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.