Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Quantum Computing

  1. Aug 1, 2006 #1
    QC requires atoms to be isolated? because the slightest disturbance can cause its state to be changed and data loss. Are the spin/ angular momentum etc. of a particle affected by em waves?
    if theres quantum fluctuations in space, so u have virtual particles/energy swapping, and its impossible to reduce this to 0, doesn't that mean you can never get an isolated atom?
  2. jcsd
  3. Aug 1, 2006 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    No, it does not.
  4. Aug 1, 2006 #3
    I thought that any environmental interaction can cause the particles to decohere so QC doesn't work unless theyre isolated.
  5. Aug 1, 2006 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Did you mean that you need a system that's fairly isolated from surroundings or that you needed individual atoms? I thought you meant the latter.

    The question of importance is "how long can we make the decoherence times?" I think typical values for some of the prospective systems are in the nanosecond range (or smaller, I'm not sure).
    Last edited: Aug 1, 2006
  6. Aug 1, 2006 #5
    There's actually a huge range of deoherence times, >15 orders of magnitude. I googled up this table:

    Ions in electromagnetic traps have macroscopic decoherence times, they're very well isolated. Nuclear dipoles interact so weakly that NMR qubits are coherent for on the order of ~10^4 seconds (essentially forever).
    Last edited by a moderator: Apr 22, 2017
  7. Aug 1, 2006 #6
    Just to add my two cents to this point..it is more about how many gate operations can be performed before the systems irrevocably decoheres.

    The distinction being that (relatively) long decoherence times do not help if it also takes a (relatively) long time to perform an operation.
  8. Aug 1, 2006 #7
    QC requires a well-isolated system, i.e. little coupling with the environment. But experimentally, that isolation is not perfect of course. However, decoherence can be "overcome" by error correction at the tradeoff of more qubits.

    In NMR, the spin is controlled via emf tuned at the proper resonant frequencies, etc.. You should look up the Bloch sphere for information on this.

    In QC processing, one has to control the qubit (ions, atoms, molecule ensemble, superconducting circuits, etc) and take measurements to get an answer. This inherently rules out a perfectly isolated qubit.
  9. Aug 3, 2006 #8
    Are their any means of performing checks to assure that decoherence is accounted for: an analogy would be, parity checks in a simple computer system, that allow for manipulation of erata in data sets.

    You'll have to excuse my ignorance about Qcomputing on this one; in a broader sense I'm asking if we have yet found a way to make quantum computing a good prospect, or is it still in the realms of the hypothetical?
  10. Aug 4, 2006 #9
    there is the repeat-until-success methodology of cluster states:

  11. Aug 4, 2006 #10


    User Avatar
    Gold Member

    wait, don't atoms spontaneously decay? Wouldn't that be unproductive to data keeping? Or is there a method/technique? Or am I misunderstanding atoma?
  12. Aug 4, 2006 #11
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook