diff --git a/src/num_results.tex b/src/num_results.tex index de69c52..40b793b 100644 --- a/src/num_results.tex +++ b/src/num_results.tex @@ -14,22 +14,33 @@ feature of the short time behavior of the bath energy flow that is visible in all simulations will be discussed and explained in \cref{sec:pure_deph}. -In the generic case where no analytic solution we nevertheless are -able to obtain consistent results as is demonstrated upon the example -of the spin-boson model \cref{sec:prec_sim}. We will also look into -the characteristics of the energy flow between system and bath -depending on the shape of the spectral density in -\cref{sec:energy-transf-char}. As the interaction energy is not -insubstantial in the regime that will be studied, we turn off the -interaction smoothly at the end of the evolution to be able to -adequately discuss our observations. +In the generic case where no analytic solution is known we +nevertheless are able to obtain consistent results as is demonstrated +upon the example of the spin-boson model \cref{sec:prec_sim}. +\Cref{sec:stocproc} investigates influence the precision of the +stochastic process sampling on the accuracy of the results. The same +analysis is performed for the hierarchy truncation in +\cref{sec:trunc}. -Finally, in \cref{sec:initial-slip-sb} we will study the short time -behavior of the spin-boson model, which will prove to be similar to -that of the harmonic oscillator discussed earlier. We will relate this -feature of the dynamics to the discussion in \cref{sec:pure_deph} and -also to the systematics of HOPS. Further, the effects of slowly and -smoothly turning on the interaction will be studied. +Subsequently, we will demonstrate some results with high consistency +\cref{sec:one_bath_cutoff} for varying bath memories. Prompted by the +results of this section we will also look into the characteristics of +the energy flow between system and bath depending on the shape of the +spectral density in \cref{sec:energy-transf-char}. As the interaction +energy is not insubstantial in the regime that will be studied, we +turn off the interaction smoothly at the end of the evolution to be +able to adequately discuss our observations. + +The feasibility of precision studies is further demonstrated for +varying coupling strengths in \cref{sec:one_bathcoup_strength}. + +Finally, in \cref{sec:initial-slip-sb,sec:moder-init-slip} the short +time behaviour of the spin-boson model is studied and will prove to be +similar to that of the harmonic oscillator discussed earlier. We will +relate this feature of the dynamics to the discussion in +\cref{sec:pure_deph} and also to the systematics of HOPS. The effects +of slowly and smoothly turning on the interaction will be studied in +\cref{sec:moder-init-slip}. % These results will strengthen the confidence in % the method so that we can turn to more complicated applications. @@ -38,6 +49,11 @@ smoothly turning on the interaction will be studied. % will turn to two applications to demonstrate these features in % \cref{sec:singlemod,sec:otto}. +However, before we begin in earnest some technical disscusions are in +order. \Cref{sec:meth} will explain our measures for the consistency +of results, as well as the spectral density used throughout the rest +of this work. + \section{Some Remarks on the Methods} \label{sec:meth} Before we begin with the applications in earnest, let us review some @@ -1404,10 +1420,107 @@ the Redfield master equation does not require the secular approximation, but only weak coupling and can therefore capture non-Markovian dynamics. +Let us now complete our precision studies of the zero temperature +spin-boson model energy flow by looking at the effect of different +coupling strengths in \cref{sec:one_bathcoup_strength}. + +\subsection{Varying the Coupling Strength}% +\label{sec:one_bathcoup_strength} +\begin{wrapfigure}[-2]{o}{0.3\textwidth}* + \centering + \includegraphics{figs/one_bath_syst/final_states_flows} + \caption{\label{fig:delta_fs_flow} The absolute value difference of + the state energies and the maximal flows for the simulations in + \cref{fig:delta_energy_overview} from their value at coupling + strength \(α(0)=0.40\) normalized by their value at + \(α(0)=1.12\).