A quantum two level system coupled to a harmonic oscillator represents a ubiquitous physical system. New experiments in circuit QED and nano-electromechanical systems (NEMS) achieve unprecedented coupling strength at large detuning between qubit and oscillator, thus requiring a theoretical treatment beyond the Jaynes Cummings model. Here we present a new method for describing the qubit dynamics in this regime, based on an oscillator correlation function expansion of a non-Markovian master equation in the polaron frame. Our technique yields a new numerical method as well as a succinct approximate expression for the qubit dynamics. We obtain a new expression for the ac Stark shift and show that this enables practical and precise qubit thermometry of an oscillator.
@article{arxiv.1203.5994,
title = {Quantum thermometry using the ac Stark shift within the Rabi model},
author = {Kieran D. B. Higgins and Brendon W. Lovett and Erik M. Gauger},
journal= {arXiv preprint arXiv:1203.5994},
year = {2013}
}