Noise and quantum measurements in a circular Josephson array qubit

We study the decoherence and the quantum measurement of charge in a qubit consisting of a circular array of Josephson junctions. The two-level system we consider couples the two charge states through a higher-order tunneling process thus making it possible to achieve a long relaxation time. Using the spin-boson Hamiltonian, we estimate decoherence due to ohmic as well as $1/f$ noise. We simulate the quantum-mechanical measurement process by studying the density matrix of the qubit and a capacitively coupled single-electron transistor that measures the charge.