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src/flow.tex
24
src/flow.tex
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@ -16,7 +16,7 @@ treated with the methods that will be developed as is shown in
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\cref{sec:general_obs}.
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The generalization to multiple baths in \cref{sec:multibath} and time
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depend Hamiltonians in \cref{sec:timedep} will present itself as
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dependent Hamiltonians in \cref{sec:timedep} will present itself as
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straight forward.
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\section{Bath Energy Change of a Zero Temperature Bath}%
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@ -161,7 +161,7 @@ Interestingly one finds that
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This expression is undesirable as it does not exist for all bath
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correlation functions\footnote{Only for BCFs that are smooth at
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\(τ=0\).} and expressions involving the process directly are alleged
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to converge slower, especially for shorter bath memories. This
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to converge more slowly, especially for shorter bath memories. This
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convergence problem is due to greater magnitude and shorter
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correlation time of the oscillations of \(\dot{η}_{t}^\ast\), as can
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be seen in \cref{fig:stocproc_comparison}.
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@ -169,7 +169,7 @@ be seen in \cref{fig:stocproc_comparison}.
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\centering
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\includegraphics{figs/analytic_comp/stocproc_comparison}
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\caption{\label{fig:stocproc_comparison} The imaginary part of ten
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realizations the stochastic process \(η^\ast\) for an ohmic BCF
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realizations of the stochastic process \(η^\ast\) for an ohmic BCF
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with different cutoff frequencies \(ω_{c}\). The process is much
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smoother and of less magnitude for smaller cutoffs. The difference
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between the cutoffs is even more severe for the derivative of the
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@ -352,7 +352,7 @@ and bath for a thermal initial condition as
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\rho(t) =
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\prod_\lambda\qty(∫\dd[2]{y_\lambda}
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\frac{\eu^{-\abs{y_\lambda}^2\bose_\lambda}}{\pi\bose_\lambda})
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U(t)D(\vb{y})\bqty{\ketbra{\psi}\otimes\ketbra{0}}D(\vb{y})^† U(t)^†,
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U(t)D(\vb{y})\bqty{\ketbra{\psi}\otimes\ketbra{0}}D^†(\vb{y}) U^†(t),
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\end{equation}
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where \(\bose_{λ}=\bose(βω_{λ})\). This is simply the
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Glauber-Sudarshan P representation of the bath state
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@ -364,7 +364,7 @@ The system state is then recovered through
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\rho_{\sys}(t) =
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\prod_\lambda\qty(∫\dd[2]{y_\lambda}
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\frac{\eu^{-\abs{y_\lambda}^2\bose_\lambda}}{\pi\bose_\lambda})
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\tr_{\bath}\bqty{U(t)D(\vb{y})\bqty{\ketbra{\psi}\otimes\ketbra{0}}D(\vb{y})^† U(t)^†}.
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\tr_{\bath}\bqty{U(t)D(\vb{y})\bqty{\ketbra{\psi}\otimes\ketbra{0}}D^†(\vb{y}) U^†(t)}.
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\end{equation}
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The usual step is now to insert \(\id =D(\vb{y})D^†(\vb{y})\) and
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@ -402,13 +402,13 @@ Remember that we want to calculate
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\begin{aligned}
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\prod_\lambda&\qty(∫\dd[2]{y_\lambda}
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\frac{\eu^{-\abs{y_\lambda}^2\bar{n}_\lambda}}{\pi\bar{n}_\lambda})\\
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&\tr[L^†\dot{B}(t)
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U(t)D(\vb{y})\ketbra{\psi}\otimes\ketbra{0}D(\vb{y})^† U(t)^†].
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&\times\tr[L^†\dot{B}(t)
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U(t)D(\vb{y})\ketbra{\psi}\otimes\ketbra{0}D^†(\vb{y}) U^†(t)].
