Extended Data Fig. 1: Topological features in the two-dimensional XY model.
From: Observation of topological phenomena in a programmable lattice of 1,800 qubits
a, The ground state, in which all rotors have equal angle θ, has continuous U(1) symmetry, as a universal rotation of θ will not change the energy of the system. b, c, A vortex or antivortex (b and c, respectively) is a point around which the angle winds (rotates completely) clockwise or anticlockwise, respectively, when traversing a closed loop around the point in a clockwise direction. Both the energy and the entropy of an isolated vortex or antivortex region of radius R are proportional to log(R/r), where r is the lattice spacing.
