  The degree of optical spatial coherence -a fundamental property of light that
describes the mutual correlations between fluctuating electromagnetic fields-
has proven challenging to control at the micrometer scale. Here we employ
surface plasmon polaritons -evanescent waves excited on both surfaces of a thin
metal film- as a means to entangle the random fluctuations of the incident
electromagnetic fields at the slit locations of a Young's double-slit
interferometer. Strong tunability of the complex degree of spatial coherence of
light is achieved by finely varying the separation distance between the two
slits. Continuous modulation of the degree of spatial coherence with amplitudes
ranging from 0% up to 80% allows us to transform totally incoherent incident
light into highly coherent light, and vice versa. These findings pave the way
for alternative methods to engineer flat optical elements with multi-functional
capabilities beyond conventional refractive- and diffractive-based photonic
metasurfaces.
