System and method for controlling superconducting qubits

A method for initializing a two-junction superconducting loop qubit, comprising:

• providing a bias current in the loop I0=±Ii for a time duration t such that the qubit enters one of its basis states.

A method for reading out a state of a qubit having two Josephson junctions that connect a mesoscopic portion of the qubit to a bulk portion, and a control Josephson junction in said bulk portion, comprising:

• providing a bias current in the loop I=IDC+IAC for some duration t;
• and
• measuring a potential drop VR across said control Josephson junction.

A method for reading a state of a qubit having two Josephson junctions that connect a mesoscopic portion of the qubit to a bulk portion of the qubit, comprising;

• applying a bias field across said mesoscopic island portion;
• and
• measuring a resulting potential related to said bias field.

A quantum register comprising;

• a first qubit, wherein said first qubit includes a superconducting loop, said superconducting loop comprising a bulk loop portion and a mesoscopic island portion, said superconducting loop further comprising at least two Josephson junctions separating said bulk loop portion from said mesoscopic island portion, and
• an apparatus for coupling said first qubit to a second qubit.

A method for performing an entanglement operation between a first qubit and a second qubit comprising;

• biasing a first qubit, wherein said first qubit comprises a first superconducting loop, said first superconducting loop comprising a first bulk loop portion and a first mesoscopic island portion, said first superconducting loop further comprising at least two Josephson junctions separating said first bulk loop portion from said first mesoscopic island portion;
• and
• biasing a second qubit, wherein said second qubit comprises a second superconducting loop, said second superconducting loop comprising a second bulk loop portion and a second mesoscopic island portion, said second superconducting loop further comprising at least two Josephson junctions separating said second bulk loop portion from said second mesoscopic island portion, wherein said first and second qubits share mutual inductance with each other and wherein said first and second qubits share flux when mutually biased.