Quantum states of small superconductors

After a brief review of recent proposals for using small superconductors wh ose charging energy is comparable to their superconducting gap as physical qubits in quantum computation, the 'Cooper-pair box' experiments of Nakamur a et al. are analysed. In these, the gate voltage on a small superconductin g island is perturbed by a pulse of height deltaV and duration Deltat and c oherent oscillations between the charge states with (n) over bar and (n) ov er bar + 1 Cooper pairs are detected. The effects of noise in deltaV and De ltat on their measurements is studied and it is found that the two kinds of noise have qualitatively different signatures in the experimental data.