Quantum entanglement stands as one of the strangest and hardest concepts to understand in physics. Two or more particles can interact in a specific ways that leave them entangled, such that a later measurement on one system identifies what the outcome of a similar measurement on the second system—no matter how far they are separated in space.

Repeated experiments have verified that this works even when the measurements are performed more quickly than light could travel between the sites of measurement: there's no slower-than-light influence that can pass between the entangled particles. However, one possible explanation for entanglement would allow for a faster-than-light exchange from one particle to the other. Odd as it might seem, this still doesn't violate relativity, since the only thing exchanged is the internal quantum state—no external information is passed.

But a new analysis by J-D. Bancal, S. Pironio, A. Acín, Y-C. Liang, V. Scarani, and N. Gisin shows that any such explanation would inevitably open the door to faster-than-light communication. In other words, quantum entanglement cannot involve the passage of information—even hidden, internal information, inaccessible to experiment—at any velocity, without also allowing for other types of interactions that violate relativity.

Experiments have definitively demonstrated entanglement, and ruled out any kind of slower-than-light communication between two separated objects. The standard explanation for this behavior involves what's called nonlocality: the idea that the two objects are actually still a single quantum system, even though they may be far apart. That idea is uncomfortable to many people (including most famously Albert Einstein), but it preserves the principle of relativity, which states in part that no information can travel faster than light.

To get around nonlocality, several ideas have been proposed over the decades. Many of these fall into the category of hidden variables, wherein quantum systems have physical properties (beyond the standard quantities like position, momentum, and spin) that are not directly accessible to experiment. In entangled systems, the hidden variables could be responsible for transferring state information from one particle to the other, producing measurements that appear coordinated. Since these hidden variables are not accessible to experimenters, they can't be used for communication. Relativity is preserved.

Hidden variable theories involving slower-than-light transfer of state information are already ruled out by the experiments that exclude more ordinary communication. Some modern variations combine hidden variables with full nonlocality, allowing for instantaneous transfer of internal state information. But could non-instantaneous, faster-than-light hidden variables theories still work?

To investigate this possibility, the authors of the new study considered the possible experimental consequences. Obviously, one way to test it would be to increase the separation between the parts of the entangled system to see if we can detect a delay in apparently instantaneous correlation we currently observe. Sufficiently fast rates of transfer, however, would still be indistinguishable from nonlocality, given that real lab measurements take finite time to perform (this assumes that both experiments happen on Earth).

The researchers took a theoretical approach instead, using something known as the no-signalling conditions. They considered an entangled system with a set of independent physical attributes, some observable, some hidden variables. Next, they allowed the state of the hidden variables to propagate faster than the speed of light, which let them influence the measurements on the separated pieces of the experiment.

However, because of the nature of quantum mechanical systems, there was a symmetry between the hidden and measurable attributes of the system—meaning if the hidden variables could transfer information faster than light, then the properties we can measure would do so as well. This is a violation of the no-signalling condition, and causes serious problems for the ordinary interpretations of quantum physics.

Of course, one conceivable conclusion would be that faster-than-light communication is possible; this result provided a possible avenue for testing that possibility. By restricting the bounds on the speed of interaction between entangled systems, future experiments could show whether any actual information is traveling or not.

However, the far more likely option is that relativity is correct. In that case, the strong ban on faster-than-light communication would rule out the possibility of faster-than-light transfer of information encoded in hidden variables, and force us to deal with nonlocality. Once again, it would seem that local realism and relativity are incompatible notions in the quantum world.

Nature Physics, 2012. DOI: 10.1038/NPHYS2460 (About DOIs).