} +\end{wrapfigure} +After having studied the dependence of the bath energy flow for +various cutoff frequencies of the BCF in \cref{sec:one_bath_cutoff,}, +we now consider the case with fixed cutoff \(ω_c=2\) but varying +coupling strength. The results presented here are mainly a +demonstration of the feasibility of high-consistency simulations for a +range of coupling strengths. We will therefore keep the discussion of +the physical implications relatively short. + +The chosen simulation parameters are the same as in +\cref{sec:one_bath_cutoff} and again consistent results have been +obtained throughout the whole range of coupling strengths as can be +gathered from \cref{fig:delta_interaction_consistency}. The +interaction strength was chosen linearly spaced and the simulation +results are presented in \cref{fig:delta_energy_overview}. + +As the shape of the BCF is not altered between the simulations, the +bath energy flows look very similar as do the interaction +energies. The main difference between the simulations is the magnitude +of the interaction energy. With increased coupling strength there is +an increased interaction energy and an increased flow which leads to +faster energy loss in the system and faster energy gain of the +bath. The stronger the coupling, the more pronounced is the +non-monotonicity in time of the interaction energy, which is reflected +in a non-monotonicity in the bath energy expectation value. + +The bath energy reaches a maximum and falls slightly for the strongest +coupling simulations (violet and brown lines). If the interaction is +strong enough, ``backflow'' can occur despite finite bath correlation +times. Here, the back flow is only occurring between interaction and +bath energy. In \cref{fig:markov_analysis_steady} the bath memory is +long, additionally to a strong coupling so that multiple oscillations +can be seen. +\begin{figure}[htp] + \centering + \includegraphics{figs/one_bath_syst/δ_energy_overview} + \caption{\label{fig:delta_energy_overview} Energy overview for the + model \cref{eq:one_qubit_model} for various coupling + strengths. The curves are converged out, and the error funnels are + not visible.} +\end{figure} + +As a task for future work it might be worthwhile to ascertain the +exact conditions under which system energy might flow back into the +system. Is resonance required and what role is played by the system +time scale? Does backflow always occur provided the coupling is strong +enough?\footnote{The author expects a negative answer to this + question.} + +Despite these differences for finite times, \cref{fig:delta_fs_flow} +demonstrates that the approximate steady state\footnote{excluding the + \(α(0)=0.4\) case} interaction energies +\(\ev{H_{\inter}}_{\mathrm{ss}}\) (blue), maximal flows +\(\abs{J}_{\mathrm{max}}\) (orange), system energies +\(\ev{H_{\sys}}_{\mathrm{ss}}\) (red) and bath energies +\(\ev{H_{\bath}}_{\mathrm{ss}}\) (green) are almost linearly dependent +on the coupling strength \(α(0)\) in the range of coupling strengths +studied. + +We find that we can control the speed of the energy transfer between +bath and system with the coupling strength at the cost of greater +steady state interaction energy. Were we to turn off the interaction +diabatically, we would have to expend this energy in the worst +case. On the other hand, more adiabatic protocol as the one used in +\cref{sec:energy-transf-char} would likely be a remedy to this +drawback. + +The cooling performance for a coupling that is being turned off at the +end would depend on the concrete protocol as we've seen in +\cref{sec:one_bath_cutoff} and a more detailed study is left to future +work. The interplay between the interaction time-scale mediated by the +coupling strength, the bath memory time and the system dynamics allows +for intricate tuning. + +Both the final system and bath energies are increasing with the +coupling strength, compensating for the interaction energy which is +the main mechanism that leads to residual system energy in the steady +state which is further and further away from the ground state, which +would be the steady state of weak coupling dynamics. + We discussed the short term dynamics for a general model in \cref{sec:pure_deph}. Now, in \cref{sec:initial-slip-sb} we will apply them the spin-boson model. +\fixme{iftime: re-run with same coupling strength, more cutoff freqs, + less samples, see, longer times for coupling strengths, more coup} + \subsection{Initial Slip}% \label{sec:initial-slip-sb} \begin{figure}[htp] @@ -1496,7 +1609,7 @@ upon the bath energy change due to the initial slip, is the subject of \subsection{Moderating the Inital Slip with Modulated Coupling}% \label{sec:moder-init-slip} -\begin{wrapfigure}[-1]{O}{0.4\textwidth}* +\begin{wrapfigure}[-1]{o}{0.4\textwidth}* \centering \includegraphics{figs/one_bath_mod/modulation_protocols_init.pdf} \caption{\label{fig:L_mod_init} The interaction is being switched on @@ -1583,105 +1696,6 @@ flow. \end{figure} -Let us now complete our precision studies of the zero temperature -spin-boson model energy flow by looking at the effect of different -coupling strengths in \cref{sec:one_bathcoup_strength}. - -\newpage -\subsection{Varying the Coupling Strength}% -\label{sec:one_bathcoup_strength} -\begin{wrapfigure}[-5]{O}{0.3\textwidth}* - \centering - \includegraphics{figs/one_bath_syst/final_states_flows} - \caption{\label{fig:delta_fs_flow} The absolute value difference of - the state energies and the maximal flows for the simulations in - \cref{fig:delta_energy_overview} from their value at coupling - strength \(α(0)=0.40\) normalized by their value at - \(α(0)=1.12\).