Propagation-invariant high-dimensional orbital angular momentum states

Abstract

Photonic states encoded in spatial modes of paraxial light fields provide a promising platform for high-dimensional quantum information protocols and related studies, where several pioneering theoretical and experimental demonstrations have paved the path for future technologies. Crucially, critical issues encountered in free-space propagation still represent a major challenge. This is the case of asynchronous diffraction between spatial modes with different modal orders, which experience variations in their transverse structure upon free-space propagation. Here we address this issue by proposing an encoding method based on the use of Laguerre-Gaussian (LG) modes of the same modal order N to define a N + 1 dimensional space. Noteworthy, such modes endowed with orbital angular momentum (OAM) experience the same propagation aberrations featuring an identical Gouy phase and wavefront curvature. We demonstrate our proposal experimentally by using time-correlated-single-photon imaging combined with a digital propagation technique. Importantly, our technique allows to eliminate, without the use of imaging systems, all issues related to asynchronous diffraction, providing an accessible way to generate propagation-invariant OAM qudits for quantum optical protocols.

Directly Determining Orbital Angular Momentum of Ultrashort Laguerre–Gauss Pulses via Spatially‐Resolved Autocorrelation Measurement

“…[25,26] Importantly, this commonly used technique requires a precise priori knowledge of some of the parameters of the unknown beam, such as its initial beam width radius w 0 and its radius of curvature R z , for an accurate characterization of the mode in question. [27][28][29] For ultrashort pulses, transverse dispersion induced by the grating of complex-amplitude modulation would bring additional issues. Although a recent work has suggested that one can measure OAM of intense ultrashort pulses by exploiting strong-field ionization, [30] a simple technique towards commercial uses has not been developed.…”