Observation of the Scharnhorst effect via multipass Casimir - Lloyd interferometry

The Scharnhorst effect – a quantum-electrodynamic prediction that light propagates faster than c between closely spaced conducting plates due to modified vacuum fluctuations – has remained experimentally inaccessible since its theoretical inception in 1990. Here, we propose and theoretically validate a novel interferometric scheme that circumvents the need for direct velocity measurement by detecting the accumulated phase shift of photons traversing a nanoscale Casimir cavity in a multipass Lloyd interferometer. By combining angular scanning, high-reflectivity mirrors, quantum-noise suppression, and resonant signal amplification, our design achieves a signal-to-noise ratio exceeding unity for realistic experimental parameters (5 nm plate separation, 10 nm XUV radiation, cryogenic operation). This work opens a feasible pathway toward the first empirical test of superluminal photon propagation in modified quantum vacuum, with implications for fundamental physics, quantum field theory in bounded geometries, and vacuum engineering.