Kinematics and rotation of a vortex lattice in a polariton fluid

We study the kinematic properties of a rotating vortex lattice imprinted on a freely expanding polariton quantum fluid. Thanks to the optical component of polaritons, we can directly measure the phase and modulus of the complex-valued macroscopic wavefunction, and extract the velocity and angular momentum profiles across the condensate. These are in agreement with quantisation prescription, although the non-uniform and unsteady background density profile may induce corrections to the vortex trajectories, and result into fractional orbital angular momentum per particle.