a Cooper-pair box (CPB). The system is almost the same as the single-electron box.there is an island to store charges, a bulk electrode to provide these charges, and a gate electrode to shift the electrostatic potential of the island with respect to the bulk electrode and to tune the number of charges thereby. The crucial difference is that now both the island and source electrodes are superconducting, and the tunnel junction that connects them is in fact a Josephson junction. There are two energies that characterize the system. The first energy is the Josephson energy of the junction.it is a periodic function of the superconducting phase difference across the junction,−EJcosϕ. It is convenient to set the phase of the lead to zero, so that the phase difference is just a phase that characterizes the superconducting state of the island. The second energy is the same as for the non-superconducting setup: the charging energy associated with discrete charge Ne in the island. Why is the system called a Cooperpairbox? Each superconductor is a coherent reservoir of Cooper pairs rather than electrons, provided the energies involved are smaller than the superconducting energy gap. In this section, we will assume that there is always an even number of extra elementary charges in the island, and that charges are always transferred in Cooper pairs. a Cooper-pair box (CPB). The system is almost the same as the single-electron box.there is an island to store charges, a bulk electrode to provide these charges, and a gate electrode to shift the electrostatic potential of the island with respect to the bulk electrode and to tune the number of charges thereby. The crucial difference is that now both the island and source electrodes are superconducting, and the tunnel junction that connects them is in fact a Josephson junction. There are two energies that characterize the system. The first energy is the Josephson energy of the junction.it is a periodic function of the superconducting phase difference across the junction,−EJcosϕ. It is convenient to set the phase of the lead to zero, so that the phase difference is just a phase that characterizes the superconducting state of the island. The second energy is the same as for the non-superconducting setup: the charging energy associated with discrete charge Ne in the island. Why is the system called a Cooperpairbox? Each superconductor is a coherent reservoir of Cooper pairs rather than electrons, provided the energies involved are smaller than the