Boffins twist light to carry 2.05 bits in one photon

2.05? You read it right: this is a new quantum crypto booster

Orbital angular momentum – in photonics short-handed as “twisted light” – can help expand the capacity of comms systems using quantum communications.

Since quantum key distribution (QKD) is already a commercial, if pricey, market segment, quantum communication science is of a more practical application today than quantum computing – but it's a relatively slow communications channel.

The latest work, announced by the University of Rochester, more than doubles the number of bits per photon from one bit-per-photon in systems using only polarisation.

To get to the 2.05 bits per photon claimed, the university's Mohammad Mirhosseini and his colleagues encoded seven symbols onto a combination of the orbital angular momentum (“twisted” light) and their angular position.

As the release puts it, the two properties form the “mutually unbiased bases” that are necessary for QKD.

Once Alice and Bob have created their key, they announce the basis – or alphabet – they're using for each symbol in the key, allowing them to compare the alphabet used to send and receive. The two ends only retain the part of the key in which they used the same alphabet.

The Rochester experimenters say they achieved a 4kHz communication channel with 93 per cent accuracy – although over the long term, they hope to reach gigahertz transmission rates.

A more immediate aim, they say, is to drive up the bits-per-photon rate to 4.17.

The work has been published in the New Journal of Physics ®