514 research outputs found
Loss Dependence on Geometry and Applied Power in Superconducting Coplanar Resonators
The loss in superconducting microwave resonators at low-photon number and low
temperatures is not well understood but has implications for achievable
coherence times in superconducting qubits. We have fabricated single-layer
resonators with a high quality factor by patterning a superconducting aluminum
film on a sapphire substrate. Four resonator geometries were studied with
resonant frequencies ranging from 5 to 7 GHz: a quasi-lumped element resonator,
a coplanar strip waveguide resonator, and two hybrid designs that contain both
a coplanar strip and a quasi-lumped element. Transmitted power measurements
were taken at 30 mK as a function of frequency and probe power. We find that
the resonator loss, expressed as the inverse of the internal quality factor,
decreases slowly over four decades of photon number in a manner not merely
explained by loss from a conventional uniform spatial distribution of two-level
systems in an oxide layer on the superconducting surfaces of the resonator.Comment: 4 pages, 5 figures, Submitted to ASC 2010 conference proceeding
Direct measurement of the electron-phonon relaxation rate in thin copper films
We have used normal metal-insulator-superconductor (NIS) tunnel junction
pairs, known as SINIS structures, for ultrasensitive thermometry at sub-Kelvin
temperatures. With the help of these thermometers, we have developed an
ac-technique to measure the electron-phonon (e-p) scattering rate directly,
without any other material or geometry dependent parameters, based on
overheating the electron gas. The technique is based on Joule heating the
electrons in the frequency range DC-10 MHz, and measuring the electron
temperature in DC. Because of the nonlinearity of the electron-phonon coupling
with respect to temperature, even the DC response will be affected, when the
heating frequency reaches the natural cut-off determined by the e-p scattering
rate. Results on thin Cu films show a behavior for the scattering rate,
in agreement with indirect measurement of similar samples and numerical
modeling of the non-linear response.Comment: to appear in Phys. Stat. Solidi (2005
Two stage superconducting quantum interference device amplifier in a high-Q gravitational wave transducer
We report on the total noise from an inductive motion transducer for a
gravitational-wave antenna. The transducer uses a two-stage SQUID amplifier and
has a noise temperature of 1.1 mK, of which 0.70 mK is due to back-action noise
from the SQUID chip. The total noise includes thermal noise from the transducer
mass, which has a measured Q of 2.60 X 10^6. The noise temperature exceeds the
expected value of 3.5 \mu K by a factor of 200, primarily due to voltage noise
at the input of the SQUID. Noise from flux trapped on the chip is found to be
the most likely cause.Comment: Accepted by Applied Physics Letters tentatively scheduled for March
13, 200
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