Superconducting quantum-bit device based on Josephson junctions

A superconducting quantum-bit based on Josephson junctions, characterized in that it has a first principal degree of freedom assigned to writing and a second principal degree of freedom assigned to reading, in that one of the first and second principal degrees of freedom is due to a charge while the other of the first and second principal degrees of freedom is due to a phase, and in that it comprises:

• a Cooper-pair box comprising first and second Josephson junctions defining a charge island of the Cooper-pair box closing up onto a superconducting loop in order to form a qubit;
• a write circuit comprising a gate electrode placed facing the said Cooper-pair box island and constituting with it a capacitor of capacitance Cg, the gate electrode capacitively coupled to the charge island, allowing this qubit to be placed in one or other of the two base states of the qubit or in a coherent superposition of these states, a bias voltage Vg being applied to the gate electrode from an adjustable voltage source;
• and
• a read circuit comprising a read Josephson junction JL inserted into the said superconducting loop and having a Josephson energy at least 50 times greater than the Josephson energy of each of the first and second Josephson junctions.