Mapping continuous-variable quantum states onto optical scalar beams

Abstract

Optical fields provide an accessible platform to explore connections between classical and quantum mechanics. We introduce a group-theoretic framework based on the $\mathrm{su}(1,1)$ Lie algebra to construct classical analogs of continuous-variable quantum states using the spatial degree of freedom of paraxial scalar beams. Our framework maps squeezed number states onto scalar beams expanded in orthonormal Gaussian modal bases, encompassing both Gaussian and non-Gaussian classical analogs, including one- and two-mode squeezed beams. To characterize the structural changes induced by squeezing, we examine phase-space redistribution through Fourier analysis and optical Wigner distribution functions. We derive analytical expressions for the waist, curvature, and Gouy phase of two-mode squeezed Laguerre-Gaussian beams, and establish a relation between the number of accessible modes and the achievable squeezing under finite numerical aperture. While squeezing introduces spatial and spectral correlations that reshape the beam structure, these beams remain constrained by the diffraction limit, as confirmed by the numerical propagation of apodized beams. These correlations give rise to classical entanglement. We establish a classical analog of the Duan--Simon inseparability criterion for continuous-variable two-mode Gaussian states. For non-Gaussian squeezed states, we analyze the marginal optical Wigner distribution functions and identify phase-space features, such as negativity, that act as witnesses of classical continuous-variable entanglement. Our framework unifies classical analogs of continuous-variable quantum states through beam engineering, enabling quantum-inspired applications in optical imaging, metrology, and communication.