Adding or Subtracting a single Photon is the same for Pure Squeezed Vacuum States

The addition of a single photon to a light field can lead to exactly the same outcome as the subtraction of a single photon. We prove that this \cterm is true for pure squeezed vacuum states of light, and in some sense only for those. We show that mixed states can show this \cterm for addition or subtraction of a photon if they are generated from incoherent sums of pure squeezed vacuum states with the same squeezing. We point out that our results give a reinterpretation to the fact that pure squeezed vacuum states, with squeezing e^-z, are formally annihilated by Bogoliubov-transformed annihilation operators: hat a_z = hat a cosh(z) - hat a^dagger sinh(z).