  At nanometer separation, the dominant interaction between an atom and a
material surface is the fluctuation-induced Casimir-Polder potential. We
demonstrate that slow atoms crossing a silicon nitride transmission nanograting
are a remarkably sensitive probe for that potential. A 15% difference between
nonretarded (van der Waals) and retarded Casimir-Polder potentials is
discernible at distances smaller than 51 nm. We discuss the relative influence
of various theoretical and experimental parameters on the potential in detail.
Our work paves the way to high-precision measurement of the Casimir-Polder
potential as a prerequisite for understanding fundamental physics and its
relevance to applications in quantum-enhanced sensing.
