It doesn't. All that matters is that the science is correct (see Ron Maimon for instance; self-taught but far more knowledgable than most physicists). However, I get in far too many discussions involving QM online (mostly Reddit) where I learn that I wasted my time arguing with a 14 year old that watched "Through the wormhole" or something.
> P.S. It turns out you were right about this
You sound surprised. My graduate research is in quantum chemistry (and related areas), so I should have at least a few concepts correct. As someone with more of a QC background, perhaps my definition of what theoretical physicists consider "entangled" is inaccurate, so I am reading up on the matter. But I'm finding their specification of "entangled" to be an unintuitive and unobvious choice of wording.
Well, I'm about to turn 50 so you don't have to worry about that. :-)
FWIW, I wrote this:
http://www.flownet.com/ron/QM.pdf
> You sound surprised.
No, I'm not. (How does one "sound surprised" in an on-line conversation?)
> perhaps my definition of what theoretical physicists consider "entangled" is inaccurate
An entangled state is one that can't be factored. See e.g.
http://www.theory.caltech.edu/people/preskill/ph229/notes/bo...
section 2.4.1.
(I am a bit nonplussed by how someone who does graduate research in quantum chemistry can not know this. It's a pretty basic concept.)
Heck, it's better than a professor I had that did DFT work who thought the single-particle wavefunction was the most fundamental aspect of quantum mechanics. Nevermind the fact that DFT is trashed for its lack of ability to account for electron correlation.
(And I do mean trashed. I was at the Gordon Research Conference in quantum chemistry this past summer where Peter Gill gave a lecture titled "the obituary of DFT". And if anyone has a right to rag on it, it's certainly him.)
Well, that would depend on how far my studies have gotten me, no? My general assumption is that anyone who actually does QM for a living knows at least as much as I do, and almost certainly a great deal more. (Case in point: I presume that QFT is quantum field theory, but what is DFT?) I prefer to view this as humility rather than passive aggression. In any case, I don't think either one of us should be losing any sleep over it :-)
But can we agree that there are states that can be factored, and states that can't, and that this is a useful distinction to make? And that this distinction is physical, i.e. that it produces observable effects? (And not just observable, but interesting and useful?)
Oh certainly. I've never disagreed on this haha.
It does make an interesting puzzle, though, how it can be physically possible to prepare a state that is both unentangled and pure. I don't actually know the answer to that off the top of my head.
P.S. It turns out you were right about this:
"it can unitarily evolve out of that configuration into one that can't be written as a tensor product"
That turns out to be true (and it retrospect it's obviously true because otherwise it would not be possible to create entangled pairs.) But you are wrong in your claim that therefore the antecedent state is also entangled. A state that can be factored is not entangled by definition.