  Zero-photon subtraction (ZPS) is a conditional measurement process that can
reduce the mean photon number of quantum optical states without physically
removing any photons. Here we show that ZPS can also be used to transform
certain super-Poissonian states into sub-Poissonian states, and vice versa.
Combined with a well-known "no-go" theorem on conditional measurements, this
effect leads to a new set of non-classicality criteria that can be
experimentally tested through ZPS measurements.
