477 research outputs found
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
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
Tunable dynamical channel blockade in double-dot Aharonov-Bohm interferometers
We study electronic transport through an Aharonov-Bohm interferometer with
single-level quantum dots embedded in the two arms. The full counting
statistics in the shot-noise regime is calculated to first order in the
tunnel-coupling strength. The interplay of interference and charging energy in
the dots leads to a dynamical channel blockade that is tunable by the magnetic
flux penetrating the Aharonov-Bohm ring. We find super-Poissonian behavior with
diverging second and higher cumulants when the Aharonov-Bohm flux approaches an
integer multiple of the flux quantum.Comment: published version, 10 pages, 10 figure
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