Quantum states of small superconductors

After a brief review of recent proposals for using small superconductors whose charging energy is comparable to their superconducting gap as physical qubits in quantum computation, the `Cooper-pair box´ experiments of Y. Nakamura et al. (1999, 2000) are analysed. In these, the gate voltage on a small superconducting island is perturbed by a pulse of height δV and duration Δt and coherent oscillations between the charge states with n¯ and n¯+1 Cooper pairs are detected. The effects of noise in δV and Δt on their measurements is studied and it is found that the two kinds of noise have qualitatively different signatures in the experimental data