The quantronium circuit is a superconducting quantum bit (qubit) made of aluminum and aluminum oxide. It includes two small tunnel junctions and a larger one inserted in a superconducting loop. The small junctions define a superconducting island capacitively coupled to a gate electrode. By applying microwave pulses to the gate, the quantronium can be prepared in any coherent superposition of its two lowest energy eigenstates, i.e. the two qubit states. These two qubit states are characterized by persistant currents flowing around the loop in opposite directions. The current loop, and consequently the quantronium state, is measured by applying a pulse of current to the large junction. This circuit has been successfully operated at 15 mK at the end of year 2001.
Dwave has improved upon the quantroniu qubit design.