Internal clock interferometry for testing fundamental physics

Atomic clocks and atom interferometers are important platforms for tests of fundamental physics. Here we propose a clock comparison scheme relying on an internal atomic clock Ramsey interferometer comprising two interfering clock states within one atom. The atoms are prepared in a superposition of two clock states and one ground state. Over the coherent evolution of this three-level system, the differential phase accumulation between the two clock states leads to high-contrast interference fringes. The ground state population exhibits a corresponding fringe visibility modulation. Because the two clocks are located within the same particle, all clock shifts are fully correlated. With no spatial splitting or recombining being involved, the full intrinsic system coherence time can be exploited. Implemented in appropriate multi-clock state systems, e.g., Yb+, this scheme opens up the path to novel differential clock spectroscopy protocols with enhanced detection sensitivities for non-Standard Model physics.