Removing exactly zero photons from a quantum optical state can produce significant changes in its statistical properties. We review our recent experimental and theoretical results in this area, which have applications in quantum communications.
The process of single-photon subtraction --essentially an implementation of the annihilation operator-- is known to be a useful tool for manipulating quantum optical states. Somewhat surprisingly, subtracting zero photons can also alter the properties of certain quantum states, and forms the bases of a noiseless attenuation process that is useful for quantum communication. Here we review our recent experimental work in this area, which uses parametric down-conversion sources and heralding signals based on the active detection of zero photons.