PME Special Quantum Seminar - A Universal Theory of Spin Squeezing and the Experimental Realization

Quantum metrology harnesses many-body entangled states to perform measurements with greater precision than would be possible using only classically correlated particles. Discerning states suitable for quantum metrology is a delicate task: nearly all states in Hilbert space are highly entangled, but nearly none of them exhibit the structured entanglement required for enhanced sensing. Identifying universal principles for finding metrologically useful states remains a long-standing challenge, especially in the context of efficiently preparing such states from unentangled product states. One such principle stems from the observation that the metrological gain from a pure state is fundamentally connected to spontaneous symmetry breaking. In this work, we apply this principle to the case of U(1) symmetry breaking and provide extensive numerical and analytical evidence for the following conjecture: Finite-temperature easy-plane ferromagnetism (i.e. XY magnets) enables scalable spin squeezing. In particular, we consider the quench dynamics of a low-energy initial state and show it undergoes squeezing as a precursor to the equilibration of long-range order. Additionally, I will present experimental pursuits to achieve scalable spin squeezing in such XY ferromagnetic systems.