  Solid state quantum bits are a promising candidate for the realization of a
scalable quantum computer, however, they are usually strongly limited by
decoherence. We consider a double quantum dot charge qubit, whose basis states
are defined by the position of an additional electron in the system of two
laterally coupled quantum dots. The coupling of these two states can be
controlled externally by a quantum point contact between the two dots. We
discuss the decoherence through coupling to the electronic leads due to
cotunneling processes. We focus on a simple Gedanken experiment, where the
system is initially brought into a superposition and then the inter-dot
coupling is removed nonadiabatically. We treat the system by invoking the
Schrieffer-Wolff transformation in order to obtain a transformed Hamiltonian
describing the cotunneling, and then obtain the dynamics of the density matrix
using the Bloch-Redfield theory. As a main result, we show that there is energy
relaxation even in the absence of inter-dot coupling. This is in contrast to
what would be expected from the Spin-Boson model and is due to the fact that a
quantum dot is coupled to two distinct baths.
