Recent content by Simon Phoenix

The point, for me at any rate, is the following : Classical physics, as we know it, is a non-starter for an explanation of QM. And perhaps more generally, local theories built upon variables that have the same properties as those of classical physics aren't good enough either. Whatever our...

Of course, but in Bell's original work and the later CHSH generalized versions, the math and the terms are all well-defined - what the issue is is that this seems to generate endless speculation on what this implies for things like 'realism'. Counterfactuals, non-contextuality, etc etc - all...

Quite possibly :smile: One of the things EPR/Bell stuff seems to generate is a whole ton of philosophical verbiage and nitpicking about the 'precise' meanings of terms. I confess I don't have too much interest in all of that. The seemingly infinite 'nuances' of the term 'realism' that folk...

I disagree : Bell's theorem states that the predictions of QM cannot wholly be replicated by any locally realistic theory. The term 'local' means that results 'here' do not depend on configuration settings 'there'. The term 'realism' is a bit harder to pin down but conceptually the crucial...

Sorry for coming to this a bit late. Been a tad busy :smile: My advice for anyone trying to understand Bell's inequality is to completely forget about QM. The inequality itself is absolutely nothing to do with QM - it is a restriction on plain old probabilities. What is the BI about then? Well...

The short answer to this question is yes. You can test for entanglement on pairs of particles (you'll need an ensemble of particle pairs all prepared in the same state) to see if there is entanglement between them. What does it matter about some 3rd particle? However, there are some quite deep...

I don't agree with you here. In practice, the projection 'postulate' is used extensively when calculating stuff (certainly in quantum optics/information anyway). Whether one views this as representative of a physical 'collapse' or merely as a convenient mathematical tool for getting the right...

That's quite a big question :wideeyed: In very general terms things become entangled when they interact - so the number of physical processes that create entanglement is quite extensive. Entanglement is not some rare and special prediction of quantum mechanics - it's pretty much par for the...

Well any measurement of the "spin-x" of particle 1 should do the trick and leave you with some entanglement of qubits 2 and 3 (unless it's a useless measurement). We can handle the case of an imperfect measurement using the POVM formalism and we can also work out what the corresponding density...

Lol - yes, I can prepare it by making a measurement :smile:

There's a nice example of this that has been demonstrated experimentally. You start with a very high-Q cavity with 'nothing' in it (so just the single mode vacuum). Send an excited 2-level atom, resonant with the cavity mode, through the cavity and tailor the interaction time such that there's a...

You have to be a bit careful here. If I have a GHZ state of 3 qubits $$ | \psi \rangle = \frac 1 {\sqrt{2}} \left( | 000 \rangle + | 111 \rangle \right) $$ where this is expressed in terms of "spin-z" eigenstates, then if I measure "spin-x" for qubit 1, I end up with qubits 2 and 3 being in a...

Sorry for coming to this late - I've been traveling over Summer with somewhat random internet access (perhaps my connection was in a mixed state of 'off' and 'on'). Anyway, you've had some good answers to your questions so far and I'm not sure I can add very much - and most of what I'm going to...

I'm not the one claiming that the Bell/CHSH inequality is wrong :wideeyed: Counterfactual definiteness is a cornerstone of classical physics - in a nutshell it is the claim that objects have properties independent of measurement. Locality is another very reasonable assumption that says that...

I think you may be missing the point. You're probably best looking at the various derivations of the CHSH inequality (the original Bell inequality is a special case of this). I'm sure you'll find one that satisfies your requirement for rigour. The key thing is that these are all classical in the...