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Boffins sling around entangled photons at telco wavelengths

1550 nanometres is the sweetest sweet spot

One of the hurdles facing photon-based quantum systems using entangled photons is that it's hard to create them at the 1550 nm wavelengths used in telecommunications systems.

Entanglements are the basis for all sorts of hot-boffin stuff: quantum information processing, repeaters, photon-based key exchange, and fully quantum communication systems.

Why does the wavelength matter? Because so far, it's been much easier to generate entangled photons at wavelengths close to visible light (around 900 nanometres) – but telecommunications systems don't use that wavelength. Signals travel through long-distance optical fibres much better at wavelengths like 1330 nm and 1550 nm.

To get their quantum dots to emit entangled photons in the 1535 – 1565 nm telecommunications C-band, the researchers from the University of Stuttgart used indium arsenic (InAs) dots embedded in indium gallium arsenide (InGaAs) barriers.

The single photons and entangled pairs leaving those dots then pass through what the boffins call a “metamorphic buffer” (also made of InGaAs) to shift the wavelength to the vital 1550 nm band without destroying the entanglement.

In their paper at Applied Physics Letters, the researchers say their measurements of the photons “clearly” proved the entanglement of the states (the fidelity of the measured Bell state was “0.61 ± 0.07, a concurrence of 0.74 ± 0.11, a tangle of 0.55 ± 0.14, and a negativity of 0.63 ± 0.12” which means more to a physicist than to this Reg hack).

As is explained in this media release, this isn't the first attempt to shift entangled photons towards the C-band, but in the past, losses to “fine structure splitting” have been too high to be useful.

The Stuttgart team's experiment, co-author Fabian Olbrich explained, “experienced less than one-fifth as much FSS as other studies in the literature”.

This system still has one big Achilles heel in mass-market terms, though: it needs cryogenic temperatures to operate. That'll keep it out of telcos' data centres for the foreseeable future. ®

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