  We develop a systematic field-theoretical approach to open quantum systems
based on condensed-matter many-body methods. The time evolution of the reduced
density matrix for the open quantum system is determined by a transmission
matrix. Developing diagrammatic perturbation theory, invoking Wick's theorem in
connection with a Caldeira-Leggett quantum oscillator environment in thermal
equilibrium, the transmission matrix satisfies a Dyson equation characterized
by an irreducible kernel. Unlike the Nakajima-Zwanzig and standard approaches,
the Dyson equation is equivalent to a general non-Markovian master equation for
the reduced density matrix, incorporating secular effects and independent of
the initial preparation. The kernel is determined by a systematic diagrammatic
expansion in powers of the interaction. We consider the Born approximation for
the kernel. Applying a condensed-matter pole or, equivalently, a
quasiparticle-type approximation, equivalent to the usual assumption of a
timescale separation, we derive a master equation of the Markov type.
Furthermore, imposing the rotating-wave approximation,we obtain a Markov master
equation of the Lindblad form. To illustrate the method, we consider the
standard example of a single qubit coupled to a thermal heat bath.
