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Physical Review Letters

Study of a quantum fermi-acceleration model

Jorge V. José, Robert Cordery

Published Jan 27, 1986• 124 citations
Publisher

Abstract

We consider the problem of a free particle in a rigid box with one wall fixed and the other oscillating periodically in time. The spectral properties of the evolution operator are calculated. We find that by a tuning of Planck's constant, the statistics of the quasienergy spectrum go continuously from showing Poisson to Gaussian-orthogonal-ensemble-like properties. This change is related to a transition from localized to extended states in energy space. We make the conjecture that recent results of microwave excitations on quasi one-dimensional highly excited hydrogen atoms may be related to this work.

DOI
10.1103/physrevlett.56.290

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•Jan 1, 2012•0 citations
We consider the problem of a free particle in a rigid box with one wall fixed and the other oscillating periodically in time. The spectral properties of the evolution operator are calculated. We find that by a tuning of Planck's constant, the statistics of the quasienergy spectrum go continuously from showing Poisson to Gaussian-orthogonal-ensemble-like properties. This change is related to a transition from localized to extended states in energy space. We make the conjecture that recent results of microwave excitations on quasi one-dimensional highly excited hydrogen atoms may be related to this work.
The quantum dynamics of a hydrogen-like atom confined in one-dimensional box with oscillating walls is studied.The description of the system is reduced to a one-dimensional Schrödinger equation for Coulomb potential with time-dependent boundary conditions, which is solved numerically.Using the obtained solution, the average kinetic energy and binding energies are calculated as a function of time.It is found that both the average kinetic energy and the binding energies are periodic in time with the period depending on the wall's oscillation parameters.The probability density is also analyzed as a function of time and coordinate.
Motivated by the Dirac idea that fundamental constants are dynamical variables and by conjectures on quantum structure of space–time at small distances, we consider the possibility that Planck constant [Formula: see text] is a time depending quantity, undergoing random Gaussian fluctuations around its measured constant mean value, with variance [Formula: see text] and a typical correlation timescale [Formula: see text]. We consider the case of propagation of a free particle and a one-dimensional harmonic oscillator coherent state, and show that the time evolution in both cases is different from the standard behavior. Finally, we discuss how interferometric experiments or exploiting coherent electromagnetic fields in a cavity may put effective bounds on the value of [Formula: see text].
Quantum localization is observed in the Fermi accelerator model with monochromatic fluctuation using the Ehrenfest description of quantum dynamics as well as through the quantum theory of motion (QTM). The effects of variations in time period and amplitude of an oscillating wall for a particle in a box, on energy and quantum phase space structures are analyzed. The effects of fluctuation decay with an increase in distance from the fluctuating wall. It is also observed that small amplitude as well as high frequency fluctuation provides enhanced quantum localization. The “quantum phase space structure” in the Ehrenfest description is constructed by following time-dependent Fourier grid Hamiltonian method while that for the grid points are formed from their trajectories obtained from QTM. © 2015 Wiley Periodicals, Inc.
We study the behavior of a quantum particle trapped in a confining potential in one dimension under multiple sudden changes of velocity and/or acceleration. We develop the appropriate formalism to deal with such situation and we use it to calculate the probability of transition for simple problems such as the particle in an infinite box and the simple harmonic oscillator. For the infinite box of length L under two and three sudden changes of velocity, where the initial and final velocity vanish, we find that the system undergoes quantum revivals for Δt=τ0≡4mL2πℏ, regardless of other parameters (Δt is the time elapsed between the first and last change of velocity). For the simple harmonic oscillator we find that the states obtained by suddenly changing (one change) the velocity and/or the acceleration of the potential, for a particle initially in an eigenstate of the static potential, are coherent states. For multiple changes of acceleration or velocity we find that the quantum expectation value of the Hamiltonian is remarkably close (possibly identical) to the corresponding classical expectation values. Finally, the probability of transition for a particle in an accelerating harmonic oscillator (no sudden changes) calculated with our formalism agrees with the formula derived long time ago by Ludwig [Z. Phys. 130(4), 468–475 (1951)], and recently modified by Dodonov [J. Russ. Laser Res. 42(3), 243–249 (2021)], but with a different expression for the dimensionless parameter γ. Our probability agrees with the one of Dodonov for γ ≪ 1 but is not periodic in time (it decays monotonously), contrary to the result derived by Dodonov.
