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

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


    User Avatar
    Science Advisor
    Gold Member

    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...
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Two state system
  1. Two-state system (Replies: 9)