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Physical Review A

High-order adiabatic representations of quantum systems through a perturbative construction of dynamical invariants

T. T. Nguyen‐Dang, Evgueni Sinelnikov, Arne Keller, Osman Atabek

Published Nov 30, 2007• 8 citations
Publisher

Abstract

A perturbative series is derived for the systematic construction of a dynamical invariant (Lewis invariant) for a time-dependent Hamiltonian which is characterized by a time-scale parameter $\ensuremath{\tau}$, as appears in the usual formulation of the adiabatic theorem. The derivations make efficient use of the quantum averaging method, and the perturbative series obtained permits the construction of the invariant in successive orders in $ϵ\ensuremath{\sim}1/\ensuremath{\tau}$, corresponding to high-order adiabatic approximations. The series can be considered and resummed analytically to all orders, yielding an exact invariant for an harmonic oscillator linearly driven by an external field. For a nondegenerate two-level system, approximate invariants have been obtained up to the fifth order and illustrate how the adiabatic approximations of increasing orders successively approach the exact dynamics. The construct is applicable also to Floquet Hamiltonians and, in this context, it furnishes high-order adiabatic representations for the time evolution of Floquet states associated with a material system in an aperiodic laser pulse. We illustrate in particular how an exact Floquet dynamical invariant, defining exact adiabatic transports of Floquet states, is obtained for a laser-driven harmonic vibrational mode.

