From Lamb shift to light shifts: Vacuum and subphoton cavity fields measured by atomic phase sensitive detection.

We have measured by Ramsey interferometry the dispersive light shifts produced on circular Rydberg states by very weak nonresonant microwave fields in a single cavity mode. This experiment yields an absolute measurement of mean photon numbers with a sensitivity of 0.1. The vacuum induced Lamb shift, equal to the light shift of ``half a photon,'' has been observed, providing a direct and absolute measurement of the zero-point field fluctuations in a cavity mode.