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Changing the spin of a Quantum particle

  1. Aug 14, 2008 #1
    I've been trying to find out if some one has found a way to meaningfully effect the spin of a
    quantum particle in such a way that it didn't destroy the particle or make the spin random. If I'm not being very clear, it may help to tell you I'm thinking about Quantum communication.
    Also, I am self taught, so if my understanding of the quantum state is grossly wrong, be kind.
  2. jcsd
  3. Aug 15, 2008 #2


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    Well, if the particle has a magnetic moment (e.g. an electron) you can just use a magnetic field. This is how NMR (and many other methods) works.
    Some systems (e.g. quantum dots) can also be manipulated using an electric field, although this is somewhat more complicated since the coupling is indirect.
  4. Aug 15, 2008 #3
    Would said particle being entangled effect the feasablity of these methods of manipulation?
  5. Aug 15, 2008 #4


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    No, not as long as you are not performing a measurement.
    When people talk about manipulating two-level systems (e.g. qubits) using "pi-pulses" etc they are usually referring controlling a quantum system using alternating magnetic fields with a certain frequency (which determines how close you are to being on resonance), amplitude (which sets the Rabi period) and duration (determines how much you are shifting the system).
    These manipulations can in the case of a single system be visualised using a Bloch sphere where the position of a vector indicates the state of the systems, using a magnetic field you can then rotate this vector.
    This can be generalized to entangled systems as well and is in fact how quantum algorithms that utilize entangled systems are implemented; a quantum gate is in fact usually just a sequence of pulses that somehow manipulates an ensemble of qubits.
  6. Aug 15, 2008 #5


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    A wave plate will shift the polarization, and will not end the entangled state of photons. This is applied routinely in Bell tests.
  7. Aug 16, 2008 #6
    Thank you for the information. This clears allot up for me.
  8. Aug 26, 2008 #7
    So can this not allow FTL communication? Because if left polarization means on (1) and right means off (0), can't you communicate in binary?
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