State tomography of toroidal light pulses

Abstract Toroidal pulses, also known as Focused or Flying Doughnuts (FDs), are few-cycle pulses of doughnut-like topology that exhibit space-polarization and space-time couplings resulting in skyrmionic field configurations and regions of energy backflow, while their light-matter interactions lead to complex multipole excitations including toroidal moments. In this work, we experimentally generate optical toroidal pulses and analyze their space-polarization and space-time couplings using tomographic techniques. By tandem projective measurements in the space and polarization degrees of freedom, we characterize the spatially varying polarization of the pulses, whereas space-time nonseparability is characterized by measuring the angular divergence of the pulse monochromatic components. Space-time and space-polarization nonseparability is quantified by concurrence and fidelity (with respect to the ideal pulses). Experimentally generated pulses show high fidelity values of 0.88 and 0.72 for their space-polarization and space-time couplings, respectively. The reported results will be of interest to the fundamental study of toroidal pulses and spatiotemporally structured light more broadly, enabling applications in telecommunications, spectroscopy, metrology, and imaging.