Quantum physics dictates that entangled particles are linked, so that if you change the property of one by measuring it, the other will instantly change, too. In theory, the particles stay linked even if they're light-years apart, so quantum entanglement effectively defies Einstein's principal that nothing can travel faster than light. Since there are still doubts that entanglement works over a long distance, the QUESS satellite team will do a so-called Bell test with entangled photons in China and Austria.

The same entanglement principal also helps cryptologists. Since measuring one entangled photon instantly changes the properties of the other, the photons can be used to create an encryption key. Any attempt to listen in by measuring a photon would instantly change the other, letting the two parties know that a breach has occurred.

Such effects have been demonstrated on the ground across a distance of 300 km (186 miles), but photons fired across fiber-optic cables or the air are eventually dispersed, making measurement impossible. The QUESS satellite, by contrast, will fire photons in clear space, theoretically increasing the test distances.

The team will run quantum experiments with the first satellite for about two years, but "if the first satellite goes well, China will definitely launch more," physicist Chaoyang Lu tells Nature. He estimates that up to 20 satellites would be needed to generate a secure quantum communications network. Similar research is also being conducted by Canadian researchers who want to create entangled photons on Earth and fire them at tiny "cube sat" microsatellites. Italian researchers, meanwhile, plan to fire entangled photons off mirrors mounted on existing satellites.

The research may yield a completely secure, space-based "internet" within a decade. Quantum satellites could also join up to form a telescope with enormous resolving power that could "read license plates on Jupiter's moon," according to NASA physicist Paul Kwiat.