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Complex Light and Optical Forces

Pendulum beams: a window into the quantum pendulum

Enrique J. Galvez, Fabio J. Auccapuclla, Kristina L. Whittler, Yingsi Qin

Published Mar 1, 2019• 4 citations
Publisher

Abstract

Parallels between the Helmholtz and Schrodinger equations can be exploited for using light beams to investigate quantum problems. We present the study of a type of non-diffracting beams known as pendulum beams, where the optical modes satisfy a form of the Helmholtz equation that is identical to the Schrodinger equation for the mechanical pendulum. We prepared optical beams in the corresponding eigenmodes and made measurements of their Fourier spectrum. We find remarkable quantitative agreement between the measured angular spectrum and the quantum mechanical probabilities.

DOI
10.1117/12.2510822

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Parallels between the Helmholtz and Schrodinger equations can be exploited for using light beams to investigate quantum problems. We present the study of a type of non-diffracting beams known as pendulum beams, where the optical modes satisfy a form of the Helmholtz equation that is identical to the Schrodinger equation for the mechanical pendulum. We prepared optical beams in the corresponding eigenmodes and made measurements of their Fourier spectrum. We find remarkable quantitative agreement between the measured angular spectrum and the quantum mechanical probabilities.
Abstract The wave equation of electromagnetism, the Helmholtz equation, has the same form as the Schrödinger equation, and so optical waves can be used to study quantum mechanical problems. The electromagnetic wave solutions for non-diffracting beams lead to the two-dimensional Helmholtz equation. When expressed in elliptical coordinates the solution of the angular part is the same as the Schrödinger equation for the simple pendulum. The resulting optical eigenmodes, Mathieu modes, have an optical Fourier transform with a spatial intensity distribution that is proportional to the quantum mechanical probability for the pendulum. Comparison of Fourier intensities of eigenmodes are in excellent agreement with calculated quantum mechanical probabilities of pendulum stationary states. We further investigate wave-packet superpositions of a few modes and show that they mimic the libration and the nonlinear rotation of the classical pendulum, including revivals due to the quantized nature of superpositions. The ability to ‘dial a wavefunction’ with the optical modes allows the exploration of important aspects of quantum wave-mechanics and the pendulum that may not be possible with other physical systems.
Physical analogies are particularly useful to study not easily accessible systems. This is the case of the quantum pendulum, whose evolution in time is not easily observed. Fortunately, its mathematical description is shared by the Pendulum beams, which can be readily created in optical laboratories. We engineered mode superpositions that upon propagation mimic the time evolution of the quantum pendulum. We stablished experimental conditions in which the analogy is valid and present results for several relevant cases, finding excellent agreement between experiment and theoretical predictions. We expect to explore other superpositions to extend our analogy to pendulums with more complex dynamics.
In the traditional prescriptions, the Fermat’s principle of least time or/and the Helmholtz equation are used to derive the beam-optical Hamiltonian. Now, we know that the Maxwell’s electromagnetism gives rise to the correct theory of light. So, we developed a formalism of beam optics starting with an exact matrix-representation of Maxwell’s equations. The resulting beam-optical Hamiltonian is not only exact but also has the mathematical structure of the Dirac equation, enabling us to employ quantum methodologies. The resulting formalism of Maxwell optics successfully describes beam optics along with polarization of light.