particular place and time, depending on its amplitude and frequency.

Current quantum communication schemes can reliably send and receive photons that have been sculpted into just two variants, determined by the polarisation of the photon ? either horizontal or vertical.

It's possible to create photons in almost any shape, such as forging one to represent each letter in the alphabet. But more complex configurations become distorted when travelling through an optical fibre, making them harder to understand on the receiving end.

Now Marco Bellini and colleagues at the National Institute of Optics in Florence, Italy, have found a way to cut through the photon fuzz. It works by mixing the photon with a laser pulse that either strengthens or cancels out the signal. The closer the laser's shape matches that of the photon, the more likely it is that the photon will be detected.

The researchers used a genetic algorithm, inspired by evolution, to tweak the parameters of the laser pulse so that it better matched the shape of the photon, repeating the process until the laser "adapted" to the photon and the particle's full shape became legible.

They tested the technique's potential for transmitting information by producing photons that were shaped according to two frequency components with a particular phase difference.

The photons were detected by a laser that had a matching phase difference, but almost none were detected when the laser had the opposite phase. This means the two phases could be used to accurately send two different pieces of information.

Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.109.053602

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