DOI
10.1103/physreva.76.052118

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A perturbative series is derived for the systematic construction of a dynamical invariant (Lewis invariant) for a time-dependent Hamiltonian which is characterized by a time-scale parameter \ensuremathτ\ensuremath{\tau}\ensuremathτ, as appears in the usual formulation of the adiabatic theorem. The derivations make efficient use of the quantum averaging method, and the perturbative series obtained permits the construction of the invariant in successive orders in ϵ\ensuremath∼1/\ensuremathτϵ\ensuremath{\sim}1/\ensuremath{\tau}ϵ\ensuremath∼1/\ensuremathτ, corresponding to high-order adiabatic approximations. The series can be considered and resummed analytically to all orders, yielding an exact invariant for an harmonic oscillator linearly driven by an external field. For a nondegenerate two-level system, approximate invariants have been obtained up to the fifth order and illustrate how the adiabatic approximations of increasing orders successively approach the exact dynamics. The construct is applicable also to Floquet Hamiltonians and, in this context, it furnishes high-order adiabatic representations for the time evolution of Floquet states associated with a material system in an aperiodic laser pulse. We illustrate in particular how an exact Floquet dynamical invariant, defining exact adiabatic transports of Floquet states, is obtained for a laser-driven harmonic vibrational mode.
The Born-Fock adiabatic theorem is extended to all orders for some quadratic quantum systems with finitely or infinitely degenerate energy spectra. A prescription is given for obtaining adiabatic invariants to any order. For any quadratic quantum system with N degrees of freedom there are 2N linear adiabatic invariant series, which correspond to the 2N exact invariants. The exact quantum mechanical solution for any nonstationary quadratic quantum system is also constructed by making use of the coherent-state representation: The Green's function, coherent states, transition amplitudes and probabilities and their generating functions are obtained explicitly. Two particular systems, the N-dimensional time-dependent general oscillator and charged particle motion in a varying and uniform electromagnetic field, are considered in greater detail as examples.
The Floquet-Magnus expansion is a widely used tool to derive effective descriptions of time-periodic quantum systems by approximating their dynamics with a time-independent Hamiltonian. However, its standard formulation is, strictly speaking, restricted to bounded Hamiltonians. In this work, we extend its definition and analysis to a broad class of time-periodic unbounded Hamiltonians. Our approach is based on an a priori distinct nonperturbative framework for the construction of effective Hamiltonians, which we show to reproduce the Floquet-Magnus expansion. A particular strength of our framework is that it allows us to prove that the resulting effective dynamics approximates the original time evolution propagators to arbitrary order in the high-frequency limit without requiring convergence of the Floquet-Magnus expansion, a condition that is already highly restrictive even in the bounded setting. We illustrate the scope of the method on representative models: the quantum Rabi Hamiltonian in the interaction picture, and the periodically driven quantum harmonic oscillator.
We introduce a variational framework for approximating the time-evolution operator U^(t)\hat{U}(t)U^(t) within a physically motivated operator manifold, reformulating quantum dynamics as a tractable problem in operator space using stationary action principle. For periodically driven systems, the resulting approximate evolution operator directly yields an effective Floquet Hamiltonian, offering a non-perturbative alternative to conventional expansion-based methods. The framework is systematically improvable by enlarging the operator pool and naturally incorporates symmetries and physical constraints. When the operator manifold is chosen from the terms of a truncated Magnus expansion, the variational procedure effectively resums the Magnus series within the restricted space, significantly enhancing accuracy. We benchmark the approach on the driven Rabi model, the driven Lipkin-Meshkov-Glick model, and the one-dimensional driven Ising chain, yielding effective Floquet Hamiltonians that are systematically more accurate than low-order Magnus expansions, particularly in regimes where the latter converge poorly, and illustrating applicability to systems with exponentially large Hilbert spaces. Although we focus here on Floquet systems, the formalism applies equally to generic time-dependent Hamiltonians, providing a versatile tool for non-equilibrium quantum dynamics.
A precise definition of an adiabaticity parameter \ensuremath{\nu} of a time-dependent Hamiltonian is proposed. A variation of the time-dependent perturbation theory is presented which yields a series expansion of the evolution operator U(\ensuremath{\tau})=${\ensuremath{\sum}}_{\mathrm{\ensuremath{\ell}}}$${\mathrm{U}}^{(\mathrm{\ensuremath{\ell}})}$(\ensuremath{\tau}) with ${\mathrm{U}}^{(\mathrm{\ensuremath{\ell}})}$(\ensuremath{\tau}) being at least of the order ${\ensuremath{\nu}}^{\mathrm{\ensuremath{\ell}}}$. In particular, U(0){\mathrm{U}}^{(0)}U(0)(\ensuremath{\tau}) corresponds to the adiabatic approximation and yields Berry's adiabatic phase. It is shown that this series expansion has nothing to do with the 1/\ensuremath{\tau} expansion of U(\ensuremath{\tau}). It is also shown that the nonadiabatic part of the evolution operator is generated by a transformed Hamiltonian which is off-diagonal in the eigenbasis of the initial Hamiltonian. This suggests the introduction of an adiabatic product expansion for U(\ensuremath{\tau}) which turns out to yield exact expressions for U(\ensuremath{\tau}) for a large number of quantum systems. In particular, a simple application of the adiabatic product expansion is used to show that for the Hamiltonian describing the dynamics of a magnetic dipole in an arbitrarily changing magnetic field, there exists another Hamiltonian with the same eigenvectors for which the Schr\"odinger equation is exactly solvable. Some related issues concerning geometric phases and their physical significance are also discussed.
We construct an explicit Lewis-Ermakov-type dynamical invariant for time-dependent two-level systems by exploiting their algebraic correspondence with the time-dependent harmonic oscillator through the su(2)su(2)su(2) and su(1,1)su(1,1)su(1,1) algebras, which share the common complexification sl(2,C)sl(2,\mathbb{C})sl(2,C). This invariant provides a closed-form evolution operator and an exact propagator for arbitrary time-dependent coupling a(t)a(t)a(t) and detuning b(t)b(t)b(t). We illustrate the method on representative scenarios, including Landau-Zener transitions, non-Hermitian dissipative processes, and adiabatic rapid passage, and we obtain inverse-engineered shortcuts to adiabaticity with fully explicit control fields when the auxiliary scaling function is taken to be real.
We present details and expand on the framework leading to the recently introduced degenerate adiabatic perturbation theory [Phys. Rev. Lett. 104, 170406 (2010)], and on the formulation of the degenerate adiabatic theorem, along with its necessary and sufficient conditions [given in Phys. Rev. A 85, 062111 (2012)]. We start with the adiabatic approximation for degenerate Hamiltonians that paves the way to a clear and rigorous statement of the associated degenerate adiabatic theorem, where the non-Abelian geometric phase (Wilczek-Zee phase) plays a central role to its quantitative formulation. We then describe the degenerate adiabatic perturbation theory, whose zeroth-order term is the degenerate adiabatic approximation, in its full generality. The parameter in the perturbative power-series expansion of the time-dependent wave function is directly associated to the inverse of the time it takes to drive the system from its initial to its final state. With the aid of the degenerate adiabatic perturbation theory we obtain rigorous necessary and sufficient conditions for the validity of the adiabatic theorem of quantum mechanics. Finally, to illustrate the power and wide scope of the methodology, we apply the framework to a degenerate Hamiltonian, whose closed-form time-dependent wave function is derived exactly, and also to other nonexactly solvable Hamiltonians whose solutions are numerically computed.