  We report the first experimental observation of quantum holographic imaging
with entangled photon pairs, generated in a spontaneous parametric
down-conversion process. The signal photons play both roles of "object wave"
and "reference wave" in holography but are recorded by a point detector
providing only encoding information, while the idler photons travel freely and
are locally manipulated with spatial resolution. The holographic image is
formed by the two-photon correlation measurement, although both the signal and
idler beams are incoherent. According to the detection regime of the signal
photons, we analyze three types of quantum holography schemes: point detection,
coherent detection and bucket detection, which can correspond to classical
holography using a point source, a plane-wave coherent source and a spatially
incoherent source, respectively. Our experiment demonstrates that the
two-photon holography in the point detection regime is equivalent to the
one-photon holography using a point source. Physically, the quantum holography
experiment verifies that a pair of non-commutable physical quantities, the
amplitude and phase components of the field operator, can be nonlocally
measured through two-photon entanglement.