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\end{aligned}
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\end{aligned}
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\end{equation}
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To make a connection to the zero temperature results we again insert a
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\(\id\), but have to additionally commute \(D(\vb{y})^†\) past
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\(\id\), but have to additionally commute \(D^†(\vb{y})\) past
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\(\dot{B}(t)\). This leads to the expression
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\begin{equation}
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\label{eq:pureagain}
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@ -419,14 +419,14 @@ To make a connection to the zero temperature results we again insert a
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\begin{aligned}
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L^†(\dot{B}(t) + \dot{ξ}(t))
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D^†(\vb{y}) &U(t)D(\vb{y})\ketbra{\psi}\\
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&\otimes\ketbra{0}D^†(\vb{y})U(t)^†D(\vb{y})
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&\otimes\ketbra{0}D^†(\vb{y})U^†(t)D(\vb{y})
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\end{aligned}
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] \\
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&=\prod_\lambda
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\qty(∫\dd[2]{y_\lambda}
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\frac{\eu^{-\abs{y_\lambda}^2\bar{n}_\lambda}}{\pi\bar{n}_\lambda})\\
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&\qquad\times\tr[L^†\qty{\dot{B}(t) + \dot{ξ}(t)}
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\tilde{U}(t)\ketbra{\psi}\otimes\ketbra{0} \tilde{U}(t)^†],
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\tilde{U}(t)\ketbra{\psi}\otimes\ketbra{0} \tilde{U}^†(t)],
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\end{aligned}
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\end{equation}
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which indeed returns us to the zero temperature formalism with a transformed
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@ -465,7 +465,7 @@ Now,
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\begin{equation}
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\label{eq:hshcomm}
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[H_{\mathrm{sys}}^{\mathrm{shift}}, H_\inter] = ξ(t) [L^†, L]
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B(t)^† + ξ^\ast(t) [L, L^†] B
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B^†(t) + ξ^\ast(t) [L, L^†] B
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\end{equation}
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and therefore
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\begin{equation}
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@ -670,7 +670,7 @@ the form
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The corresponding version of~\cref{eq:f_ex_zero} would only explicitly
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depend on the zeroth order state and the stochastic processes. It has
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been observed that expressions involving the stochastic process
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directly tend to converge slower. However, this statement comes
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directly tend to converge more slowly. However, this statement comes
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without empirical proof and its verification may be left to future
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study. An explanation may be that the first hierarchy states fluctuate
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about their average dynamics whereas the stochastic process fluctuates
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@ -458,20 +458,20 @@ we can define the \(\vb{k}th\) hierarchy state
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The origin of the normalization chosen in \cref{eq:d_op_hier} is the
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desire to give all hierarchy states the same unit and to formulate the
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final hops equations unified into one equation in an enlarged Hilbert
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final HOPS equations unified into one equation in an enlarged Hilbert
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space~\cite{Gao2021Sep}. We refrain from going into details here and
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refer to \cref{sec:multihops} instead.
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For this state the following equation of motion can be
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derived~\cite{Suess2014Oct,Hartmann2021Aug}
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\begin{equation}
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\begin{multline}
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\label{eq:singlehops}\tag{HOPS}
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\ket{\dot{ψ}^{\vb{k}}} = \qty[-\iu H_\sys + \vb{L}\cdot\vb{η}^\ast -
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∑_{μ=1}^{M}k_{μ}W_μ]\ket{ψ^{\vb{k}}} +
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\iu ∑_{μ=1}^{M}\sqrt{G_μ}\qty[\sqrt{k_{μ}} L \ket{ψ^{\vb{k} -
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∑_{μ=1}^{M}k_{μ}W_μ]\ket{ψ^{\vb{k}}} \\
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+ \iu ∑_{μ=1}^{M}\sqrt{G_μ}\qty[\sqrt{k_{μ}} L \ket{ψ^{\vb{k} -
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\vb{e}_{μ}}} + \sqrt{\qty(k_{μ} + 1)} L^† \ket{ψ^{\vb{k} +
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\vb{e}_{μ}}} ],
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\end{equation}
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\end{multline}
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where \(\vb{k}=(k_{1}, k_{2}, \ldots, k_{M})\) with \(k_{μ}\geq 0\) is
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a multi index and \(\pqty{\vb{e}_{μ}}_{ν} = δ_{μ,ν}\). The term
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\({\vb{k} - \vb{e}_{μ}}\) is evaluated only if \(k_{μ}\geq 1\). The
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@ -529,7 +529,7 @@ The main code repository for this work can be found under
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\url{https://github.com/vale981/master-thesis}.
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The directory \path{python/energy_flow_proper} contains several
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project subdirectory with literate programming notebooks in the
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project subdirectories with literate programming notebooks in the
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\texttt{org} format~\cite{EricSchulte2022Sep}. A detailed listing
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linking subprojects to chapters can be found in
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\cref{tab:code_structure}.
