Superconducting phase qubits with shadow-evaporated Josephson junctions

We develop a fabrication process for the superconducting phase qubits in which Josephson junctions for both the qubit and superconducting quantum interference device（SQUID） detector are prepared by shadow evaporation with a suspended bridge. Al junctions with areas as small as 0.05 μm^2 are fabricated for the qubit, in which the number of the decoherencecausing two-level systems（TLS） residing in the tunnel barrier and proportional to the junction area are greatly reduced. The measured energy spectrum shows no avoided crossing arising from coherent TLS in the experimentally reachable flux bias range of the phase qubit, which demonstrates the energy relaxation time T1 and dephasing time Tφ on the order of 100 ns and 50 ns, respectively. We discuss several possible origins of decoherence from incoherent or weakly-coupled coherent TLS and further improvements of the qubit performance.

The Superconducting Phases in Bi-Sr-Ca-Cu-O and Bi-Pb-Sr-Ca-Cu-O Superconductors at High Temperatures

The addition Of Pb enhances the hightemperature stability as well as theproportion of the high Tphase in Bi-Sr-Ca-Cu-O superconductor.The sample A with nominal composi-tion of BiSrCaCuOwas synthesized bySolid state reaction.It was sintered in airat 1153K for 24h,followed by annealingat 773K for 24h and furnace-cooling.The sample B with a nominal compositionof BiPbSrCaCuOwas prepared bymixing PbO with the powder ofBiSrCaCuOwhich had been prefired at1113K for 24h,pressing,sintering at

Effect of heat treatment on electronic phase in underdoped La_(2-x)Sr_xCuO_4 single crystal

Superconducting La1.937Sr0.063CuO4 crystals grown by the travelling-solvent floating-zone technique were thermally treated under various temperatures and oxygen pressures for moderately adjusting the oxygen content. The response of intrinsic electronic property of the crystals to the change of hole density in La2-xSrxCuO4 in the vicinity of the magic doping of x= 1/16 （= 0.0625） is studied in detail by magnetic measurements under various fields up to 1 T. It is found that when the superconducting critical temperature （Tc） increases with the oxygen content, there appears also a new subtle electronic state that can be detected from the differential curves of diamagnetic susceptibility dx/dT of the crystal sample. In contrast with the intrinsic state, the new subtle electronic state is very fragile under the magnetic fields. Our results indicate that a moderate change in oxygen doping does not significantly modify the intrinsic electronic state originally existing at the magic doping level.

Temperature Dependence of phase Transitions and Superconductivity in YBa_2Cu_3O_(7-X)

By means of X-ray diffraction investigations and electric resistivity measurements the tempera- ture dependence of phase structure and supercon- ductivity in YBa_2Cu_3O_(7_x) over the temperature range of 20℃-950℃ have been studied.The lattice parameters a,b and c as well as conductivity as a function of annealing temperatures can be roughly divided into three zones which are superconductive orthorhombic structure 1(a<b(?)c/3) at 20℃-500℃,normal conductive orthorhombic structure 2(a<b<c/3)from 550℃ to about 750℃ and semiconductive-like tetragonal structure (a=b<c/3)from about 800℃ to 950℃.However, the boundaries of the three zones is not much distinctive because there is a continuous transi- tion from the orthorhombic to tetragonal struc- ture.Furthermore,the processes of orthorhom- bic-tetragonal transition in the range of 350℃-950℃ are not only continuous but also reversible.The isothermal transition of orthorhombic 2 to orthorhombic 1 occurs at 350℃-500℃,while the maximum rate of transition and high T_c superconductivity can be obtained at about 460℃.This process of isothermal transition depends on the ordering of oxygen atoms or vacancies and the thermodynamic equilibrium to allow the oxygen diffusion.