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Modern Physics Letters B

CONTROL OF QUANTUM ROTOR WITH SPIN-1/2

Alexandre G. M. Schmidt, Bruno N. Martins

Published Jan 18, 2012• 0 citations
Publisher

Abstract

In this work, we study the time evolution of a quantum rotor with spin-1/2 interacting with a time-dependent magnetic flux. Choosing an adequate magnetic flux and applying it at certain special switching times ti and tf, we show how to drive a wave-packet from a starting position to a final one maximizing its probability.
DOI
10.1142/s0217984911500217

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We study the possibility of controlling a quantum rotor interacting with a time-dependent external field, namely, using a continuous or a piecewise continuous (bang-bang control) magnetic flux. Using the exact eigenfunctions we show how to steer a wave-packet constructed as a superposition of these eigenstates. In the first case we show how to steer a Gaussian wave-packet (GWP) to two other positions: one of them which would not be realized if the evolution were free, and the second is the one where wave-packet revival takes place. In the piecewise approach we implement a kind of tracking control, i.e. before steering it to the final target we achieve another given GWP.
The techniques of controlling the free evolution of a nonrelativistic charged particle by time-dependent magnetic fields are proposed and the possibility of more general operations upon the Schrödinger wave packet is discussed. It is found that a properly programmed sequence of magnetic pulses can invert the free evolution process, forcing an arbitrary wave packet to ‘‘go back in time’’ to recover its past shape. Our manipulation prescriptions hold also for nonrelativistic particles of arbitrary spin.
We look at time evolution of a physical system from the point of view of dynamical control theory. Normally we solve motion equation with a given external potential and we obtain time evolution. Standard examples are the trajectories in classical mechanics or the wave functions in Quantum Mechanics. In the control theory, we have the configurational variables of a physical system, we choose a velocity field and with a suited strategy we force the physical system to have a well defined evolution. The evolution of the system is the 'premium' that the controller receives if he has adopted the right strategy. The strategy is given by well suited laboratory devices. The control mechanisms are in many cases non linear; it is necessary, namely, a feedback mechanism to retain in time the selected evolution. Our aim is to introduce a scheme to obtain Quantum wave packets by control theory. The program is to choose the characteristics of a packet, that is, the equation of evolution for its centre and a controlled dispersion, and to give a building scheme from some initial state (for example a solution of stationary Schroedinger equation). It seems natural in this view to use stochastic approach to Quantum Mechanics, that is, Stochastic Mechanics [S.M.]. It is a quantization scheme different from ordinary ones only formally. This approach introduces in quantum theory the whole mathematical apparatus of stochastic control theory. Stochastic Mechanics, in our view, is more intuitive when we want to study all the classical-like problems. We apply our scheme to build two classes of quantum packets both derived generalizing some properties of coherent states.
We present a new effect of position-dependent mass (PDM) systems: the possibility of creating squeezed wave packets at the partial revival times. We solve exactly the PDM Schrödinger equation for the two-dimensional quantum rotor with two effective masses μ(θ), both free and interacting with a uniform electric field, and present their energy eigenvalues and eigenfunctions in terms of Mathieu functions. For the first one, in order to squeeze the wave packet it is necessary to apply an electric field; for the second one such an effect can be achieved without the field.