  We propose a variant of the heterodyne holography scheme that combines the
properties of off-axis and phase-shifting holography. This scheme makes it
possible to filter off numerically the zero-order image alias and the technical
noise of the reference. It is then possible to record and reconstruct
holographic images at an extremely low signal level. We show experimentally
that the sensitivity of the method is limited only by the quantum nature of
photons.
