Researchers hope to send an experiment to the International Space Station (ISS) by the middle of the next decade that would pave the way for transcontinental transmission of secret messages encoded using the mysterious quantum property of entanglement.

When two particles such as photons are born from the same event, they emerge entangled, meaning they can communicate instantaneously no matter how far apart they are. Transmitting entangled pairs of photons reliably is the backbone of so-called quantum key distribution—procedures for converting those pairs into potentially unbreakable codes. Quantum cryptography, as it is known, could appeal to banks, covert government agencies and the military, and was tested in a 2007 Swiss election.

Photons can travel perhaps 100 miles (160 kilometers) or so along today's fiber-optic cables before their quantum character breaks down. That limit vanishes aboveground. Last year, a team led by physicist Anton Zeilinger of the University of Vienna transmitted quantum keys up to 89.5 miles (144 kilometers) between a pair of telescopes in Spain's Canary Islands.

Now Zeilinger and colleagues want to make the jump to hundreds of miles or more. The Vienna group is spearheading an international project, called Space-QUEST (Quantum Entanglement for Space Experiments), to prove that a system for generating pairs of entangled photons can fit the constraints imposed by the ISS.

Quantum keys distributed from orbit could be transmitted to any two points in the station's line of sight, limited only by the ability of transmitters and receivers to keep a tight lock on one another and to distinguish entangled photons from background light. Earlier this year, Zeilinger and colleagues showed they could detect single photons reflected from an Italian satellite 3,700 miles (5,955 kilometers) above Earth. 

The tools for the experiment would be an ultraviolet laser, a special type of crystal for splitting ultraviolet photons into pairs of infrared photons and equipment for keeping the two components aligned during the rigors of spaceflight. To make it into space, the instruments, including telescopes to transmit the photons, must weigh less than about 220 pounds (100 kilograms), consume no more than 250 watts of power and be no larger than 4.6 feet (1.4 meters) on its longest side.

The 10-country Space-QUEST group hopes to build a prototype device and gather preliminary data in time for a November meeting of the European Space Agency (ESA), where officials will decide which projects will receive funding and deserve a chance to fly in space, says Rupert Ursin, a postdoctoral researcher in Zeilinger's lab.

Ursin says the team's proposal has scored high in preliminary assessments by European funding agencies including the ESA. "We can do a lot of physics experiments with a relatively small budget" of a few hundred million euros, he says.

In addition to the potential practical application, the Space-QUEST project would give researchers a chance to probe the reaches of entanglement in a new way. Theory says entangled should be unlimited in range, Ursin says, but to know for sure, "we have to test it."