  Through the $\eta$-inner product, we investigate $\cal PT$-symmetric quantum
mechanics from the viewpoint of superposition and coherence theory. It is
argued that $\cal PT$-symmetric quantum systems are endowed with stationary
superposition or superposition-free properties. A physical interpretation of
$\eta$-inner product in $\mathbb C^2$ is given through the Stokes parameters,
showing the difference between broken and unbroken $\cal PT$-symmetric quantum
systems.
