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We describe the design of an inductor-capacitor (LC) network to increase the isolation of a dc SQUID phase qubit from its current bias leads and thereby increase the relaxation time T1 and coherence time T2. One junction in the SQUID acts as an ideal phase qubit while the second junction and the SQUID loop inductance act as a broadband inductive filter to isolate the first junction from the current bias leads. The LC isolation network provides an additional isolation factor at the junction plasma frequency and allows flexibility in the choice of SQUID parameters. Our thin-film on-chip LC isolation network has a 10 nH inductor and an 80 pF capacitor. The combination of the broadband filter and LC filter provides a maximum nominal isolation factor of about 108 at a qubit plasma frequency of about 6.7 GHz. To reduce dielectric loss and two level systems in the qubit junction, we use a relatively small area (4 mum2) Al/AlOx/Al qubit junction built on sapphire and add an external capacitor with 100 nm thick SiNx dielectric layers. Measurements revealed Rabi oscillations with an envelope decay time constant of about 42 ns, and an energy relaxation time of 32 ns, consistent with a loss tangent tan(delta)=7times10-4 in the SiNx.

We report measurements of spectroscopic linewidth and Rabi oscillations in three thin-film dc SQUID phase qubits. One device had a single-turn Al loop, the second had a 6-turn Nb loop, and the third was a first order gradiometer formed from 6-turn wound and counter-wound Nb coils to provide isolation from spatially uniform flux noise. In the 6 - 7.2 GHz range, the spectroscopic coherence times for the gradiometer varied from 4 ns to 8 ns, about the same as for the other devices (4 to 10 ns). The time constant for decay of Rabi oscillations was significantly longer in the single-turn Al device (20 to 30 ns) than either of the Nb devices (10 to 15 ns). These results imply that spatially uniform flux noise is not the main source of decoherence or inhomogenous broadening in these devices.

We have used 2 ns pulses of current to read out the states of a Nb/AlOx/Nb dc SQUID phase qubit at 25 mK. Plotting the number of escape events that occur versus the size of the current pulse reveals a series of steps corresponding to the occupancy of different energy levels. After calibrating the measurement pulses, we fit these steps to determine the population in each level. Rabi oscillations were viewed using this technique. The single-shot measurement fidelity was theoretically analysed and the optimal single pulse in our device was determined.

We present measurements of the spectroscopic coherence time, T2* of a Nb-AlOx-Nb dc SQUID phase qubit. T2* is calculated from the spectroscopic width of the |0rang to |1rang transition measured at 25 mK. We find that T2*increases at higher transition frequencies corresponding to measurements deeper in the potential well. The measured are consistent with root mean square current noise saturating at 4 nA.

We calculate the effective capacitance of a Cooper-pair box and demonstrate that the Cooper-pair box can be used as a variable capacitor. The gate voltage modulates the charge transfer through the small junction in the Cooper-pair box, leading to a gate-dependent capacitance. We describe ongoing experiments to use an Al/AlO/sub x//Al Cooper-pair box as a tunable circuit element at milli-Kelvin temperatures and discuss possible applications to quantum computing as a variable coupling element between two Josephson junction qubits.

This paper discusses some of the major experimental features of microwave nonlinearity in high temperature superconductors, both intrinsic and extrinsic. The case is made for solving the problem of extrinsic nonlinearity through the use of localized measurements of microwave surface impedance and electromagnetic fields. Along these lines, a brief introduction is given to our work on scanning near-field microwave microscopy.

We have developed a scanning microwave microscope which can image features with 20 /spl mu/m spatial resolution. The microscope consists of a section of open-ended coaxial cable which is scanned over the surface of a planar sample. Images can be made in either passive mode, in which the reflectivity of the probe tip is measured as a function of position, or in active mode, in which stray fields from the sample are picked up by the scanning probe and measured with a vector demodulation circuit. We have imaged reflectivity variations of metallic and superconducting samples in passive mode to determine the spatial resolution of the technique. Images are also presented in active mode of a superconducting microwave device taken at liquid nitrogen temperature.

