  The three-wave mixing processes in a second-order nonlinear medium can be
used for imaging protocols, in which an object field is injected into the
nonlinear medium together with a reference field and an image field is
generated. When the reference field is chaotic, the image field is also chaotic
and does not carry any information about the object. We show that a clear image
of the object be extracted from the chaotic image field by measuring the
spatial intensity correlations between this field and one Fourier component of
the reference. We experimentally verify this imaging protocol in the case of
frequency downconversion.

Quantum Physics