Direct comparison of two spin-squeezed optical clock ensembles at the 10⁻¹⁷ level

Building scalable quantum systems that demonstrate performance enhancement based on entanglement is a major goal in quantum computing and metrology. The main challenge arises from the fragility of entanglement in large quantum systems. Optical atomic clocks utilizing a large number of atoms have pushed the frontier of measurement science, building on precise engineering of quantum states and control of atomic interactions. However, state-of-the-art optical atomic clocks are limited by a fundamental source of noise stemming from fluctuations of the population of many atoms—the quantum projection noise. Here, we present an optical clock platform integrated with collective strong-coupling cavity quantum electrodynamics for quantum non-demolition measurements. Optimizing the competition between spin measurement precision and loss of coherence, we measure a metrological enhancement for a large ensemble of atoms beyond the initial coherent spin state. Furthermore, a movable lattice allows the cavity to individually address two independent subensembles, enabling us to spin squeeze two clock ensembles successively and compare their performance without the influence of clock laser noise. Although the clock comparison remains above the effective standard quantum limit, the performance directly verifies 1.9(2) dB clock stability enhancement at the 10⁻¹⁷ level without subtracting any technical noise contributions.