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Physical Review A

Nonlinear pseudospin dynamics on a noncompact manifold

Dora M. Jezek, E. Susana Hernández

Published Jul 1, 1990• 13 citations
Publisher

Abstract

We describe the motion of an SU(1,1) pseudospin vector in the frame of the mean-field approximation induced by the variational principle on linear-plus-quadratic Hamiltonians. The dynamics of the SU(1,1) states of the Perelomov type obeys a nonlinear Bloch or torquelike equation, and each orbit can be interpreted as the intersection of two quadrics, one representing the energy surface and the other the group manifold, both in the space of the averaged algebra generators or semiclassical pseudospin. The fixed points of the flow can be also determined by resorting to strictly geometric considerations. The evolution of the phase diagram in parameter space is investigated as well for selected Hamiltonians. The bifurcation sets are constructed and the nonthermodynamic phase transitions can be clearly identified for the systems under consideration.

DOI
10.1103/physreva.42.96

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We describe the motion of an SU(1,1) pseudospin vector in the frame of the mean-field approximation induced by the variational principle on linear-plus-quadratic Hamiltonians. The dynamics of the SU(1,1) states of the Perelomov type obeys a nonlinear Bloch or torquelike equation, and each orbit can be interpreted as the intersection of two quadrics, one representing the energy surface and the other the group manifold, both in the space of the averaged algebra generators or semiclassical pseudospin. The fixed points of the flow can be also determined by resorting to strictly geometric considerations. The evolution of the phase diagram in parameter space is investigated as well for selected Hamiltonians. The bifurcation sets are constructed and the nonthermodynamic phase transitions can be clearly identified for the systems under consideration.
We analyze the possible bifurcations of the stationary points of the self-consistent flow on the Bloch sphere. We propose a generalized SU(2) model Hamiltonian for which, by means of a geometrically simple classification, we find the topologically invariant regions of the mean field flow in the space of interaction parameters. The borders of these regions are the bifurcation sets corresponding to general nonthermodynamic phase transitions. When these separatrices are crossed, the mean field flow undergoes a qualitative change. We also examine the consequences of the catastrophical configurations on the exact dynamics in quasispin space.
The semiclassical Dicke model and the ideal parametric amplifier are described by Hamiltonians linear in the SU(2) and SU( 1,1) group generators, respectively. We use the usual pseudospin vector as well as its SU(1,1) analog1 to give a unified description of the dynamics of the coherent states in these models. The pseudospin dynamics is governed by a precession (or pseudoprecession) equation, hence simple geometrical arguments can be used to infer the evolution of the coherent state parameters. To study fluctuations and squeezing we introduce a correlation tensor which is a higher-order generalization of the pseudospin vector. The correlation tensor obeys a generalized precession equation which can be factored into scalar, vector, and quadrupole parts and solved exactly by application of the Cayley-Hamilton theorem. Our approach to the calculation of squeezing and fluctuations provides an alternative to the more traditional approach employing disentangling theorems. It also provides an attractive and closely parallel geometrical description of coherent state evolution and squeezing in the two models.
The semi-classical Dicke model and the ideal parametric amplifier are described by Hamiltonians linear in the SU(2) and SU(1, 1) group generators, respectively. We use the usual pseudospin vector as well as its SU(1, 1) analog to give a unified description of the dynamics of the coherent states in these models. The pseudospin vector obeys a precession (or pseudoprecession) equation; hence simple geometrical arguments can be used to relate its evolution to that of the coherent-state parameters. To study fluctuations and squeezing we introduce the pseudospin tensor, which is a higher-order generalization of the pseudospin vector. The pseudospin tensor obeys a generalized precession equation that can be factored into scalar, vector, and quadrupole parts and solved exactly by application of the Cayley–Hamilton theorem. Squeezing is related directly to the behavior of the. pseudospin tensor and can therefore be calculated without invoking the group-theoretical disentangling formulas. Our approach is thus elementary in character. It also provides an attractive and closely parallel geometrical account of coherent-state evolution and squeezing in the two models.
In this paper, a nonlinear semiquantum Hamiltonian associated to the special unitary group SU(2) Lie algebra is studied so as to analyze its dynamics. The treatment here applied allows for a reduction in: 1) the system’s dimension, as well as 2) the number of system’s parameters (to only three). We can now discern clear patterns in: 1) the complete characterization of the system’s fixed points and 2) their stability. It is shown that the parameter associated to the uncertainty principle, which constitutes a very strong constraint, is the key one in determining the presence of fixed points and bifurcation curves in the parameter’s space.
Spectral properties of the Hamiltonian function which characterizes a trapped ion are investigated. In order to study semiclassical dynamics of trapped ions, coherent state orbits are introduced as sub-manifolds of the quantum state space, with the Kähler structure induced by the transition probability. The time dependent variational principle (TDVP) is applied on coherent states' orbits. The Hamilton equations of motion on Kähler manifolds of the type of classical phase spaces naturally arise. The associated classical Hamiltonian is obtained from the expected values on symplectic coherent states of the quantum Hamiltonian. Spectral information is thus coded within the phase portrait. We deal with the bosonic realization of the Lie algebra of the SU(1,1) group, which we particularize for the case of an ion confined in a combined, Paul and Penning trap. This formalism can be applied to Hamiltonians which are nonlinear in the infinitesimal generators of a dynamical symmetry group, such as the case of ions confined in electrodynamic traps. Discrete quasienergy spectra are obtained and the corresponding quasienergy states are explicitly realized as coherent states parameterized by the stable solutions of the corresponding classical equations of motion. A correspondence between quantum and classical stability domains is thus established, using the Husimi representation.
We investigate dynamics of the SU (1,1) coherent states with the use of the group transformations diagonalizing the coherence preserving Hamiltonian driving the physical system. The model physical system we consider may be viewed as a particular case of the generalized time-dependent harmonic oscillator, or as a generalization of the degenerate parametric amplifier, with the pumping field having modulated amplitude and a nonresonant phase. A Hamiltonian of such a system is given as a linear combination of the SU (1,1) generators with time-dependent coefficients, and the group transformations, mentioned above, transform this Hamiltonian to an expression containing only one generator, i.e. diagonalize the Hamiltonian. Trajectories of the complex coherent state parameter in the phase space (Lobachevskii plane) can be divided into two classes: compact trajectories never approaching the unit circle (boundary of the phase space) and noncompact trajectories approaching the unit circle from inside asymptotically, after sufficiently long time. The character of the dynamics is reflected by the time behaviour of the parameters of the group transformation diagonalizing the Hamiltonian. The main observation is that in the case of noncompact dynamics absolute values of the group parameters increase indefinitely, although in some cases they may exhibit a singular behaviour with regions of rapid variations, sudden changes of their values or cusplike singularities. For compact dynamics the group transformation parameters remain bounded and exhibit an oscillatory behaviour as functions of time.