  We report on the passive measurement of time-dependent Green's functions in
the optical frequency domain with low-coherence interferometry. Inspired by
previous studies in acoustics and seismology, we show how the correlations of a
broadband and incoherent wave-field can directly yield the Green's functions
between scatterers of a complex medium. Both the ballistic and multiple
scattering components of the Green's function are retrieved. This approach
opens important perspectives for optical imaging and characterization in
complex scattering media.
