# String theory entangled

## Equations can be retooled to describe a strange quantum effect

Physicists looking for a way to test their theory about strings might make more progress if they tangle them up.

String theory — equations that aspire to explain all of nature’s particles and forces — has extended its reach to the strange quantum behavior known as entanglement, physicists report September 2 in *Physical Review Letters*. Repurposing string theory mathematics allowed physicists to solve a hard problem involving entanglement, a strange feature at the heart of quantum mechanics. In doing so, the new study also points out a way to test whether the co-opted string theory equations are actually correct.

“String theory has not had a lot of success in making falsifiable predictions,” says study coauthor Michael Duff of Imperial College London. “But in the field of quantum information theory, it can.”

One of the hallmarks of quantum information is that particles carrying it can interact in a way that makes them “entangled,” so that measuring one seems to instantaneously affect the other, even at great distances. Over the last few years, Duff and his colleagues began to notice similarities between string theory — the idea that particles of matter and force are tiny vibrating loops or strands — and the equations that govern entangled particles.

In a paper published last year, the physicists noted that the string theory math describing black holes is surprisingly similar to the equations for a group of three entangled particles. The new study takes the analogy a step further, tackling the more difficult problem of how four pieces of quantum information, called qubits, behave when they’re entangled. Because experiments disagree, physicists aren’t sure about how many ways four qubits can be entangled. The answer, according to string theory, is 31.

Duff and his team can’t yet explain why the formulas apply to this system. “We don’t understand why it works,” he says. “At some deep level we’re mystified by it.”

Although the reasons remain unclear, the surprising result identifies a way to put string theory to the test. In the past, string theory has been used to describe black holes, which are notoriously problematic study subjects. But now, entangled particles — which can be created and studied in laboratories around the world — may serve as stand-ins for string theory experiments.

Theoretical physicist and black hole expert Sergio Ferrara says the new result may convince physicists that string theory can be useful in a variety of settings. Experiments confirming that string theory equations worked as advertised for qubit entanglement would provide “sound confirmation” that the black hole-qubit duality is real, says Ferrara, of CERN, the European nuclear research laboratory in Geneva.

The surprise appearance of string theory mathematics in entangled particles may signal a deeper connection between quantum mechanics, which is typically observed on very tiny scales, and the universe. On the other hand, the connection might turn out to be just “a quirky mathematical coincidence,” Duff says.

Even if experiments turn out exactly as the string theory equations predict, Duff cautions, physicists won’t be crowing that string theory has unlocked the deepest mysteries of the world. The results would confirm that string theory got it right for entanglement, but not necessarily for everything else, too. “We have nothing to say about whether string theory is the theory of everything,” Duff says.

L. Borsten et al. Four-qubit entanglement from string theory. Physical Review Letters.Vol. 105, September 2, 2010. Doi: 10.1103/PhysRevLett.105.100507