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Quantum entanglement means that multiple particles are linked together in a way such that the measurement of one particle's quantum state determines the possible quantum states of the other particles. But my question is that if we randomly pick up any pair of particles from anywhere, will it be entangled? Does it mean that all particles in the universe are entangled to each other? For example- I pick up an electron on earth and an electron which is 1000 million light years away from the electron on earth, will both of those electrons be entangled? Also does this entanglement work only with similar particles, can two different type of particles like an electron and proton be entangled? (Extra question: does quantum entanglement work with only spin of a particle?)

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  • $\begingroup$ ...why would you think that two "randomly" picked particles are entangled? $\endgroup$ – ACuriousMind Mar 25 '16 at 17:10
  • $\begingroup$ it was wheeler who talks of the "great smoky dragon" and how old photons from distant stars are entangled. $\endgroup$ – vzn Mar 25 '16 at 17:12
  • $\begingroup$ @ Acuriousmind, I would say that is a slim but not impossible (e.g. the two he picked might happened to have interacted and entangled some hours ago and then fly away to their current locations). The main point I think is then how to tell which type of correlation will imply entanglement $\endgroup$ – Secret Mar 25 '16 at 17:14
  • $\begingroup$ It's a "yes, but". If you have a single photon emitted by an atom somewhere, the state of that photon and the atom would be entangled (for a very little while because the atom will interact strongly with other photons). The experimentally generated two-photon states that we use to demonstrate decoherence are very special in that they do not interact strongly and are therefor isolated from rapid decoherence. They are, if you will, an artifact. $\endgroup$ – CuriousOne Mar 25 '16 at 17:15
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To answer your questions one by one:

  1. No. All the particles are not entangled. If they were so, we'd have quantum computers by now. Entanglement is a phenomena very sensitive to environment and even the slightest interference from outside can diminish the entanglement between two particles.

  2. Different type of particles can be entangled. In a pair production the produced electron-positron are entangled.

  3. Again No. Any quantum mechanical property of the system can be entangled. Not only spin but position ,momenta, total angular momenta etc.

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  • $\begingroup$ This answer seems to put a nail in the coffin of the idea that the reason "observation" plays a part in quantum theory is due to entanglement between the "observer" and the "observed". Am I understanding this correctly?? $\endgroup$ – Jack R. Woods Jun 20 '18 at 16:25
  • $\begingroup$ I've never heard of this idea being discussed seriously in academia. So, I can't comment on this. $\endgroup$ – Ari Jun 22 '18 at 9:30

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