Nakajima–Zwanzig equation

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The Nakajima–Zwanzig equation (named after the physicists Sadao Nakajima and Robert Zwanzig) is an integral equation describing the time evolution of the "relevant" part of a quantum-mechanical system. It is formulated in the density matrix formalism and can be regarded a generalization of the Master equation.

The equation belongs to the Mori–Zwanzig theory within the statistical mechanics of irreversible processes (named after Hazime Mori). By means of a projection operator the dynamics is split into a slow, collective part (relevant part) and a rapidly fluctuating irrelevant part. The goal is to develop dynamical equations for the collective part.

Derivation[edit]

The starting point[1] is the quantum mechanical Liouville equation (von Neumann equation)

\partial_t \rho = \frac{i}{\hbar}[\rho,H] = L \rho,

where the Liouville operator L is defined as L A = \frac{i}{\hbar}[A,H].

The density operator (density matrix) \rho is split by means of a projection operator \mathcal{P} into two parts \rho =\left( \mathcal{P}+\mathcal{Q} \right)\rho , where \mathcal{Q}\equiv 1-\mathcal{P}. The projection operator \mathcal{P} projects onto the aforementioned relevant part, for which an equation of motion is to be derived.

The Liouville – von Neumann equation can thus be represented as

{\partial_t}\left( \begin{matrix}
   \mathcal{P}  \\
   \mathcal{Q}  \\
\end{matrix} \right)\rho =\left( \begin{matrix}
   \mathcal{P}  \\
   \mathcal{Q}  \\
\end{matrix} \right)L\left( \begin{matrix}
   \mathcal{P}  \\
   \mathcal{Q}  \\
\end{matrix} \right)\rho +\left( \begin{matrix}
   \mathcal{P}  \\
   \mathcal{Q}  \\
\end{matrix} \right)L\left( \begin{matrix}
   \mathcal{Q}  \\
   \mathcal{P}  \\
\end{matrix} \right)\rho.

The second line is formally solved as

\mathcal{Q}\rho ={{e}^{\mathcal{Q}Lt}}Q\rho (t=0)+\int_{0}^{t}dt'{e}^{\mathcal{Q}Lt'}\mathcal{Q}L\mathcal{P}\rho (t-{t}').

By plugging the solution into the first equation, we obtain the Nakajima–Zwanzig equation:

\partial_t \mathcal{P}\rho =\mathcal{P}L\mathcal{P}\rho +\underbrace{\mathcal{P}L{{e}^{\mathcal{Q}Lt}}Q\rho (t=0)}_{=0}+\mathcal{P}L\int_{0}^{t}{dt'{{e}^{\mathcal{Q}Lt'}}\mathcal{Q}L\mathcal{P}\rho (t-{t}')}.

Under the assumption that the inhomogeneous term vanishes[2] and using

\mathcal{K}\left( t \right)\equiv\mathcal{P}L{{e}^{\mathcal{Q}Lt}}\mathcal{Q}L\mathcal{P},
\mathcal{P}\rho \equiv {{\rho }_\mathrm{rel}}, as well as
\mathcal{P}^2=\mathcal{P},

we obtain the final form

\partial_t{\rho }_\mathrm{rel}=\mathcal{P}L{{\rho}_\mathrm{rel}}+\int_{0}^{t}{dt'\mathcal{K}({t}'){{\rho }_\mathrm{rel}}(t-{t}')}.

References[edit]

  • E. Fick, G. Sauermann: The Quantum Statistics of Dynamic Processes Springer-Verlag, 1983, ISBN ISSN 3-540-50824-4.
  • Heinz-Peter Breuer, Francesco Petruccione: Theory of Open Quantum Systems. Oxford, 2002 ISBN ISSN 970-0-19-852063-4
  • Hermann Grabert Projection operator techniques in nonequilibrium statistical mechanics, Springer Tracts in Modern Physics, Band 95, 1982
  • R. Kühne, P. Reineker: Nakajima-Zwanzig's generalized master equation: Evaluation of the kernel of the integro-differential equation, Zeitschrift für Physik B (Condensed Matter), Band 31, 1978, S. 105–110, Abstract

Original works[edit]

  • Sadao Nakajima (1958), "On Quantum Theory of Transport Phenomena" (in German), Progress of Theoretical Physics 20 (6): pp. 948–959
  • Robert Zwanzig (1960), "Ensemble Method in the Theory of Irreversibility" (in German), Journal of Chemical Physics 33 (5): pp. 1338–1341
  • original paper

Notes[edit]

  1. ^ A derivation analogous to that presented here is found, for instance, in Breuer, Petruccione The theory of open quantum systems, Oxford University Press 2002, S.443ff
  2. ^ Such an assumption can be made if we assume that the irrelevant part of the density matrix is 0 at the initial time, so that the projector for t=0 is the identity.