A circuit comprising a superconducting qubit and a resonant control system that
is characterized by a resonant frequency. The resonant frequency of the control
system is a function of a bias current. The circuit further includes a superconducting
mechanism having a capacitance or inductance. The superconducting mechanism coherently
couples the superconducting qubit to the resonant control system. A method for
entangling a quantum state of a first qubit with the quantum state of a second
qubit. In the method, a resonant control system, which is capacitively coupled
to the first and second qubit, is tuned to a first frequency that corresponds to
the energy differential between the lowest two potential energy levels of the first
qubit. The resonant control system is then adjusted to a second frequency corresponding
to energy differential between the lowest two potential energy levels of the second qubit.