TeX source:

\Pr\{m|i\} = |\mathbf{M}_m|\psi_i\rangle|^2 = \langle\psi_i|\mathbf{M}_m^\dagger\mathbf{M}_m|\psi_i\rangle = \text{tr}(\mathbf{M}_m^\dagger\mathbf{M}_m|\psi_i\rangle\langle\psi_i|).