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One of the great mysteries of modern physics is the link between quantum mechanics and general relativity or gravity. But quantum phenomena generally occur on the very smallest scales while gravity generally crops on the largest scales. Never the twain shall meet.

At least, not without some clever thinking. One idea is to entangle a pair of photons, hang on to one and send the other across a distance so vast that gravity is significant, in other words, far enough for the gravitational curvature of space to come into play.

The issue in question is whether the entanglement–a purely quantum phenomenon–‘feels’ this curvature in the same way as purely classical things, like humans.  

The required distance isn’t that far–a few hundred kilometres should do the trick.

But there’s a problem. The furthest scientists have sent entangled photons is just 144 kilometres. and because of atmospheric losses and the curvature of the Earth’s surface, the only way to go further is to fire photons straight upwards, into space. 

Today, Thomas Scheidl at the Austrian Academy of Sciences in Vienna and a couple of pals suggests a simple and relatively cheap way of doing these kinds of experiments for the first time using the International Space Station (ISS), which orbits at an altitude of about 400 kilometres.  

Their plan is to create entangled photons on the ground and beam them up to the ISS. That gets around one important problem with this kind of work, which is that much of the hardware needed for creating entangled photons–the lasers, nonlinear materials etc–are not yet  qualified for use in space and getting such a qualification is an expensive business.  

So leaving all this stuff on the ground is a sensible idea. All you need in space is a sensor capable of detecting photons and their polarisation. In other words, a camera.

As it turns out, the International Space Station isn’t a bad place to put such a device. It already has a camera mount in its cupola, the seven-windowed viewing area that gives astronauts an enviable view of the planet.

The cupola has windows made of four vacuum-separated panes of glass. Nevertheless, Scheidl and co calculate that enough photons ought to get through to do good science. 

And the mount is motorised to counteract the movement of the space station allowing astronauts to take extraordinary images of our world.

Scheidl and co’s plan is to build a camera-sized photon sensor that fits on this motorised mount and watches for photons fired from labs in Austria or in the Canary Islands.

They calculate that the ISS ought to pass over their labs seven or eight times a month each offering a 20-second window  to send the entangled photons for the on board sensor to capture.

Such a camera would be straightforward to build at relatively low cost and could easily be chucked into the back of resupply ship for astronauts to set up when they have free moment.

That looks like a sensible plan to test the technologies for relatively little money. And there are other reasons to try.

In addition to the problems of fundamental physics, this kind of work is an important stepping stone towards setting up a quantum internet that can carry perfectly secure messages from one part of the world to another via satellite links.  

But perhaps most important of all is the possibility it offers of putting the ISS to good use. NASA has always sold the International Space Station to the American public as a laboratory in space that will do important science. For those in the know, that’s always been a laughable claim. 

So a cheap quantum optics experiment with the potential to change our ideas about physics and to pave the way for a new communications technology gives NASA the chance to redress the balance.

Ref: Optics Experiments To The International Space Station ISS: A Proposal

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