Nonequilibrium Green's functions approach to the sub-femtosecond dynamics
of plasmas and atoms in intense x-ray fields

Michael Bonitz,
Institute for Theoretical Physics and Astrophysics
Christian-Albrechts University Kiel, Germany

In this talk I discuss the idea of nonequilibrium Green's functions (NEGF)
and recent theoretical and computational results. I will start by
introducing the real-time (Keldysh) Green's functions for a many-body
system of bosons or fermions and their equations of motion - the
Keldysh-Kadanoff-Baym equations (KBE). Their main advantage is that
they guarantee the relevant conservation laws, are applicable to fields
of arbitrary intensity as well as to arbitrarily short pulses. There are
two main lines of research:

1. Use the KBE to derive general quantum kinetic equations for the Wigner
function or density matrix. These equations contain collisions including
nonlinear field effects, finite collision durations etc.
I will illustrate this by examples from dense laser plasmas.

2. Direct solution of the KBE for the two-time NEGF. Here numerical results
were obtained for the correlated electron gas and for electron-hole plasmas
in semiconductors in excited by optical pulses. These results can directly
be extended to small atoms interacting with short x-ray pulses. Here first
results have been obtained by our group, in particular by my affiliate
Karsten Balzer. I will give a brief overview and discuss the capabilities
of this method.