Microscopic two-level systems ͑TLSs͒ are known to contribute to loss in resonant superconducting microwave circuits. This loss increases at low power and temperatures as the TLSs become unsaturated. We find that the loss is dependent on both the substrate-superconductor interface and the roughness of the surfaces. A native, oxide-free interface reduced the loss due to TLSs. However, a rough surface in the CPW gap did not cause more TLS loss, but the overall loss was significantly increased for the roughest surfaces.

Abstract. We present a fabrication scheme and testing results for epitaxial submicrometer Josephson junctions. The junctions are made using a high-temperature (1170 K) "via process" yielding junctions as small as 0.8 µm in diameter by use of optical lithography. Sapphire (Al 2 O 3 ) tunnel-barriers are grown on an epitaxial Re/Ti multilayer base-electrode. We have fabricated devices with both Re and Al top electrodes. While room-temperature (295 K) resistance versus area data are favorable for both types of top electrodes, the low-temperature (50 mK) data show that junctions with the Al top electrode have a much higher subgap resistance. The microwave loss properties of the junctions have been measured by use of superconducting Josephson junction qubits. The results show that high subgap resistance correlates to improved qubit performance.

This paper demonstrates direct detection of weak signals at microwave frequencies based on parametric frequency conversion. The atomic medium is optically pumped by a resonant light field and prepared in a coherent atomic superposition by a weak microwave field. The coherent atomic superposition causes a parametric modulation of the polarization of a probe light field at the microwave field frequency. An upper limit magnetic field component sensitivity of 1.2(1.0) pT/Hz 1/2 , corresponding to 3.7(3.1)-µV/cm/Hz 1/2 electric field component sensitivity, is achieved at ∼6.835 GHz with a 33-mm 3 vapor cell.

We demonstrate a remote sensing design of phase qubits by separating the control and readout circuits from the qubit loop. This design improves measurement reliability because the control readout chip can be fabricated using more robust materials and can be reused to test different qubit chips. Typical qubit measurements such as Rabi oscillations, spectroscopy, and excited-state energy relaxation are presented.Superconducting phase qubits are one of the most promising technologies for a scalable quantum computer.

Magnetic field distributions were measured for test stripline structures with anisotropic magnetoresistive ͑AMR͒, thin film Permalloy sensors. Linear arrays of eight barber-pole-type AMR sensors were used to increase the speed of the imaging and stability of scanning. The magnetic field distributions were converted to the distribution of electric current density in a stripline by inverting Maxwell's equations.