  In the lectures we will be concerned with some aspects of physical
implementations of quantum gate operations which are necessary for quantum
information processing. We will discuss two possible realizations. One of them
is based on qubits being encoded in atomic degrees of freedom where the atoms
are manipulated in optical lattices above atom chips. The other realization is
based on photonic qubits and measurement-induced nonlinearities in linear
optics. Both implementations have in common that their main decoherence
mechanism is absorption in dielectric materials. The quantum theory of light in
absorbing media and its implications to decoherence will form the last part of
the lectures.
