We have measured the current-phase relationship I(ϕ) of symmetric 45 • YBa2Cu3O7−x grain boundary Josephson junctions. Substantial deviations of the Josephson current from conventional tunnel-junction behavior have been observed: (i) The critical current exhibits, as a function of temperature T , a local minimum at a temperature T * . (ii) At T ≈ T * , the first harmonic of I(ϕ) changes sign. (iii) For T < T * , the second harmonic of I(ϕ) is comparable to the first harmonic, and (iv) the ground state of the junction becomes degenerate. The results are in good agreement with a microscopic model of Josephson junctions between d-wave superconductors.The most important phenomenological difference between the high-T c cuprates and conventional superconductors regards the orbital symmetry of the superconducting order parameter. In the cuprates the pair potential changes sign depending on the direction in momentum space according to 1,2 ∆(ϑ) = ∆ 0 cos 2(ϑ − θ), where ϑ is the angle between the wave vector and the (laboratory) x-axis, while θ is the angle between the Cu-Cu bond direction of the superconductor and the x-axis. This unconventional d-wave symmetry was predicted 3 and experimentally confirmed 1,2 to be directly measurable in the Josephson effect between a high-T c and a conventional superconductor. Another consequence of the dwave symmetry is that mid-gap states (MGS) with energy ε = 0 should form on the free surface of a d-wave superconductor if ∆(ϑ) has opposite signs on incident and reflected electronic trajectories. 4 The MGS density must be maximal for (110)-like surfaces and this prediction has in fact been confirmed by STM microscopy on YBCO single crystals 5 which revealed the MGS contribution to the YBCO tunneling density of states. The presence of the MGS is expected to influence in a spectacular way also the Josephson effect in junctions between d-wave superconductors with different crystallographic orientations. Yet no clear manifestation of the MGS in the Josephson effect in such junctions has been observed so far, which is a challenge for the concept of d-wave superconductivity in the cuprates.Moreover, due to possible applications in quantum computing, 6,7 there is substantial interest in Josephson junctions and circuits with a doubly degenerate ground state. Such a state was predicted in an asymmetric 45 • junction (θ 1 = 0 • and θ 2 = 45 • , the angles θ 1,2 are defined in Fig. 1), since odd harmonics of the Josephson current I(ϕ) = n I n sin nϕ are suppressed by symmetry. 8,9The current-phase relation observed in Ref. 10 indeed showed a substantial contribution of the second harmonic I 2 . However, there is a finite supercurrent flowing along the interface in the ground state of asymmetric 45 • junctions. 9 Therefore they do not lead to completely quiet qubits in the sense of Ref. 6. Motivated by the search for both, the MGS in high-T c Josephson junctions and a quiet qubit, we have studied symmetric 45 • junctions (i.e. junctions with θ 1 = −θ 2 = 22.5 • ). In this paper we report the fir...

We study the loss rate for a set of lambda/2 coplanar waveguide resonators at millikelvin temperatures (20 mK - 900mK) and different applied powers (3E-19 W - 1E-12 W). The loss rate becomes power independent below a critical power. For a fixed power, the loss rate increases significantly with decreasing temperature. We show that this behavior can be caused by two-level systems in the surrounding dielectric materials. Interestingly, the influence of the two-level systems is of the same order of magnitude for the different material combinations. That leads to the assumption that the nature of these two-level systems is material independent.Comment: 3 pages, 5 figures, Submitted to Applied Physics Letter

The current-phase relation (CPR) for asymmetric 45 • Josephson junctions between two d-wave superconductors has been predicted to exhibit an anomalous periodicity. We have used the singlejunction interferometer to investigate the CPR for this kind of junctions in YBa2Cu3O7−x thin films. Half-fluxon periodicity has been experimentally found, providing a novel source of evidence for the d-wave symmetry of the pairing state of the cuprates.

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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.