Superconducting Casimir Forces on a Chip: Ultimate Parallelism with STM Readout

Abstract: Whether superconductivity changes the Casimir force is still an open question, with implications for fundamental electrodynamics, high-T_c superconductivity, and even speculative links to quantum gravity. Experiments have so far been unable to observe superconducting Casimir effects, because it is extremely hard to keep two surfaces parallel and to separate the Casimir force from other forces. We address this with a microchip platform: suspended NbTiN membranes held above a fixed backgate. This forms the most parallel plate-plate Casimir cavity to date by orders of magnitude, with nanometer-scale gaps that remain stable through the superconducting transition and no alignment uncertainty.
To read out the motion, we use a scanning tunneling microscope (STM) as a force sensor for the first time. It probes an atomic-scale spot on a 700 × 700 µm membrane. This local, minimally invasive readout lets us experimentally cancel all non-Casimir contributions, including electrostatic, van der Waals, and thermal forces. The resulting Casimir forces are orders of magnitude larger than in previous experiments. In this talk I will present the fabrication, the measurement protocol, and the first systematic exploration of Casimir forces in superconductors.