String theory has finally made a prediction that can be tested with experiments -- but in a completely unexpected realm of physics.

The theory has long been touted as the best hope for a unified "theory of everything," bringing together the physics of the vanishingly small and the mindbendingly large. But it has also been criticized and even ridiculed for failing to make any predictions that could be checked experimentally. It's not just that we don't have big enough particle accelerators or powerful enough computers; string theory's most vocal critics charge that no experiment could even be imagined that would prove it right or wrong, making the whole theory effectively useless.

Now, physicists at Imperial College London and Stanford University have found a way to make string theory useful, not for a theory of everything, but for quantum entanglement.

"We can use string theory to solve problems in a different area of physics," said theoretical physicist Michael Duff of Imperial College London. "In that context it's actually useful: We can make statements which you could in principle check by experiment." Duff and his colleagues describe their findings in a paper in *Physical Review Letters* September 2.

String theory suggests that matter can be broken down beyond electrons and quarks into tiny loops of vibrating strings. Those strings move and vibrate at different frequencies, giving particles distinctive properties like mass and charge. This strange idea could unite all the fundamental forces, explain the origins of fundamental particles and connect Einstein's general relativity to quantum mechanics. But to do so, the theory requires six extra dimensions of space and time curled up inside the four that we're used to.

To understand how these extra dimensions could hide from view, imagine a tightrope walker on a wire between two high buildings. To the tightrope walker, the wire is a one-dimensional line. But to a colony of ants crawling around the wire, the rope has a second dimension: its thickness. In the same way that the tightrope walker sees one dimension where the ants see two, we could see just three dimensions of space while strings see nine or ten.

Unfortunately, there's no way to know if this picture is real. But although string theorists can't test the big idea, they can use this vision of the world to describe natural phenomena like black holes.

Four years ago, while listening to a talk at a conference in Tasmania, Duff realized the mathematical description string theorists use for black holes was identical to the mathematical description of certain quantum systems, called quantum bits or qubits.

Qubits form the backbone of quantum information theory, which could lead to things like ultrafast computers and absolutely secure communication. Two or more qubits can sometimes be intimately connected in a quantum state called entanglement. When two qubits are entangled, changing one's state influences the state of the other, even when they're physically far apart.

"As I listened to his talk, I realized the kind of math he was using to describe qubit entanglement was very similar to mathematics I had been using some years before to describe black holes in string theory," Duff said. When he looked into it, the mathematical formulation of three entangled qubits turned out to be exactly the same as the description of a certain class of black holes.