Entanglement-Enhanced Frequency Comparison of Two Optical Atomic Clocks

Optical atomic clocks are our most precise tools to measure time and frequency [1]–[3]. They enable precision frequency comparisons between atoms in separate locations to probe the space-time variation of fundamental constants [4] and the properties of dark matter [5], [6], to perform geodesy [4], [7], [8] and to evaluate systematic clock shifts. Measurements on independent systems are limited by the standard quantum limit (SQL); measurements on entangled systems, in contrast, can surpass the SQL to reach the ultimate precision allowed by quantum theory - the so-called Heisenberg limit. While local entangling operations have been used to demonstrate this enhancement at microscopic distances [9]–[14], frequency comparisons between remote atomic clocks require rapid high-fidelity entanglement between separate systems that have no intrinsic interactions.