Quantum XY Magnetism in a Two-dimensional Rydberg Atom Array

Optical tweezer arrays of strongly-interacting Rydberg atoms are an emerging platform for studying quantum magnetism. In this talk, we present an experimental and theoretical investigation of a new two-dimensional, square lattice system of up to 100 atoms, where each effective spin-1/2 is encoded in a pair of Rydberg states. The ordinary dipole-dipole interaction between two such Rydberg atoms then manifests as a long-range XY Hamiltonian, featuring a continuous U(1) spin-rotation symmetry. We design and use an adiabatic preparation scheme to realize low-temperature states of this XY model - for both ferromagnetic and antiferromagnetic XY couplings - and validate the efficacy of our protocol with extensive numerical simulations. We also demonstrate how to inject additional energy into the isolated system via a controlled quantum quench. Remarkably, with ferromagnetic XY coupling we observe off-diagonal long-range order in the spin correlation functions, even at finite effective temperature. Spontaneously breaking a continuous symmetry in this way is ordinarily forbidden by the Hohenberg-Mermin-Wagner theorem; here, the underlying long-range interactions are key.