  Light shining through wall experiments (in the optical as well as in the
microwave regime) are a powerful tool to search for light particles coupled
very weakly to photons such as axions or extra hidden sector photons. Resonant
regeneration, where a resonant cavity is employed to enhance the regeneration
rate of photons, is one of the most promising techniques to improve the
sensitivity of the next generation of experiments. However, doubts have been
voiced if such methods work at very low regeneration rates where on average the
cavity contains less than one photon. In this note we report on a demonstration
experiment using a microwave cavity driven with extremely low power, to show
that resonant amplification works also in this regime. In accordance with
standard quantum mechanics this is a demonstration that interference also works
at the level of less than one quantum. As an additional benefit this experiment
shows that thermal photons inside the cavity cause no adverse effects.
