Qubits based on the single Cooper-pair box The Single Cooper-pair Box (SCB) is a small superconducting island connected via two Josephson junctions to a reservoir. It is described by its Josephson coupling energy EJ, and by its charging

The Single Cooper-pair Box (SCB) is a small superconducting island connected via two Josephson junctions to a reservoir. It is described by its Josephson coupling energy EJ, and by its charging energy EC=e/2Cqb, where Cqb is the total capacitance of the box and e is the electron charge. In the SCB it is possible to create superpositions of charge states involving a discrete number of excess Cooper-pairs in the box, which we denote |0>, |1>, |2> etc. Because of the Josephson coupling energy, the states form Bloch bands as a function of an external voltage Vqb. If EJ is much smaller than EC, only neighboring states are relevant and with an appropriate Vqb two states can be selected, for example |0> and |1>. The superconducting gap ? of the box has to be larger than the charging energy in order to avoid quasi-particle states, and finally temperature has to be small compared to all these energy scales. Thus the inequality ? >EC>EJ>kBT has to be satisfied for the system to act as a well defined two level system. The SCB has two control knobs; the gate voltage Vqb which changes the energy difference between the two charge states, and the magnetic flux which threads the loop containing the island which tunes the Josephson coupling energy. Quantum coherence in a SCB, was demonstrated in an experiment by Nakamura et al. [1]. They showed how the states could be manipulated using very fast voltage pulses, and they observed coherent oscillations between the two states by varying the pulse duration. We can routinely fabricate SCBs both at Chalmers and at KTH.