We study quantum dynamics of one-electron atom confined in a spherical impenetrable box with time-dependent radius. The behavior of the atomic electron interacting with the moving walls of the box is analyzed by computing average binding energy, average force and pressure. Linearly extending, contracting and harmonically breathing boxes are considered in the non-adiabatic regime i.e. when the wall of the box moves with high velocities. It is shown that linearly extending box leads to de-excitation of the atom, while the rapidly contracting box causes the creation of very high pressure on the atom and transition of the atomic electron into the unbound state. In harmonically breathing box diffusive excitation of atomic electron and ionization may occur in analogy with that for atom in a microwave field.
The idea of wave packet, that clarified many conceptual difficulties, has more recently been revisited to address interferometric and dynamical issues.Present work focuses on the spatio-temporal dynamics of the quantum wave packets in the Fermi-Ulam Accelerator Model.The specific system considered here is a one-dimensional box under periodic oscillations at one boundary. Such a seemingly simple system can be mapped to non-trivial problems like BEC trapped in an oscillating cavity or dynamics of atomic wave packet in different potential in presence or absence of gravitational field.The system is shown to display rich dynamical features including transitions from regular to stochastic and further to chaotic domain in classical phase space. We have obtained the approximated eigen-value solutions of the problem through time dependent Schrodinger’s equation under rescaling transformations that effectively replace moving boundary with a static one. Consequently the running time becomes a nonlinear function of the continuously changing boundary.Based on these solutions, revivals or the resurrection of the original wave packet, in the driven and un-driven Fermi-Ulam model are presented. Such revivals correspond to the constructive interference whereas the wave packet’s collapse owes to destructive interference. Furthermore, interplay of constructive and destructive interferences also readily mimics the fractional revivals.It is further demonstrated that initial position and momentum play a highly crucial role in the emergent revival and fractional revival patterns.It is shown that the symmetries exhibited by such structures in un-driven systems are usually lost under driven conditions and the revival/fractional revival times can only be predicted through a transcendental equation whose solution mainly depends on the strength of the external driving force.In present thesis we have proposed a comprehensive mathematical technique to calculate these hitherto unpredictable times.In addition, a graphical method is also suggested to observe these asymmetric revival/fractional revival times.This apparent asymmetry, do display an overall periodicity over a comparatively long span covering many times of revivals.It is found that whenever the ratio between the period of wall oscillation and quantum revival time `α/β' is a rational number then the revivals time shows a periodicity after β number of revival times over α wall oscillation. Standard tools of autocorrelation and quantum carpets, a consequence of quantum interference, have been employed for visual manifestation of these revivals.These carpets amply demonstrate the explicit asymmetric shifting of revival times for the driven case.These micro changes exhibited in quantum carpet morphologies consequently yield amended phase velocities associated with the canals and ridges in the driven framework.Furthermore, to grasp the quantum phase space dynamics in its entirety, we have also utilized the more sophisticated techniques of the Wigner function and information entropy.It is demonstrated mathematically that revivals not apparent via autocorrelation become prominent in the Wigner phase space description along with the appearance of smooth mini wave packets in the fractional revivals.These interference patterns emerge due to the state vector formed through the linear combination of Gaussians with different positions and momenta having pure Gaussian as well as oscillatory terms. Moreover, the information entropy, like the Wigner function also predicts the revivals when and where they happen.Thus a rich pattern of fractional revivals, including some altogether new ones, are observed through this tool in both driven and un-driven situations.Finally we have elucidated various scenarios including cold atoms in magneto-optical traps where such a system can be experimentally realized along with the suggestions for future work in this specific domain