Photon-number parity
= Photon-number parity
{title2=$\Pi=(-1)^N$}
Even and odd occupation sectors have <eigenvalues> $+1$ and $-1$. Pair creation preserves parity, whereas one <photon addition> or <photon subtraction> reverses it. An <optical squeezed vacuum> is even and a <squeezed one-photon state> is odd.