A real-time functional-integral approach at finite temperature is used for self-consistent formulation of the dynamics and superconducting phase transition of preformed composite bosons in the extended negative-U Hubbard model in the large-U limit. By employing the representation in terms of the amplitude and phase of the bound-pair field we derive the effective phase Hamiltonian and exploit the effects associated with coherent bound-pair tunneling at the microscopic level---responsible for the phase-coherent state, as well as the dissipative effects due to quasiparticle charge transfer. Striking formal similarity of the physical picture in the large-U limit to those modeled by Josephson-coupled arrays with charging effects is pointed out.
We present investigations of Josephson charge-phase qubits of superconducting quantum interference device (SQUID) configuration, inductively coupled to a radio-frequency-driven tank circuit, enabling the readout of the states by measuring the Josephson inductance of the qubit. The circuits, including junctions with linear dimensions of $60\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\ifmmode\times\else\texttimes\fi{}60\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and $80\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\ifmmode\times\else\texttimes\fi{}80\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, are fabricated from
Nb∕AlOx∕Nb trilayer and allow the determination of relevant sample parameters at liquid helium temperature. The observed partial suppression of the circulating supercurrent in the qubit loop at
4.2K is explained within the framework of a quantum-statistical model. We have probed the ground-state properties of qubit structures with different ratios of the Josephson coupling to Coulomb charging energy at
20mK, demonstrating both the magnetic control of phase and the electrostatic control of charge of the qubit.
. In circuits of underdamped, small-capacitance Josephson junctions, the charge and phase variables behave quantum mechanically. Superpositions of charge states and flux states have been demonstrated, as well as microwave induced quantum dynamics (see e.g. refs [3, 4]). These e#ects have their origin in the conjugation of the phase and charge variables of Josephson circuits. The qubit design that we present here also contains small-capacitance Josephson junctions. However, even though charge-phase conjugation underlies the quantum behavior of the qubit, it was explicitly designed to be very insensitive to charge e#ects. In practice, random background charges and 1/f charge noise are always present in Josephson circuits, but these charge e#ects have negligible influence on the qubit. The qubit loop contains three Josephson junctions. Two junctions have Josephson coupling E J , the third junction has a coupling of #E J .Weaim