We report spectroscopic measurements of discrete two-level systems (TLSs) coupled to a dc superconducting quantum interference device phase qubit with a 16μ m 2 area Al/ AlO x /Al junction. Applying microwaves in the 10-11 GHz range, we found eight avoided level crossings with splitting sizes from 10 to 200 MHz and spectroscopic lifetimes from 4 to 160 ns. Assuming the transitions are from the ground state of the composite system to an excited state of the qubit or an excited state of one of the TLS states, we fit the location and spectral width to get the energy levels, splitting sizes, and spectroscopic coherence times of the phase qubit and TLSs. The distribution of splittings is consistent with noninteracting individual charged ions tunneling between random locations in the tunnel barrier and the distribution of lifetimes is consistent with the AlOx in the junction barrier having a frequency-independent loss tangent. To check that the charge of each TLS couples independently to the voltage across the junction, we also measured the spectrum in the 20-22 GHz range and found tilted avoided level crossings due to the second excited state of the junction and states in which both the junction and a TLS were excited.

Rabi oscillations have been observed in many superconducting devices, and represent prototypical logic operations for quantum bits (qubits) in a quantum computer. We use a three-level multiphoton analysis to understand the behavior of the superconducting phase qubit (current-biased Josephson junction) at high microwave drive power. Analytical and numerical results for the ac Stark shift, single-photon Rabi frequency, and two-photon Rabi frequency are compared to measurements made on a dc SQUID phase qubit with Nb/AlOx/Nb tunnel junctions. Good agreement is found between theory and experiment., 4 pages, 4 figures, accepted for publication in IEEE Trans. Appl. Supercond

The dc SQUID qubit can be viewed as a single current biased Josephson junction attached to an inductiveisolation network. Excellent broadband isolation is possible and is adjustable in situ . The isolation networkincreases the eective shunt resistance due to the lead impedance allowing for long energy dissipation times T 1 .We present data on Rabi oscillations, and macroscopic quantum tunneling as isolation from the bias leads isvaried.Keywords: Quantum Computing, Josephson Junctions, Rabi Oscillations 1. INTRODUCTION Quantum computation requires a two-level system with long coherence time. The states need to be repeatedlymanipulated and read out with high “delity. Remarkable progress towards achieving a viable quantum bit(qubit) has been achieved with a wide variety of circuits containing Josephson tunnel junctions. 1 5 Despitethese strides, energy dissipation times, T 1 , and quantum coherence times, T 2 , in these qubits are still shorterthan desirable. Understanding the causes of dissipation and decoherence will be the key to obtaining a workingquantum computer.In this paper, we discuss one particular Josephson junction qubit design, the dc SQUID qubit.

We describe a technique for initializing the flux state of an inductively isolated Josephson junction, fulfilling an essential requirement for using the device as a qubit. By oscillating the applied magnetic flux with a specified amplitude and offset, we can select any of the allowed long-lived metastable flux states. We applied this technique to $\mathrm{Nb}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{Nb}$ and $\mathrm{Al}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{Al}$ devices with from 10 to over 100 distinct flux states at temperatures as low as $25\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. In a ten-state system with an initial probability $p=0.13$ to be in the desired flux state, we achieved $p=0.999\phantom{\rule{0.2em}{0ex}}96$ after 50 oscillations at $22.6\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$. The technique is generally applicable to other systems with multiple metastable wells (including rf SQUIDs), requires no additional readout or bias lines, involves minimal energy dissipation, and appears to scale favorably with the number of qubits.

We study the quantum mechanical behavior of a macroscopic, three-body, superconducting circuit. Microwave spectroscopy on our system, a resonator coupling two large Josephson junctions, produced complex energy spectra well explained by quantum theory over a large frequency range. By tuning each junction separately into resonance with the resonator, we first observe strong coupling between each junction and the resonator. Bringing both junctions together into resonance with the resonator, we find spectroscopic evidence for entanglement between all three degrees of freedom and suggest a new method for controllable coupling of distant qubits, a key step toward quantum computation., Comment: 4 pages, 3 figures

Superconducting microwave devices are now beginning to show significant potential for cellular telephone applications. The key limitations to this new technology, besides reliable inexpensive cryogenic cooling, are the problems of power dependence and nonlinearity. We have employed imaging techniques, including scanning near-field microwave microscopy of devices and materials, to begin examining the microscopic origins of nonlinearity.

We describe the near-field microwave microscopy of microwave devices on a length scale much smaller than the wavelength used for imaging. Our microscope can be operated in two possible configurations, allowing a quantitative study of either material properties or local electric fields., 4 pages, 8 figures, minor corrections to text and 2 figures