Application of Magnus expansion for the quantum dynamics of $Λ$-systems under periodic driving and assessment of the rotating wave approximation

Abstract: Employing a sixth order expression for the differential time evolution operator based on the Magnus expansion (ME), we conducted quantum dynamics calculations of a $\Lambda$-system driven by two sinusoidal time dependent fields. For a closed system dynamics, we confirmed the equivalence of the dynamics in the Hilbert space and the Liouville space numerically. We also conducted open system quantum dynamics calculation by generalizing the ME to the non-Hermitian dynamics in the Liouville space for the case where the effects of photonic bath are represented by Lindblad operators. In both cases, the accuracy of the rotating wave approximation (RWA) was assessed. We found significant errors of RWA during initial stages of the dynamics for representative cases where electromagnetically induced transparency or coherent population trapping can be observed. The presence of bath for open system quantum dynamics reduces the errors of RWA, but significant errors for off-diagonal elements of the density operator can still be seen. We also found that approaches to steady state limits of exact dynamics are slower than those for RWA. These results demonstrate the utility of the ME as a general and reliable tool for closed and open system quantum dynamics for time dependent Hamiltonians, and expose potential issues of drawing conclusions based solely on RWA.