} -\end{wrapfigure} -After having studied the dependence of the bath energy flow for -various cutoff frequencies of the BCF in \cref{sec:one_bath_cutoff,}, -we now consider the case with fixed cutoff \(ω_c=2\) but varying -coupling strength. The results presented here are mainly a -demonstration of the feasibility of high-consistency simulations for a -range of coupling strengths. We will therefore keep the discussion of -the physical implications relatively short. - -The chosen simulation parameters are the same as in -\cref{sec:one_bath_cutoff} and again consistent results have been -obtained throughout the whole range of coupling strengths as can be -gathered from \cref{fig:delta_interaction_consistency}. The -interaction strength was chosen linearly spaced and the simulation -results are presented in \cref{fig:delta_energy_overview}. - -As the shape of the BCF is not altered between the simulations, the -bath energy flows look very similar as do the interaction -energies. The main difference between the simulations is the magnitude -of the interaction energy. With increased coupling strength there is -an increased interaction energy and an increased flow which leads to -faster energy loss in the system and faster energy gain of the -bath. The stronger the coupling, the more pronounced is the -non-monotonicity in time of the interaction energy, which is reflected -in a non-monotonicity in the bath energy expectation value. - -The bath energy reaches a maximum and falls slightly for the strongest -coupling simulations (violet and brown lines). If the interaction is -strong enough, ``backflow'' can occur despite finite bath correlation -times. Here, the back flow is only occurring between interaction and -bath energy. In \cref{fig:markov_analysis_steady} the bath memory is -long, additionally to a strong coupling so that multiple oscillations -can be seen. -\begin{figure}[htp] - \centering - \includegraphics{figs/one_bath_syst/δ_energy_overview} - \caption{\label{fig:delta_energy_overview} Energy overview for the - model \cref{eq:one_qubit_model} for various coupling - strengths. The curves are converged out, and the error funnels are - not visible.} -\end{figure} - -As a task for future work it might be worthwhile to ascertain the -exact conditions under which system energy might flow back into the -system. Is resonance required and what role is played by the system -time scale? Does backflow always occur provided the coupling is strong -enough?\footnote{The author expects a negative answer to this - question.} - -Despite these differences for finite times, \cref{fig:delta_fs_flow} -demonstrates that the approximate steady state\footnote{excluding the - \(α(0)=0.4\) case} interaction energies -\(\ev{H_{\inter}}_{\mathrm{ss}}\) (blue), maximal flows -\(\abs{J}_{\mathrm{max}}\) (orange), system energies -\(\ev{H_{\sys}}_{\mathrm{ss}}\) (red) and bath energies -\(\ev{H_{\bath}}_{\mathrm{ss}}\) (green) are almost linearly dependent -on the coupling strength \(α(0)\) in the range of coupling strengths -studied. - -We find that we can control the speed of the energy transfer between -bath and system with the coupling strength at the cost of greater -steady state interaction energy. Were we to turn off the interaction -diabatically, we would have to expend this energy in the worst -case. On the other hand, more adiabatic protocol as the one used in -\cref{sec:energy-transf-char} would likely be a remedy to this -drawback. - -The cooling performance for a coupling that is being turned off at the -end would depend on the concrete protocol as we've seen in -\cref{sec:one_bath_cutoff} and a more detailed study is left to future -work. The interplay between the interaction time-scale mediated by the -coupling strength, the bath memory time and the system dynamics allows -for intricate tuning. - -Both the final system and bath energies are increasing with the -coupling strength, compensating for the interaction energy which is -the main mechanism that leads to residual system energy in the steady -state which is further and further away from the ground state, which -would be the steady state of weak coupling dynamics. - - -\fixme{iftime: re-run with same coupling strength, more cutoff freqs, - less samples, see, longer times for coupling strengths, more coup} - \section{Conclusion}% \label{sec:conclusion-1}