One of the lesser known cornerstones of modern physics is Claude Shannon’s mathematical theory of communication which he published in 1948 while juggling and unicycling his way around Bell Labs.

Shannon’s theory concerns how a message created at one point in space can be reproduced at another point in space. He calls the conduit for such a process a channel and the limits imposed by the universe on this process the channel capacity.

The capacity of a communications channel is hugely important idea. It tells you, among other things, the rate at which you can send information from one location to another, without loss. If you’ve ever made a phone call, watched television or surfed the internet you’ll have benefited from the work associated with this idea.

In recent years, our ideas about communication have been transformed by the possibility of using quantum particles to carry information. When that happens the strange rules of quantum mechanics govern what can and cannot be sent from one region of space to another. This kind of thinking has has spawned the entirely new fields of quantum communication and quantum computing.

But ask a physicist what the capacity is of a quantum information channel and she’ll stare at the floor and shuffle her feet. Despite years of trying, nobody has been able to update Shannon’s theory of communication with a quantum version.

Which is why a paper today on the arXiv is so exciting. Graeme Smith at the IBM Watson Research Center in Yorktown Heights NY (a lab that has carried the torch for this problem) and Jon Yard from Los Alamos National Labs have made what looks to be an important breakthrough by calculating that two zero-capacity quantum channels can have a nonzero capacity when used together.

That’s interesting because it indicates that physicists may have been barking up the wrong tree with this problem: perhaps the quantum capacity of a channel does not uniquely specify its ability for transmitting quantum information. And if not, what else is relevant?

That’s going to be a stepping stone to some interesting new thinking in the coming months and years. Betcha!

Ref: arxiv.org/abs/0807.4935: Quantum Communication With Zero-Capacity Channels

Entanglement is the strange and beautiful property of certain quantum particles to become so deeply linked that they share the same existence. According to quantum theory, that link should be maintained whatever the distance between the particles, whether the width of an atom or the diameter of the universe.

This led Einstein to claim that the instantaneous effects of entanglement would lead to “spooky action-at-a-distance” in violation of special relativity which prevents faster-than-light signals.

Nobody knows how the different predictions of relativity and quantum mechanics can be resolved. However, entanglement has been measured in numerous experiments over relatively short distances on Earth. The tests involve two entangled particles, photons say, being sent to distant experimenters who then perform measurements on them.

In every one of these tests, the results agree entirely with the predictions of quantum mechanics. And yet naysayers continue to unearth loopholes that allow them to claim that there is a way in which the results are fixed, perhaps because quantum mechanics works only only over the short distances that can be exploited on Earth or by the existence some kind of hidden variable that determines in advance how the particles will behave when they are separated.

There is one way to settle the matter for sure: send entangled photons to two orbiting astronauts on board different spacecraft with large relative velocities. That leaves no room for hidden variable theories or any other fix because the peculiarities of special relativity allow both astronauts to claim the measurement on their photon was performed before the other.

Today Anton Zeilinger from the University of Vienna in Austria, says he wants to try just such an experiment and has put together an impressive international team to design and promote idea. The team has submitted its proposal, called Space-QUEST, to the European Space Agency in the hope that one end of the experiment could hosted on the Columbus module, Europe’s orbiting laboratory attached to the International Space Station.

The other observer need only be on the ground since Zeilinger has already proven that single photons can be bounced off orbiting satellites and detected on the ground.

That should please mission planners for the International Space Station which has yet to host a single significant experiment in space. Zeilinger’s Space-QUEST experiment looks like a genuine attempt to push the envelope of physics. The quicker they get it into orbit, the better.

Ref: arxiv.org/abs/0806.0945: Space-QUEST: Experiments with Quantum Entanglement in Space