Effects of initial system-environment correlations on state preparation and variational polaron method for driven open quantum systems

• Main Authors: Chien-Chang Chen, 陳建彰
• Other Authors: Hsi-Sheng Goan
• Format: Others
• Language: en_US
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/ntsee5

博士 === 國立臺灣大學 === 物理學研究所 === 105 === We investigate the preparation of a target initial state for a two-level (qubit) system from a system-environment equilibrium or correlated state by an external field. The system-environment equilibrium or correlated state results from the inevitable interaction of the system with its environment. An efficient method in an extended auxiliary Liouville space is introduced to describe the dynamics of the non-Markovian open quantum system in the presence of a strong field and an initial system-environment correlation. By using the time evolutions of the population difference, the state trajectory in the Bloch sphere representation, and the trace distance between two reduced system states of the open quantum system, the effect of initial system-environment correlations on the preparation of a system state is studied. We introduce an upper bound and a lower bound for the trace distance within our perturbation formalism to describe the diverse behaviors of the dynamics of the trace distance between various correlated states after the system state preparation. These bounds, that are much more computable than similar bounds in the literature, give a sufficient condition and a necessary condition for the increase of the trace distance and are related to the witnesses of non-Markovianity and initial system-bath correlation. In addition, we develop a practical method that combines the traditional variational polaron master equation approach with the counter-rotating-hybridized rotating-wave (CHRW) method to describe the dynamics of a driven open quantum system in a weak or strong system-environment coupling regime. Our method, called the CHRW variational polaron method, is valid over a broad range of parameters beyond the rotating wave approximation (RWA), i.e., also capable of dealing with the cases of strong driving and far off-resonance (large detuning). We illustrate our method through a driven spin-boson model and discuss the criteria for and give the boundaries of regimes of validity of our CHRW variational polaron master equation. We show that the results we obtain reduce to those by the weak-system-bath-coupling method and the full-polaron method in their appropriate parameter limits. Thus our versatile CHRW variational polaron method is able to interpolate between these two methods and, combining with the weak-system-bath-coupling method, can capture the correct behaviors over a very wide parameter space for driven open quantum systems.