We have observed signatures of resonant tunneling in an Al three-junction qubit, inductively coupled to a Nb LC tank circuit. The resonant properties of the tank oscillator are sensitive to the effective susceptibility (or inductance) of the qubit, which changes drastically as its flux states pass through degeneracy. The tunneling amplitude is estimated from the data. We find good agreement with the theoretical predictions in the regime of their validity.PACS numbers: 85.25. Cp, 85.25.Dq, 84.37.+q, 03.67.Lx Several groups, using different devices, have by now established that superconductors can behave as macroscopic quantum objects.1-3 These are natural candidates for a qubit, the building block of a quantum computer. Qubits are effectively two-level systems with timedependent parameters. One of them is a superconducting loop with low inductance L, including three Josephson junctions (a 3JJ qubit).4 Its potential energy, U = For suitable parameters, U (φ 1 , φ 2 ) has two minima corresponding to qubit states Ψ l and Ψ r , carrying opposite supercurrents around the loop. These become degenerate for Φ x = 1 2 Φ 0 . The Coulomb energy E C (≡ e 2 /2C, with C the capacitance of junction 1) introduces quantum uncertainty in the φ j . Hence, near degeneracy the system can tunnel between the two potential minima.(Since E C ≪ E J ≡ E J1 , we deal with a flux qubit; E C ≫ E J yields a charge qubit. Coherent tunneling was demonstrated in both.)In the basis {Ψ l , Ψ r } and near Φ x = 1 2 Φ 0 , the qubit can be described by the Hamiltonian∆ is the tunneling amplitude. At bias ǫ = 0 the two lowest energy levels of the qubit anticross [ Fig. 1(a)], with a gap of 2∆. Increasing ǫ slowly enough, the qubit can adiabatically transform from Ψ l to Ψ r , staying in the ground state E − . Since dE − /dΦ x is the persistent loop current, the curvature d

We investigated both theoretically and experimentally dynamic features of a phase-biased charge qubit consisting of a single-Cooper-pair transistor closed by a superconducting loop. The effective inductance of the qubit was probed by a high-quality tank circuit. In the presence of a microwave power, with a frequency of the order of the qubit energy level separation, an alteration of the qubit inductance was observed. We demonstrate that this effect is caused by the redistribution of the qubit level population. The excitation of the qubit by one-, two-, and three-photon processes was detected. Quantitative agreement between theory and experimental data was found.

We implemented experimentally an interferometer-type charge qubit consisting of a single-Cooper-pair transistor closed by a superconducting loop that is in flip-chip configuration inductively coupled to a radiofrequency tank circuit. The tank permits us to readout the qubit state, acting as an inductance measuring apparatus. By applying continuous microwave power to the quantum device, we observed inductance alterations caused by redistributions of the energy-level populations. From the measured data we extracted the energy gap between ground and upper levels as a function of the transistor quasicharge as well as the Josephson phase across both junctions.