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Boffins brew 'Stop Light' that turns photons in fibre into memory

Faster-than-light is easy if you slow the photons down

By Richard Chirgwin, 12 May 2015

Sorry sci-fi fans, we're still not travelling even as far as Alpha Centauri. Rather, scientists at the Kastler Brossel Laboratory in Paris have pushed fibre-optic technology along by storing and re-emitting photons in a purely optical system.

In other words, the mechanism the boffins have demonstrated constitutes an all-fibre memory. If it can be taken out of the laboratory, that would mean an optical network could have buffers without converting photons to electricity and vice-versa.

As the researchers explain in their paper – accepted by Physical Review Letters and available as an Arxiv pre-print – prior work has demonstrated that photons can be coupled with atoms, but only with limited connectivity to fibre networks.

Their optical memory isn't something you'd buy off-the-shelf quite yet, because it's “based on the interaction of cold cesium atoms with the evanescent field surrounding an optical nanofibre”.

The light was propagating through an almost-ordinary glass fibre, the only modification being that the researchers heat-stretched the fibre slightly in the area they wanted the interaction to happen.

However, that interaction is enough to keep storage completely in the optical domain: the photon captured by the cesium cloud can then be released on demand.

A trick called electronically-induced transparency in the fibre allowed the photons to create a quantum superposition in the cesium atoms, in what's called a “collective excitation”.

As lead author of the paper, graduate student Baptiste Gouraud, says in the Kastler Brossel media release (at R&D Magazine here), “This work provides a demonstration of an all-fibered memory for light. We have been able to store the light and release it later into the fibre.

“Previous demonstrations were based on free-space ensembles of atoms, not on a wave-guided implementation compatible with fibres used in networks.”

Even down at the single photon level, the researchers claim a 20:1 signal-to-noise ratio, and the single photon can be stored for up to 5 microseconds – equivalent to travelling for about 1 km if the light hadn't been stopped along the way. ®

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