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Two state system

  1. Sep 6, 2007 #1
    I have started to read from Sakurai's modern QM.At very first,he refers to two state systems in context of Stern Gerlach experiment.Indeed he says that this system is a two state system.Can anyone please tell me what is meant by a two state system?
  2. jcsd
  3. Sep 6, 2007 #2
  4. Sep 6, 2007 #3


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    In QM a two state system (sometimes called a two-level system) is a system which can be in one of just wo states, and these states are separated by an energy barrier.
    The most archetypical two-state system is a free electron which have either spin +1/2 or spin -1/2. This is the reason why you see two beams in a S-G experiment.

    However, many systems can be MODELLED as two-state systems, e.g. an atom participating in a process where only two energy levels are involved.
    Another example would be a quantum bit (qubit), which, by definition is a two-state system. Thi is why a qubit can be descibed using formalism that was originally developed to describe spin; even it the qubit is actually implemented using e.g. an electronic circuit..
  5. Sep 6, 2007 #4
    The Feynman Lectures Vol III (Quantum) does an excellent job with the Stern-Gerlach approach. Very worthwhile read if your interested in such.
  6. Sep 7, 2007 #5
    So,there is an energy barrier between the two possible states of the free electron?

    Now,I can see more clearly.Thank you...
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