Researchers in Vienna have demonstrated that it's possible to create a system that behaves in an entangled way, minus any actual quantum entanglement. That is, two things in a quantum system can be correlated in a way unexplainable by anything we understand in nature, and it doesn't take entanglement for it to happen. Two bits of a system knowing each other a little too well for normal physics to explain. They have a paper out in Nature this month.First, a bit of background.Separate two particles from the same system. Photons, bits of light. Fire them in different directions and they will remain related in a way that Einstein termed "spooky action at a distance," notably a way he rejected. For good reason. A weird property of quantum mechanics has taken over the relationship between the particles: a measurement performed on one photon will affect the possible outcomes of the other. At any distance and instantaneously. It's the central weirdness of quantum entanglement.You can like to imagine that the relationship is explained by something hidden within the particles, a property that is "local" to them. Imagine that they have their own bits of DNA that determine these correlated results. These are called "local hidden variables" and they provide an "out" for quantum entanglement that allows its effects to follow the rules of Einstein's relativity, which says that information can't travel faster than light and certainly not the instantaneously of entanglement. It wouldn't have to because all of the information dictating the weird relationship was traveling with the particles all the time.John Bell did a calculation of probabilities – a relatively simple calculation – and discovered that local realism is impossible here. There's a mismatch between the probabilities possible with local realism and the probabilities possible with this faster-than-light other thing of quantum mechanics. Entangled particles will share probabilities, and that sharing is non-local. Particles are not smuggling hidden information. Information must come from one particle instantly to account for the properties of its entangled partner. This is pretty well established now, and it's pretty awesome.It has a new twist, what's being dubbed "entanglement without 'spooky action at a distance.'" Not that it is any less spooky, but the researchers have successfully removed the distance: rather than entangle two particles of the same system and transported away from each other, they've shown that the same probabilistic weirdness that cannot occur in a classical system can occur without entanglement. In their experiment it was done within a qutrit, a three bit system consisting of a single photon that can take on three different states. It's a system that very notably does not allow for entanglement.Yet if you were to make a set of "pairwise" measurements on the system, you will get results that do not make sense classically. Does this make "spooky action" – still some kind of faster-than-light communication – more weird or more spooky? Got me. It makes the phenomenon more general. Which is bad for the normal old description of the world, at the very least.Connected:
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