One of the great questions in physics is how the quantum rules that govern the universe on the smallest scale are related to general relativity which governs the universe on the largest scale. Physicists would dearly love to test their ideas about this but the problem they come up against is in designing experiments in which both sets of rules can be brought into play.

One possibility is to send entangled photons over distances in which the curvature of space-time becomes important. The entanglement is governed by quantum mechanics but the curvature of space-time governed by general relativity. So this kind of experiment is an attractive prospect.

But there is a problem. The only place that such an experiment could be done is in space. And so far, nobody has built the gear necessary for creating entangled photons and measuring them in this environment.

Enter Zhongkan Tang at the National University of Singapore and a few pals. These guys have built a device for generating and measuring pairs of entangled photons that is specifically designed to fit inside a standard CubeSat, a tiny satellite that takes the form of a 10 cm cube weighing only 1 kg.

In the standard design, each CubeSat has a 2 Watt power source and can be stacked to create larger spacecraft. They are designed to be launched at low cost carrying bespoke experiments into low Earth orbit.

Entangled photons are made by taking one high-energy photon and splitting it into two lower energy ones that are entangled. This splitting is done by passing the high energy photon through a non-linear crystal. To be fair, this technology is standard in any Earth-based quantum optics lab.

The question that Zhongkan and co aimed to answer was whether this equipment could safely be used in space. They chose to generate the high-energy photons using a 405 nm laser diode. These photons then pass on to crystal of beta-barium-borate which splits each photon into a pair of 750 nm and 867 nm photons.

All this requires various filters, beam splitters, liquid crystal units and a printed circuit board for rotating and analysing the photon pairs. The device also needs a heater to maintain the equipment at its operating temperature of between 20 and 30°C.

Zhongkan and co glued all this stuff into an aluminium base which is designed to fit into the standard CubeSat bay, 10 x 10 x 3cm in size.

Zhongkan and co then tested their gear by shaking it at all the frequencies it is likely to experience during a launch and at forces up to 7.8 g. They also tested it in a vacuum, their fear being that outgassing from the glue would end up on sensitive optical components inside the device. That fear proved to be unfounded.

Finally, the attached the device to a helium balloon and released it into the atmosphere to see whether it would work at high altitude. When the balloon burst and equipment parachuted safely back to ground, they found that all had been well throughout the duration of the flight, which reached an altitude of 35 km.

That leaves one remaining step—to fly this thing in space. For that, Zhongkan and co will need to book some space on a rocket travelling to low Earth orbit. Or perhaps, sell their device to somebody who already has a slot. That should allow the first tentative steps towards exploiting entanglement in space.

Incidentally, testing the laws of physics isn’t the only application for this kind of gear. Another important role for these devices is in quantum key distribution that allows messages to be transmitted around the world with perfect secrecy.

If the funding is not available for fundamental physics experiments, then Zhongkan and co might find deeper pockets in the kind of organisations interested in this kind of secrecy.

Ref: arxiv.org/abs/1404.3971 : Near-Space ﬂight Of A Correlated Photon System