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@ -564,7 +564,7 @@ merged into the original repository
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The extensive and well tested existing HOPS code of the Theoretical
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Quantum Optics group\footnote{Available upon reasonable request.} was
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created by Richard Hartmann. Some improvements have been made in the
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course of this work this work. Documentation, type hints and unit
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course of this work. Documentation, type hints and unit
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tests have been introduced. The parallelization mechanism was
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overhauled and now uses the \href{https://www.ray.io}{\texttt{Ray}}
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library. The structure of the code was further modularized, allowing
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@ -11,7 +11,7 @@ verify the results of \cref{chap:flow} in
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\cref{sec:hopsvsanalyt}. Excellent consistency of the analytical and
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numeric solutions for the QBM models will be demonstrated. A common
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feature of the short time behaviour of the bath energy flow that is
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visible in all simulations will be discussed and explained in
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visible in all simulations that will be discussed and explained in
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\cref{sec:pure_deph}.
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In the generic case where no analytic solution is known we
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@ -80,7 +80,7 @@ For the estimation of mean and standard deviation from trajectory
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data, Welford's online algorithm is employed to avoid catastrophic
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numerical cancellation~\cite{Welford1962Aug,Knuth1997}.
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In all simulations discussed use an Ohmic spectral density
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In all simulations, we will use an Ohmic spectral density
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\begin{equation}
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\label{eq:ohmic_sd}
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J(ω)=η ω \eu^{-\frac{ω}{ω_{c}}}\quad (ω>0)
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@ -567,7 +567,7 @@ We solve the model with the Hamiltonian (Schr\"odinger picture)
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\label{eq:puredeph}
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H = L^†(t) B + L(t) B^† + H_\bath
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\end{equation}
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with \(L(t)=L(t)^†\), \([L(t), L(s)] = 0\;\forall t,s\) (so that the
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with \(L(t)=L^†(t)\), \([L(t), L(s)] = 0\;\forall t,s\) (so that the
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Heisenberg Hamiltonian matches \cref{eq:puredeph}) and \(B,\,H_\bath\)
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as in \cref{sec:nmqsd_basics}.
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@ -630,9 +630,9 @@ match the zero temperature case in which the bath has minimal energy
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in the initial state.
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A thermodynamically useful model should feature significant system
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dynamics which is always given, assuming that the coupling is no too
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dynamics which is always given, assuming that the coupling is not too
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strong. Non hermitian coupling may also have this effect, but in the
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literature most effective qubit models tend to favour Hermitian
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literature most effective two-level models tend to favour Hermitian
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couplings
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\cite{Aurell2019Apr,Hita-Perez2021Nov,Hita-Perez2021Aug,MacQuarrie2020Sep,Andersen2017Feb,Mezzacapo2014Jul}. For
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the spin-boson model, non Hermitian coupling is the result of the
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@ -793,11 +793,11 @@ be witnessed in the inset of \Cref{fig:stocproc_systematics}.
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Only in the simulation with precision \(\varsigma=10^{-6}\) (blue)
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however, the compatibility condition of \cref{sec:meth} is satisfied
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for the given trajectory count. This is due to the fact that multiple
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quantities are being obtained in different ways, namely system energy
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and the flow. The system energy can be calculated directly from the
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system state whereas the flow has to be integrated numerically to
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obtain the total bath energy change.
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for the given trajectory count. The system energy can be calculated
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directly from the system state whereas the flow has to be integrated
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numerically to obtain the total bath energy change and depends
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directly on the first hierarchy states. The accuracy must be high for
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the results to be consistent.
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\begin{figure}[htp]
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\centering
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\includegraphics{figs/one_bath_syst/stocproc_systematics_bath_energy}
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@ -821,7 +821,7 @@ plotted in \cref{fig:stocproc_consistency_dev}. Only for the highest
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precision (blue) case good consistency is given continually. For lower
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precision (green, orange), the consistency fluctuates and only
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occasionally surpasses \(68\%\). Initially compatibility is being
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demonstrated (until about \(N=10^4\)) but a divergence diverge from
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demonstrated (until about \(N=10^4\)) but a divergence from
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the most precise result occurs. It is therefore important to consider
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the dependence of the compatibility on the sample count \(N\) to judge
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the veracity of the simulation results.
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@ -854,7 +854,7 @@ convergence as is also demonstrated in
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\subsection{Hierarchy Truncation}
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\label{sec:trunc}
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As the systematics of the truncation depth has already been studied
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As the systematics of the truncation depth have already been studied
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thoroughly in~\cite{RichardDiss,Hartmann2021Aug}, we will keep the
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discussion short. We chose \(N=4.5 \cdot 10^5\) trajectories and an
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Ohmic BCF with \(α(0)=0.8\) and \(ω_c=2\). Again, a BCF expansion with
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