Short light

They're short, lasting just 65 femtoseconds, but the light pulses produced by Oxford scientists could be very important for quantum computing.

Why? Well that's almost fifty times shorter than any single photon previously produced and, in a quantum information device based on light (where photonic logic gates replace electronic ones), having a stream of identical, high-quality single photons on tap is vital.

Peter Mosley of Oxford's Ultrafast Group, co-author of a Physical Review Letters paper on the research, explained: ’It is possible to make photons in pairs by sending laser light through special crystals. When a pair has been created, the detection of one of these photons heralds the presence of its twin. However these twin photons are entangled, meaning that the properties of one photon are inextricably linked to those of its partner and detecting one can ruin the quantum state of the other.'

‘Our technique minimises the effects of this entanglement, enabling us to prepare single photons that are extremely consistent and, to our knowledge, have the shortest duration of any photon ever generated. Not only is this a fascinating insight into fundamental physics but the precise timing and consistent attributes of these photons also makes them perfect for building photonic quantum logic gates and conducting experiments requiring large numbers of single photons.'

In the Oxford experiment the pairs of photons made had a central wavelength of about 830 nm, at the border between visible and near-infrared light. Each of these photons was about 65 femtoseconds (65 millionths of a billionth of a second) long. In units of space, they were about 20 microns long. The shortest previously produced single photon was about 1 picosecond long.

'Creating single photons even under controlled conditions is extremely challenging,' said Peter. 'Even the purest laser light beam consists of many photons all bunched together. Our approach enables us to generate individual photon replicas, identical packets of light of very short duration that are ideal for quantum computing.'

Peter Mosley is a member of the Ultrafast Group, part of Oxford University's Department of Physics

Top image: Set-up used to create very short light pulses. Above: Diagram showing theory behind 'short light' generation.

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