810 nm QKD in telecom fibers

• Post by: admin
• July 27, 2010
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We have a new paper, ‘Quantum entanglement distribution with 810 nm photons through telecom fibers‘, in Applied Physics Letters as of today (also available on the arXiv here). The experiment has been done in Thomas Jennwein’s group at IQC in Canada and demonstrates that entangled photons at 810 nm can be transmitted through 1550 nm, standard telecom fibers with reasonably high fidelity.

Most quantum information processing experiments are performed at wavelengths around 810 nm where silicon avalanche photodetectors are most efficient. The standard wavelength used in the telecom industry is however 1550 nm, which is where fiber loss is minimal. In our new experiment we show that, even though 1550 nm fibers are bi-modal for 810 nm, they can still be used for the transmission of entanglement at that shorter wavelength. As it turns out, the cross-talk between the two supported modes in the fiber is not critical and we can employ simple mode filtering, either temporal or spatial, to get a high-fidelity signal. In other words, for medium distance quantum information experiments, you might just as well use the much cheaper (and easily available) telecom fibers instead of expensive custom single-mode fibers for shorter wavelengths.

This idea goes back to efforts by Paul Townsend and then Gerald Buller and co-workers, who have pioneered high clock-rate quantum key distribution protocols in telecom fibers with weak coherent states at 850 nm (see this recent paper, for example).

The fact that telco fibers are bi-modal around 810 nm and that the cross-talk between these modes is low begs the question whether one could transmit spatially encoded qu-dits through standard telecom fibers, which would allow QKD with higher coding density. This is certainly a long stretch in practical terms. Interestingly, the group of Han Woerdman in Leiden has demonstrated just such an experiment in a hollow-core fiber, coincidentally pre-published on the arXiv the same day as our paper, so it’s definitely an idea worth thinking about.