No matter how many times researchers try, there's just no getting around the weirdness of quantum mechanics.



In the latest attempt, researchers at the University of Geneva in Switzerland tried to determine whether entanglement—the fact that measuring a property of one particle instantly determines the property of another—is actually transmitted by some wave-like signal that's fast but not infinitely fast.



Their test involved a series of measurements on pairs of entangled photons (particles of light) that were generated in Geneva (satellite view at left) and then split apart by optical fiber to two villages 18 kilometers (11 miles) apart where the team had set up photon detectors. (In 2007, researchers transmitted entangled light 144 kilometers between two of the Canary Islands.)



The idea in the new experiment is that the photons in each entangled pair are hitting the distant detectors simultaneously, so there's no time for them to exchange a signal. By comparing results from the two detectors, the researchers determined whether the photons were entangled or not, using a test known as Bell's inequalities.



The photons were indeed entangled, the group reports in Nature. But in reality, no experiment is perfect, so what they end up with is a lower limit on how fast the entanglement could be traveling: 10,000 times the speed of light.



To appreciate the weirdness of entanglement, consider that the outcome of a single quantum measurement is random. By all tests, a photon *has* no definite polarization until it hits a detector capable of measuring it. So it's like the entangled particles share one big quantum state.



There's one other subtlety to the experiment. If entanglement is traveling through space like some kind of faster-than-light wave, that would violate Einstein's theory of special relativity, which says the laws of nature are the same no matter which way you're moving with respect to anything else.



So the group had to run their experiment repeatedly for more than 24 hours, counting on Earth's rotation to sample all the different orientations relative to the stars. (Imagine a laser pointer shining into space along the direction of the optical fiber.)



It's always conceivable that quantum mechanics might break down (read: show some signs of everyday normalcy) if experimenters could test it the right way. In a 2007 study, researchers in Vienna tested the idea that maybe the instantaneous-ness of entanglement (called nonlocality) was consistent with hidden "variables" that can explain the randomness of quantum measurements. But no dice for that idea.



Theoretical physicist Terence Rudolph of Imperial College London, author of a commentary on the new paper, says that putting bounds on faster-than-light entanglement is useful for researchers trying to imagine theories that might extend beyond quantum mechanics.



What might such a theory look like? Rudolph says we're probably stuck with instantaneous entanglement, which seems impossible to us because we're stuck in everyday space and time. "We need to understand how quantum mechanics sees space and time," he says. "I think there's probably much deeper issues."

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Image credit: NASA Earth Observatory