A 100 GHz Josephson mixer using resistively-shunted Nb tunnel junctions

The authors describe preliminary mixer results using resistively shunted Nb/AlOx/Nb tunnel junctions in a 100-GHz waveguide mixer mount. The mixer utilizes robust, lithographically defined devices which have nonhysteretic I-V curves. A receiver temperature of 390 K (DSB) has been obtained with a conversion loss of -6.5 dB. The receiver's behavior agrees qualitatively with the behavior predicted by the resistively shunted junction model. Substantial improvements in performance are expected with the use of better-optimized shunted junctions and numerical simulations suggest that, if devices with higher ICRN (critical-current normal-resistance) products can be obtained. Josephson effect mixers could be competitive with superconductor-insulator-superconductor (SIS) mixers at high frequencies

Circuit QED and engineering charge based superconducting qubits

The last two decades have seen tremendous advances in our ability to generate and manipulate quantum coherence in mesoscopic superconducting circuits. These advances have opened up the study of quantum optics of microwave photons in superconducting circuits as well as providing important hardware for the manipulation of quantum information. Focusing primarily on charge-based qubits, we provide a brief overview of these developments and discuss the present state of the art. We also survey the remarkable progress that has been made in realizing circuit quantum electrodynamics (QED) in which superconducting artificial atoms are strongly coupled to individual microwave photons.Comment: Proceedings of Nobel Symposium 141: Qubits for Future Quantum Informatio

The millimeter-wave properties of superconducting microstrip lines

We have developed a novel technique for making high quality measurements of the millimeter-wave properties of superconducting thin-film microstrip transmission lines. Our experimental technique currently covers the 75-100 GHz band. The method is based on standing wave resonances in an open ended transmission line. We obtain information on the phase velocity and loss of the microstrip. Our data for Nb/SiO/Nb lines, taken at 4.2 K and 1.6 K, can be explained by a single set of physical parameters. Our preliminary conclusion is that the loss is dominated by the SiO dielectric, with a temperature-independent loss tangent of 5.3 ± 0.5 x 10^(-3) for our samples

Direct access to quantum fluctuations through cross-correlation measurements

Detection of the quantum fluctuations by conventional methods meets certain obstacles, since it requires high frequency measurements. Moreover, quantum fluctuations are normally dominated by classical noise, and are usually further obstructed by various accompanying effects such as a detector backaction. In present work, we demonstrate that these difficulties can be bypassed by performing the cross-correlation measurements. We propose to use a pair of two-level detectors, weakly coupled to a collective mode of an electric circuit. Fluctuations of the current source accumulated in the collective mode induce stochastic transitions in the detectors. These transitions are then read off by quantum point contact (QPC) electrometers and translated into two telegraph processes in the QPC currents. Since both detectors interact with the same collective mode, this leads to a certain fraction of the correlated transitions. These correlated transitions are fingerprinted in the cross-correlations of the telegraph processes, which can be detected at zero frequency, i.e., with a long time measurements. Concerning the dependance of the cross-correlator on the detectors' energy splittings, the most interesting region is at the degeneracy points, where it exhibits a sharp non-local resonance, that stems from higher order processes. We find that at certain conditions the main contribution to this resonance comes from the quantum noise. Namely, while the resonance line shape is weakly broadened by the classical noise, the height of the peak is directly proportional to the square of the quantum component of the noise spectral function.Comment: Added discussion of the time scales in the introduction and one figure. 14 pages, 8 figure

Optimal configurations for normal-metal traps in transmon qubits

Controlling quasiparticle dynamics can improve the performance of superconducting devices. For example, it has been demonstrated effective in increasing lifetime and stability of superconducting qubits. Here we study how to optimize the placement of normal-metal traps in transmon-type qubits. When the trap size increases beyond a certain characteristic length, the details of the geometry and trap position, and even the number of traps, become important. We discuss for some experimentally relevant examples how to shorten the decay time of the excess quasiparticle density. Moreover, we show that a trap in the vicinity of a Josephson junction can reduce the steady-state quasiparticle density near that junction, thus suppressing the quasiparticle-induced relaxation rate of the qubit. Such a trap also reduces the impact of fluctuations in the generation rate of quasiparticles, rendering the qubit more stable.Comment: 16 pages, 7 figures; to appear in Phys. Rev. Applie

High Frequency Quantum Admittance and Noise Measurement with an On-chip Resonant Circuit

By coupling a quantum detector, a superconductor-insulator-superconductor junction, to a Josephson junction \textit{via} a resonant circuit we probe the high frequency properties, namely the ac complex admittance and the current fluctuations of the Josephson junction at the resonant frequencies. The admittance components show frequency dependent singularities related to the superconducting density of state while the noise exhibits a strong frequency dependence, consistent with theoretical predictions. The circuit also allows to probe separately the emission and absorption noise in the quantum regime of the superconducting resonant circuit at equilibrium. At low temperature the resonant circuit exhibits only absorption noise related to zero point fluctuations, whereas at higher temperature emission noise is also present.Comment: 15 pages, 15 figure

Measurement of the Noise Spectrum Using a Multiple-Pulse Sequence

A method is proposed for obtaining the spectrum for noise that causes the phase decoherence of a qubit directly from experimentally available data. The method is based on a simple relationship between the spectrum and the coherence time of the qubit in the presence of a pi-pulse sequence. The relationship is found to hold for every system of a qubit interacting with the classical-noise, bosonic, and spin baths.Comment: 8 pages (4 pages + 4 pages Supplemental material), 1 figur