Entanglement beating in free space through spin-orbit coupling

摘要

It is well known that the entanglement of a quantum state is invariant under local unitary transformations.This rule dictates,for example,that the entanglement of internal degrees of freedom of a photon remains invariant during free-space propagation.Here,we outline a scenario in which this paradigm does not hold.Using local Bell states engineered from classical vector vortex beams with non-separable degrees of freedom,the so-called classically entangled states,we demonstrate that the entanglement evolves during propagation,oscillating between maximally entangled (purely vector) and product states (purely scalar).We outline the spin-orbit interaction behind these novel propagation dynamics and confirm the results experimentally,demonstrating spin-orbit coupling in paraxial beams.This demonstration highlights a hitherto unnoticed property of classical entanglement and simultaneously offers a device for the on-demand delivery of vector states to targets,for example,for dynamic laser materials processing,switchable resolution within stimulated emission depletion (STED) systems,and a tractor beam for entanglement.