12,211 research outputs found
Quantum Heating of a nonlinear resonator probed by a superconducting qubit
We measure the quantum fluctuations of a pumped nonlinear resonator, using a
superconducting artificial atom as an in-situ probe. The qubit excitation
spectrum gives access to the frequency and temperature of the intracavity field
fluctuations. These are found to be in agreement with theoretical predictions;
in particular we experimentally observe the phenomenon of quantum heating
Circuit QED with a Nonlinear Resonator : ac-Stark Shift and Dephasing
We have performed spectroscopic measurements of a superconducting qubit
dispersively coupled to a nonlinear resonator driven by a pump microwave field.
Measurements of the qubit frequency shift provide a sensitive probe of the
intracavity field, yielding a precise characterization of the resonator
nonlinearity. The qubit linewidth has a complex dependence on the pump
frequency and amplitude, which is correlated with the gain of the nonlinear
resonator operated as a small-signal amplifier. The corresponding dephasing
rate is found to be close to the quantum limit in the low-gain limit of the
amplifier.Comment: Paper : 4 pages, 3 figures; Supplementary material : 1 page, 1 figur
Characterization of a two-transmon processor with individual single-shot qubit readout
We report the characterization of a two-qubit processor implemented with two
capacitively coupled tunable superconducting qubits of the transmon type, each
qubit having its own non-destructive single-shot readout. The fixed capacitive
coupling yields the \sqrt{iSWAP} two-qubit gate for a suitable interaction
time. We reconstruct by state tomography the coherent dynamics of the two-bit
register as a function of the interaction time, observe a violation of the Bell
inequality by 22 standard deviations after correcting readout errors, and
measure by quantum process tomography a gate fidelity of 90%
Transport phenomenology for a holon-spinon fluid
We propose that the normal-state transport in the cuprate superconductors can
be understood in terms of a two-fluid model of spinons and holons. In our
scenario, the resistivity is determined by the properties of the holons while
magnetotransport involves the recombination of holons and spinons to form
physical electrons. Our model implies that the Hall transport time is a measure
of the electron lifetime, which is shorter than the longitudinal transport
time. This agrees with our analysis of the normal-state data. We predict a
strong increase in linewidth with increasing temperature in photoemission. Our
model also suggests that the AC Hall effect is controlled by the transport
time.Comment: 4 pages, 1 postscript figure. Uses RevTeX, epsf, multico
Charge dynamics in the phase string model for high-Tc superconductors
An understanding of the anomalous charge dynamics in the high-Tc cuprates is
obtained based on a model study of doped Mott insulators. The high-temperature
optical conductivity is found to generally have a two-component structure: a
Drude like part followed by a mid-infrared band. The scattering rate associated
with the Drude part exhibits a linear-temperature dependence over a wide range
of high temperature, while the Drude term gets progressively suppressed below a
characteristic energy of magnetic origin as the system enters the pseudogap
phase. The high-energy optical conductivity shows a resonancelike feature in an
underdoped case and continuously evolves into a 1/\omega tail at higher doping,
indicating that they share the same physical origin. In particular, such a
high-energy component is closely correlated with the \omega-peak structure of
the density-density correlation function at different momenta, in systematic
consistency with exact diagonalization results based on the t-J model. The
underlying physics is attributed to the high-energy spin-charge separation in
the model, in which the "mode coupling" responsible for the anomalous charge
properties is not between the electrons and some collective mode but rather
between new charge carriers, holons, and a novel topological gauge field
controlled by spin dynamics, as the consequence of the strong short-range
electron-electron Coulomb repulsion in the doped Mott insulator.Comment: 19 pages, 13 figures; final version to appear in Phys. Rev.
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