(Admittedly, the cute kitten was added by my editor.)

Albert Einstein described quantum entanglement in 1935, along with colleagues Boris Podolsky and Nathan Rosen, and used it as an argument against quantum mechanics. Entanglement is the phenomenon by which two widely separated systems act as a single system, due to interactions in the past; a measurement on one system reveals the outcome of a similar experiment performed on the other, no matter how far they are separated in space. Since the 1980s, a wide variety of experiments have showed (contra Einstein) that entanglement is a real feature of quantum physics, though no information usable by an experimenter can be extracted—which preserves relativity and causality (that is, future events can’t cause past events).

A new theorem has showed even more that entanglement truly seems to be the result of separated systems actually being parts of a single unbroken system, however you end up interpreting that (and I don’t want to get into that today!).

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. [Read more…]