Commutation Relations in Adiabatic Elimination

TL;DR

This paper investigates the adiabatic elimination method in quantum optics, revealing that naive application can lead to incorrect commutation relations due to improper treatment of high-frequency noise, and proposes a more accurate approach.

Contribution

It identifies the flaw in the standard adiabatic elimination procedure related to noise treatment and clarifies how to correctly perform elimination without losing physical accuracy.

Findings

Incorrect commutation relations arise from high-frequency noise treatment.

High-frequency noise contributes minimally when averaged over the bath.

Proper cutoff of vacuum noise frequency preserves physical reliability.

Abstract

The method of adiabatic elimination has been widely adopted in quantum optics in the past several decades. In the study of cavity-based light-matter interactions, the bad-cavity limit is often encountered, where the damping rate of the cavity is much larger than the interaction strength. The fast-damped cavity will quickly relax to a quasi-stationary state, and one can eliminate the cavity from the equation of motion by setting its time derivative to zero. Elimination of the cavity in the bad-cavity limit can reduce the dimensionality of the equations of motion of the system. However, we find that the adiabatic elimination procedure performed in the quantum Langevin equation leads to an incorrect commutation relation, which was rarely discussed in the former studies, as far as we know. Here, we show the incorrect commutation relation arises from the fact that the high frequency of the vacuum noise should be cut off to perform adiabatic elimination, but the noise with high frequency cutoff is still treated as white noise with infinite bandwidth and delta commutation relation. We also study the correlation function and show that the high frequency part of noise contributes very little when averaged over the bath. Therefore, the adiabatic elimination method can reduce the complexity of the calculations while maintaining physical reliability.