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Physical Review A

Fractional quantum revivals in the atomic gravitational cavity

Wenyu Chen, Gerard J Milburn

Published Mar 1, 1995• 22 citations
Publisher

Abstract

In this paper we discuss the quantum dynamics and fractional quantum revivals of an integrable nonlinear system, consisting of an atom bouncing vertically from an evanescent field, for two cases with the simplified infinite-potential and the more practical exponential potential, respectively. We study the two cases separately, then contrast and compare the results and reach the conclusion that provided the starting position of the atoms is not too close to the reflecting surface supporting the evanescent wave (this condition is always satisfied in present experiments in this field), the two cases will produce the same results. This means that the idealized infinite potential is a good approximation to the more realistic exponential potential. Because the quantum analysis of the infinite-potential case is quite simple and straighforward (since its Schr\"odinger equation has analytical solutions), this will greatly simplify the quantum analysis of the more complicated exponential potential case and hence has practical significance.

DOI
10.1103/physreva.51.2328

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•Jan 1, 2012•0 citations
In this paper we discuss the quantum dynamics and fractional quantum revivals of an integrable nonlinear system, consisting of an atom bouncing vertically from an evanescent field, for two cases with the simplified infinite-potential and the more practical exponential potential, respectively. We study the two cases separately, then contrast and compare the results and reach the conclusion that provided the starting position of the atoms is not too close to the reflecting surface supporting the evanescent wave (this condition is always satisfied in present experiments in this field), the two cases will produce the same results. This means that the idealized infinite potential is a good approximation to the more realistic exponential potential. Because the quantum analysis of the infinite-potential case is quite simple and straighforward (since its Schr"odinger equation has analytical solutions), this will greatly simplify the quantum analysis of the more complicated exponential potential case and hence has practical significance.
We calculate the quantum revival time for a wave packet initially well localized in a one-dimensional potential in the presence of an external periodic modulating field. The dependence of the revival time on various parameters of the driven system is shown analytically. As an example of an application of our approach, we compare the analytically obtained values of the revival time for various modulation strengths with the numerically computed ones in the case of a driven gravitational cavity. We show that they are in very good agreement.
We calculate the modes in a stable gravitational cavity where the atomic mirror at the bottom of the bounce is caused by an internally reflected intense Gaussian-mode laser beam. In contrast to Wallis, Dalibard, and Cohen-Tannoudji [Appl. Phys. B 54, 407 (1992)], who treated the bottom bouncing mechanism as due to an infinite potential, we include the effect of the exponentially decaying potential caused by the evanescent wave. Nevertheless, we find that above this evanescent wave potential the mode structure corresponds exactly to that obtained using an infinite potential. The allowed energies differ slightly due to the phase change through the evanescent wave. The broadening of the modes due to the damping effects of spontaneous emission was calculated and because of the intrinsic transverse width of the modes was found to depend strongly on energy and transverse excitation, which differs from a simple one-dimensional classical calculation. The finesse and QQQ of the cavity due to the damping effects of spontaneous emission were also calculated and in contrast to an optical Fabry-P\'erot cavity were found to depend strongly on energy.
Considerable progress has recently been made in controling the motion of free atomic particles by means of light pressure exerted by laser radiation. The free fall of atoms and bouncing on a reflecting surface made from evanescent wave formed by internal reflection of a quasiresonant laser beam at a curved glass surface in the presence of homogeneous gravitational field has been observed. In this paper we present the energy quantization of this system by making use the asymptotic expansion method. It is shown that for large nnn the levels go like n2/3n^{2/3}n2/3 which may be compared with n2n^2n2 for the infinite square well.
Wave-packet fractional revivals is a relevant feature in the long time-scale evolution of a wide range of physical systems, including atoms, molecules, and nonlinear systems. We show that the sum of information entropies in both position and momentum conjugate spaces is an indicator of fractional revivals by analyzing three different model systems: (i) the infinite square well, (ii) a particle bouncing vertically against a wall in a gravitational field, and (iii) the vibrational dynamics of hydrogen iodide molecules. This description in terms of information entropies complements the usual one in terms of the autocorrelation function.
We have investigated quantum revivals of wave packets in a one-dimensional infinite square well potential containing a $\ensuremath{\delta}$ well in the middle. The time-dependent Schr\"odinger equation for this composite potential admits formally exact solutions. We present analytical results for revival properties in three physically motivated approximations: wave packets containing eigenstates with large numbers in the presence of an arbitrary $\ensuremath{\delta}$ well, ``shallow'' and ``deep'' $\ensuremath{\delta}$ wells. Analytical results in the case of a ``shallow'' $\ensuremath{\delta}$ well have been tested numerically.
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