Architecture-agnostic analysis of partially coherent light with programmable photonics.

Precise characterization of the spatial degree of coherence in a radiation field is essential for evaluating its suitability in applications such as optical communications and advanced imaging. Conventional methods for spatial coherence characterization require complex and phase-sensitive interferometric setups that are highly susceptible to noise and challenging to scale into integrated platforms. To address this, this Letter presents an architecture-agnostic approach compatible with any universal programmable photonic unitary circuit for the analysis of partially coherent light, independent of the internal topology. Leveraging the Schur-Horn theorem, our method diagonalizes the incoming coherence matrix, enabling direct extraction of its eigenvalues from output power measurements alone. We numerically validate this framework across various universal topologies and demonstrate its efficacy even in under-parameterized, non-universal architectures with only minor loss in precision. Finally, our black-box approach proves inherently resilient to arbitrary optical losses and component deviations, paving the way for robust, lower-depth, and programmable spatial coherence analyzers.