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

Beam Splitter - Commutation relations

  1. Nov 24, 2013 #1
    Hi guys, why does the following mean B is unitary?

    if we have two rotations such that;

    b1 = B11a1 + B12a2
    b2 = B21a1 + B22a2

    and the following commutator results are;

    [b1, b1(dagger)] = |B11|^2 + |B12|^2 --> 1

    [b2, b2(dagger)] = |B21|^2 + |B22|^2 --> 1

    [b1, b2(dagger)] = [B11 B*21] + B12 B*22 --> 0

    thus B is unitary.

    I'm assuming it's something to do with the probabilities adding to 1, but i'm hoping for a more 'visual' understanding.

    Thanks in advance.
  2. jcsd
  3. Nov 24, 2013 #2


    User Avatar
    Science Advisor
    Gold Member

    A transformation is unitary if the inner product between any two vectors remains unchanged before and after the transformation.

    So if a1 and a2 are orthonormal vectors, and the transformation is unitary, then b1 and b2 will also be orthonormal vectors.

    This is also a necessary and sufficient condition, so you can prove the transformation is unitary, if the inner products always remain the same.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook