In
Quantum Mechanics, an operator is called unitary if its
hermitian adjoint is equal to its
inverse, ie. U
+U = UU
+ = 1.
Unitary operators can be used to describe
symmetries of the space of quantum states; due to their property that they preserve the hermitian
inner product: that is to say, if under some transfromation |A
> goes to U|A
> and |B
> goes to U|B
>, then
<B| goes to
<B|U
+, and then