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

Spin density matrix

  1. Aug 10, 2009 #1
    Could any one help me to understand how the spin density matrix is invariant under unitary transformation?
     
  2. jcsd
  3. Aug 12, 2009 #2
    hi,
    if you mean how a general density matrix is invariant under unitary transformation, the consider that density matrices are used to evaluate mean values of an operator A, <A R>= Tr(AR), where R is the density matrix and Tr denotes the trace operation. Then performing a unitary transformation yields the same result: Tr(SAS^-1 SRS^-1) = Tr(SARS^-1) = Tr(S^-1SAR) = Tr(AR) because of invariance under cyclic permutation of the trace.
    If you mean something else, please forget my answer and post again.
    thanks
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Spin density matrix
  1. Spin matrix (Replies: 2)

  2. Spin matrix (Replies: 30)

  3. Spin matrix (Replies: 1)

Loading...