  We present an efficient and robust method for the reconstruction of photon
number distributions by using solely thermal noise as a probe. The method uses
a minimal number of pre-calibrated quantum devices, only one on/off
single-photon detector is sufficient. Feasibility of the method is demonstrated
by the experimental inference of single-photon, thermal and two-photon states.
The method is stable to experimental imperfections and provides a direct,
user-friendly quantum diagnostics tool.
