Sixteen shades of grey survive Viennese quantum optics test

'Twisted light' test transcends turbulence, suggesting secure comms tool

Grey-scale Motzart transmitted by twisted light

Video Viennese researchers have taken another important step in the development of communications systems using optical angular momentum (OAM), a.k.a. “twisted light”, as the basis of the modulation.

What's different in the latest work is that the group, from the University of Vienna and the Institute for Quantum Optics and Quantum Communication, managed to maintain a relatively low error rate over a distance of three kilometres, in spite of turbulence.

The work is published in the New Journal of Physics (abstract here, article in full here).

OAM is a handy technique in both quantum and classical optical communications, since it offers many more possible encoding states than polarisation. However, in lab experiments, it's proven highly susceptible to turbulence.

The Viennese researchers say they focussed on developing neural network techniques to remove the effects of turbulence, allowing them to detect the 16 angular states of light they used to modulate their grey-scale images of Mozart, Ludwig Boltzmann and Erwin Schrödinger.

As the paper notes, quantum key distribution is one possible application of the technology, since OAM encoding has “the potential to provide high key-generation rates and a higher tolerance to eavesdropping” than systems using polarisation alone.

Getting OAM-based communications to work in free-space transmissions avoids some of the problems encountered in trying to preserve OAM information in optical fibres, the paper says.

The test in Vienna yielded an error rate of 1.7 per cent, which is high for a practical communications medium but a useful starting point for an emerging technology. ®

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