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I think either you're confused on an issue or I'm confused on what you're trying to say. Quantum interference is observable with even a single particle. > bec

I think either you're confused on an issue or I'm confused on what you're trying to say. Quantum interference is observable with even a single particle. > because entangled particles do not self-interfere Entangled particles certainly can, and do interfere (as mentioned, electrons in an atom are entangled and definitely interfere; there's a whole field devoted to calculating this effect), so I'm not sure exactly what you mean here. Can you give me, in ket notation, an example of an unentangled state? (And if you're referring to a direct tensor product of two single particle states, I'm still considering that an entangled state since it can unitarily evolve out of that configuration into one that can't be written as a tensor product).

bmnmasdas 12y ago
> (And if you're referring to a direct tensor product of two single particle states, I'm still considering that an entangled state since it can unitarily evolve out of that configuration into one that can't be written as a tensor product) This reasoning does not make sense. The notion of entanglement is not invariant under "global" operations, nor should it be.
lisper 12y ago
> Quantum interference is observable with even a single particle. Yes, but only if that particle is in an unentangled state. If that particle is entangled then the only way to observer the interference is to make measurements on both the particle whose interference you wish to observe and the particle with which it is entangled. This is simply the old canard about measurement "destroying" interference (because measurement and entanglement are the same thing). > Entangled particles certainly can, and do interfere That's true, but the way in which they interfere is different, and the procedure you need to perform to observe the interference is likewise different. > Can you give me, in ket notation, an example of an unentangled state? |U> + |D> Or if you want two unentangled particles: (|U1> + |D1>)(|U2> + |D2>) as opposed to the entangled state: |U1>|D2> + |D1>|U2> (Where U and D are intended to designate something like spin Up or spin Down, but that doesn't really matter.) > if you're referring to a direct tensor product of two single particle states, I'm still considering that an entangled state since it can unitarily evolve out of that configuration into one that can't be written as a tensor product That's news to me. How does that happen?
- Xcelerate 12y ago
  This debate has really been bothering me and making me anxious the last couple of days. I haven't been able to sleep. I need to know something, are you self-taught in quantum mechanics or do you have a PhD in theoretical physics? I'm not doubting your argument, (for all I know you're a well-known physics professor), I would just really like an answer to this question, as I think it would put my mind at ease.
  - lisper 12y ago
    Self-taught. Why does that matter? (I do have a Ph.D. but it's in CS.) 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.
    - Xcelerate 12y ago
      > Self-taught. Why does that matter? 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.
      - lisper 12y ago
        > I wasted my time arguing with a 14 year old 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 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/book.ps 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.)

bmnmasdas 12y ago

> (And if you're referring to a direct tensor product of two single particle states, I'm still considering that an entangled state since it can unitarily evolve out of that configuration into one that can't be written as a tensor product) This reasoning does not make sense. The notion of entanglement is not invariant under "global" operations, nor should it be.

lisper 12y ago

> Quantum interference is observable with even a single particle. Yes, but only if that particle is in an unentangled state. If that particle is entangled then the only way to observer the interference is to make measurements on both the particle whose interference you wish to observe and the particle with which it is entangled. This is simply the old canard about measurement "destroying" interference (because measurement and entanglement are the same thing). > Entangled particles certainly can, and do interfere That's true, but the way in which they interfere is different, and the procedure you need to perform to observe the interference is likewise different. > Can you give me, in ket notation, an example of an unentangled state? |U> + |D> Or if you want two unentangled particles: (|U1> + |D1>)(|U2> + |D2>) as opposed to the entangled state: |U1>|D2> + |D1>|U2> (Where U and D are intended to designate something like spin Up or spin Down, but that doesn't really matter.) > if you're referring to a direct tensor product of two single particle states, I'm still considering that an entangled state since it can unitarily evolve out of that configuration into one that can't be written as a tensor product That's news to me. How does that happen?

Xcelerate 12y ago
This debate has really been bothering me and making me anxious the last couple of days. I haven't been able to sleep. I need to know something, are you self-taught in quantum mechanics or do you have a PhD in theoretical physics? I'm not doubting your argument, (for all I know you're a well-known physics professor), I would just really like an answer to this question, as I think it would put my mind at ease.
- lisper 12y ago
  Self-taught. Why does that matter? (I do have a Ph.D. but it's in CS.) 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.
  - Xcelerate 12y ago
    > Self-taught. Why does that matter? 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.
    - lisper 12y ago
      > I wasted my time arguing with a 14 year old 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 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/book.ps 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.)