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How stable/fragile is Quantum Entanglement?

  1. Jun 20, 2012 #1
    How stable/fragile is Quantum Entanglement? with respect to time, space and various events.


    Space - We know that experiments have validated QE working across say 150 kms and it is generally believed that they can work across very large distances. Does gravity effect QE over large distances?

    Time - How long can Alice and Bob keep their entangled photons bouncing of mirrors? (if that is how entangled photons are kept trapped)

    The interesting part is that between the two entangled photons there is so much background noise (in the form of EM radiations, other energy forms, matter and the various fundamental forces) yet entanglement survives.

    Yet QE "breaks" as soon as we try to measure one of the twins and it's believed, by some, that the photons at that point get entangled with something else (like parts of the measurement apparatus).

    It seems as if entangled particles are "connected" via a different mechanism/channel/dimension? and hence the noise does not effect them, yet events like measurement/detection does?
     
    Last edited: Jun 20, 2012
  2. jcsd
  3. Jun 20, 2012 #2
    Are you sure that entanglement isn't affected by noise? In fact, entanglement is very sensitive to the effects of noise. In a finite time interval, some combinations of noise can even cause entanglement to drop to zero within a finite time - "entanglement sudden death" (Yu and Eberly, 2004)
     
  4. Jun 20, 2012 #3
    ok, so entanglement is effected by noise. Now the question is what kind of things effect (and don't effect) quantum entanglement? Could this tell us more about the nature/properties of entanglement?
     
  5. Jun 20, 2012 #4
    If you are interested in finding out more about entanglement and have time, I refer you to this excellent review paper by the Horodecki family (yes, a whole family of physicists):
    http://arxiv.org/abs/quant-ph/0702225
     
  6. Jun 20, 2012 #5
    "Entanglement" is actually a fairly broad term. While it is true that entanglement that is used for things like quantum computation is usually pretty fragile and easily disturbed by noise, this is not true for all entangled state. In fact, entangled states can also be used to make an otherwise sensitive qubit, completely insensitive to certain types of noise, by putting it in what is called a decoherence free subspace.

    Also, some entangled states are even created BY noise, and can stay in this noisy environment indefinitely (see e.g. work by E. Polzik). However, one must remember that not all entangled states are useful in the sense of quantum computing and communication (think there was a paper by J. Eisert stating that most entangled states were not useful in fact), so the question of whether an entangled state is useful and how sensitive to noise it is, is very context dependent.
     
  7. Jun 20, 2012 #6
    Interesting information. Thanks Zargon and Fightfish, I will go through the references, you provided.
     
  8. Jun 20, 2012 #7
    Schrodinger said " One disposes provisionally (untill the entanglement is resolved by actual observation) of only a common description of the two in that space of higher dimension. This is the reason that the knowledge of the individual systems can decline to the scientist, even to zero, while that of the combined system remains continually maximal. best possible knowledge of a whole does not include best possible knowledge of its parts - and this is what keeps coming back to haunt us"

    What I think "Has it occurred to anyone that the two particles are getting entangled in a completely different dimension so that they are observable only as a unit and as soon as we try to study them as individual systems it should be zero as our instruments cannot detect any activity in those dimensions."

    am I right or wrong please do help me understand as I am new in this field.
     
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