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Complex Light and Optical Forces

Propagating pendulum beams to simulate the quantum-pendulum time evolution

Valeria Rodríguez-Fajardo, Thao P. Nguyen, Enrique J. Galvez

Published Mar 15, 2023• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.2649330

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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.
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.
Summary form only given. The quantum driven pendulum can be implemented using laser cooled rubidium atoms which are exposed to a far detuned amplitude modulated standing wave. This system allows controlled variation of the effective Planck's constant and the amount of coupling to the environment (decoherence). Therefore it is possible to undertake experiments in both classical and quantum regimes of the dynamics of these atoms. The potential can be extremely well approximated as one-dimensional, making the system ideal for simulation using a master equation or quantum trajectories. The dynamics of the system feature distinct regions of regular motion (second order resonances) bounded by a sea of chaos. The resonances are observed as side peaks in the atomic momentum distribution, which each contain up to 30% of the atoms initially in the trap. We report on the results of our experiments to achieve quantum phase space tunneling. After adiabatic elimination of the excited state, the Hamiltonian describing the system is given. The evolution of the quantum driven pendulum can be described with the Floquet operator. Eigenstates of the Floquet operator may be identified with fixed points in phase space. Starting the evolution in a superposition of two Floquet states which is localised in one resonance, time evolution leads to localisation in the other fixed point. This is a coherent tunneling process forbidden by classical dynamics. To observe this tunneling process it is important to localise the superposition of Floquet states in one fixed point. We discuss the effectiveness of alternative methods of loading most of the atoms into a single resonance in order to observe quantum phase space tunneling.
Taking the classical pendulum as its starting point, this paper unearths the profound physics hidden within this familiar model and constructs an investigative path that links macroscopic classical mechanics to microscopic quantum mechanics. Using MATLAB, we first perform numerical simulations and visual analyses of the dynamical behavior of ideal, large-angle, damped, and externally driven pendulums. By next studying a pendulum whose pivot is subjected to periodic vertical vibration, we introduce and elucidate the classical phenomenon of parametric resonance. Building on this bridge, we extend the model into the quantum realm and demonstrate that, in the one-dimensional quantum harmonic oscillator, periodic modulation of a system parameter (e.g., the mass) can excite particles from the quantum vacuum—an effect known as “quantum vacuum amplification.” Simulations of particle-number evolution and Wigner functions carried out with Python corroborate the theoretical predictions. This study provides an intuitive yet inspiring case for undergraduate and graduate physics education.
We present an analytical description of the tunneling dynamics between two coupled Bose-Einstein condensates in the Josephson regime. The model relies on the classical analogy with a rigid pendulum and focuses on two dynamical modes of this system: Josephson oscillations and macroscopic quantum self-trapping. The analogy is extended to include an energy difference between the two superfluids caused by an asymmetry in the trapping potential. The model is compatible with the mean-field predictions of the two-mode Bose-Hubbard model. It gives insights on the mean-field model by involving experimentally measurable parameters with reduced correlations. For agreement with recent experimental observations, we establish heuristic formulas including a dissipation. We conclude with a convincing application of the model to several sets of experimental results.
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.