The impressive experiments described in a preprint by Ronald Hanson at Delft and colleagues have been widely reported (for example, here and here) as if to imply that they confirm quantum “spooky action at a distance” (in other words, entanglement). With all due respect to my excellent colleagues (who of course don’t write their own headlines), this is not true.
Einstein’s phrase is of course too nice to resist. But there’s a clue here. Einstein? You, know, the guy who wasn’t convinced by the Copenhagen Interpretation of quantum mechanics that reality is just what we can measure, and that nothing deeper lies “beneath”? Einstein, who suspected that there might be “hidden variables” that restore local properties to the quantum world?
Einstein’s “spooky action at a distance” was predicated on that view. It was action at a distance if, via this thing we call (that is, which Schrödinger called) entanglement, an influence at one location (via a measurement) is transmitted instantaneously to another. Only in some kind of local or hidden-variables view do you need to invoke that picture.
Quantum nonlocality – which is what is supported by a violation of Bell’s inequality, and what the new experiments now confirm by closing another of the loopholes that could have permitted a violation in other circumstances – is not spooky action at a distance, but the alternative to it. It says that we can’t always characterise the properties of a particle in ways local to that particle: its state is a smeared-out thing (to put it crudely) that may be correlated with the state of another distant particle. And so it appears to be. In this view, there is no action at a distance when we make a measurement on one particle – rather, there are nonlocal quantum correlations with the state of another. It is hard to find words for this. But they are not “spooky action at a distance.”
I don’t expect these words to make a blind bit of difference, but here they are anyway.