Superconducting π qubit with three Josephson junctions

We propose a qubit consisting of a superconducting ring with two zero junctions and one ferromagnetic pi junction. In the system, degenerate states appear in the phase space without an external magnetic field because of a competition between the zero and pi states. Quantum tunneling between the degenerate states leads to a formation of bonding and antibonding states which are used as a bit. For manipulating the states, only a small magnetic field around zero is required. This feature leads to a large-scale integration and a construction of the smaller-sized qubit which is robust to the decoherence by external noises. (c) 2006 American Institute of Physics.