  We report on the experimental demonstration of single photon state generation
and characterization in an electron microscope. In this aim we have used low
intensity relativistic (energy between 60kV and 100 keV) electrons beams
focused in a ca 1 nm probe to excite diamond nanoparticles. This triggered
individual neutral Nitrogen-vacancies (NV0) centers to emit photon which could
be gathered and sent to a Hanbury Brown Twiss intensity interferometer. The
detection of a dip in the correlation function at small time delays clearly
demonstrates antibunching and thus the creation of non-classical light states.
Specifically, we have also demonstrated single photon state detection. We
unveil the mechanism behind quantum states generation in an electron
microscope, and show that it clearly makes cathodoluminescence the nanometer
scale analog of photoluminescence rather than electroluminescence. By using an
extremely small electron probe size and the ability to monitor its position
with sub nanometer resolution, we also show the possibility of measuring the
quantum character of the emitted beam with deep sub wavelength resolution.
