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Journal of Modern Optics

Time-dependent state populations with and without the rotating wave approximation: a model-based study

László Péter Biró, András Csehi

Published Aug 27, 2018• 4 citations
Publisher

Abstract

We present a benchmark study by comparing time-dependent electronic state populations calculated in the rotating wave approximation (RWA) to those obtained in numerical exact simulations without the application of the RWA. A wide range of laser intensities and energies are considered in order to explore the limits of the RWA. Our model system consists of a single electron confined to a finite potential well of atomic size so that there are five bound electronic states. The presented numerical study is carried out in the dipole approximation considering either two or all the five electronic states of the system.

DOI
10.1080/09500340.2018.1512673

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We present a benchmark study by comparing time-dependent electronic state populations calculated in the rotating wave approximation (RWA) to those obtained in numerical exact simulations without the application of the RWA. A wide range of laser intensities and energies are considered in order to explore the limits of the RWA. Our model system consists of a single electron confined to a finite potential well of atomic size so that there are five bound electronic states. The presented numerical study is carried out in the dipole approximation considering either two or all the five electronic states of the system.
A rotating-wave approximation (RWA) is developed to describe the interaction of a two-level system, which has permanent dipole moments, with two pulsed lasers. The RWA expressions for the time-dependent populations of the molecular states are applied to model laser-molecule interactions and tested by comparison with exact results. The results are used to discuss the pulsed-laser phase control of molecular excitation through the interplay of competing one- and two-photon resonances involving the effects of a nonzero difference d between the permanent dipoles of the two states involved in the transition; the competition vanishes if d=0.d=0.d=0.
The Tavis-Cummings model for more than one qubit interacting with a common oscillator mode is extended beyond the rotating wave approximation (RWA). We explore the parameter regime in which the frequencies of the qubits are much smaller than the oscillator frequency and the coupling strength is allowed to be ultrastrong. The application of the adiabatic approximation introduced by Irish et al. [Phys. Rev. B 72, 195410 (2005)] for a single-qubit system is extended to the multiqubit case. For a two-qubit system, we identify three-state manifolds of close-lying dressed energy levels and obtain results for the dynamics of intramanifold transitions that are incompatible with results from the familiar regime of the RWA. We exhibit features of two-qubit dynamics that are different from the single-qubit case, including calculations of qubit-qubit entanglement. Both number state and coherent state preparations are considered, and we derive analytical formulas that simplify the interpretation of numerical calculations. Expressions for individual collapse and revival signals of both population and entanglement are derived.
A rotating-wave approximation (RWA) is developed to describe the evolution of a two-level system, which has permanent dipole moments, interacting with a pulsed laser. Comparisons with exact calculations for one- and two-photon excitations involving the two lowest vibrational states of the ground electronic state of HeH+{\mathrm{HeH}}^{+}HeH+ are given to illustrate the validity of the RWA formula.
Dipole squeezing in the Jaynes-Cummings model with and without the rotating wave approximation (RWA) has been numerically studied when restricted to the following initial conditions: the field in the superposition of the vacuum and one-photon state, and the atom in the ground state. Numerical results show that even if under the condition for which the RWA is considered to be valid, there is a significant effect caused by the virtual-photon field on the dipole squeezing; in the cases of sufficiently strong coupling of the atom-field interaction, the interference between the virtual-photon processes and the real-photon processes quenches the dipole squeezing.
The problem of a single two level atom in its excited state, placed in a cavity in which no photons are present initially, and interacting with N modes of the electromagnetic field is solved exactly in the electric dipole, rotating wave approximation. Expressions for the energy eigenvalues and eigenvectors are derived, and an exact representation for the time dependent wavefunction of the system is given. These quantities are evaluated analytically for N=1,2 and 3, and the time dependent properties of the system for these values of N are investigated and discussed.
The generalized rotating-wave approximation (GRWA) is presented for the two-qubit quantum Rabi model. The analytical expressions in the zeroth-order approximation recover the previous adiabatic ones. The counter-rotating-wave terms can be eliminated by performing the first-order corrections. An effective solvable Hamiltonian with the same form as the ordinary RWA one is then obtained, giving very accurate eigenvalues and eigenstates. Energy levels in the present GRWA are in accordance with the numerical exact diagonalization ones in a wide range of coupling strengths. The population dynamics in the GRWA are in quantitative agreement with the numerical results and exhibit the absence of collapses clearly, revealing the effects of the counter-rotating wave.