Flow-equation approach to quantum systems driven by an amplitude-modulated time-periodic force

TL;DR

This paper introduces a flow equation method to analyze quantum systems under amplitude-modulated, time-periodic drives, enabling systematic high-order expansions and effective Hamiltonian derivation.

Contribution

The paper develops a flow equation approach that handles time-dependent envelopes in driven quantum systems, facilitating automated, high-order high-frequency expansions.

Findings

Effective Hamiltonian and micromotion operators derived

Flow generator prevents additional Fourier harmonics

Automated high-order expansions implemented in computer algebra

Abstract

We apply the method of flow equations to describe quantum systems subject to a time-periodic drive with a time-dependent envelope. The driven Hamiltonian is expressed in terms of its constituent Fourier harmonics with amplitudes that may vary as a function of time. The time evolution of the system is described in terms of the phase-independent effective Hamiltonian and the complementary micromotion operator that are generated by deriving and solving the flow equations. These equations implement the evolution with respect to an auxiliary flow variable and facilitate a gradual transformation of the quasienergy matrix (the Kamiltonian) into a block diagonal form in the extended space. We construct a flow generator that prevents the appearance of additional Fourier harmonics during the flow, thus enabling implementation of the flow in a computer algebra system. Automated generation of otherwise cumbersome high-frequency expansions (for both the effective Hamiltonian and the micromotion) to an arbitrary order thus becomes straightforward for driven Hamiltonians expressible in terms of a finite algebra of Hermitian operators. We give several specific examples and discuss the possibility to extend the treatment to cover rapid modulation of the envelope.