Frozen differential scattering in reconfigurable complex media

The sensitivity of transmission to the input wavefront is a hallmark feature of complex media and the basis for wavefront shaping techniques. Yet, intriguing special cases exist in which the output wavefront is "frozen" (agnostic to the input wavefront). This happens when special structure in the complex medium collapses the rank of its transmission matrix to unity. Here, we unveil that an analogous phenomenon exists more universally for differential scattering (including reflection) in reconfigurable complex media. Specifically, for a localized perturbation, the differential scattering matrix of any complex medium has rank one. One consequence is that the differential output signal is perfectly coherent irrespective of the input wavefront's coherence. Moreover, the thermal noise emitted into the frozen differential output mode has a particular structure that can be exploited for thermal noise management. We experimentally evidence frozen differential scattering in a rich-scattering wireless link parametrized by a programmable meta-atom. Then, we demonstrate "customized freezing" by optimizing the configuration of additional programmable meta-atoms that parametrize the wireless link, as envisioned for 6G networks. We impose particular shapes of the frozen differential output mode, and maximize its signal-to-thermal-noise ratio. Potential applications include filtering and stabilization of differential wavefronts, as well as imaging, sensing, and communication in complex media.