Special optical fibre preserves spatial entanglement

Researchers in the Netherlands and Germany have shown that it is possible to send spatially entangled photons down an optical fibre and maintain the entanglement. The trick is to have the right kind of fibre.

This is potentially a very useful breakthrough. It is all very well being able to spatially entangle photons and build yourself a quantum computer, but if you can’t send the data anywhere, your options are limited. (No twitter, for a start.)

Photons can be entangled spatially, using their polarization, or even temporally. Please don’t ask usto explain this last one and it's shades of Star Trek.

Regardless: polarization is good, but of limited use because it has only two degrees of freedom. A photon can be spatially entangled in many more dimensions, though, so can potentially be used to encode more data per photon. As Wolfgang Löffler, a researcher from Leiden University in The Netherlands, explains at PhysOrg: "The spatial structure of a single photon field has basically infinite dimensions."

So how do you build a fibre that doesn’t upset all this delicate entanglement? Mainly you build it with holes in so that light can be guided down a hollow core in the fibre. The team found one multimodal fibre that would allow the entangled pair to travel undisturbed, but not for huge distances. Tests have only been conducted with two dimensionally entangled photons. He plans to continue experiments with higher dimensions of entanglement and over longer distances.

The work is published in issue 24 of Physical Review Letters.