341 research outputs found
Effects of Interactions on the Critical Temperature of a Trapped Bose Gas
We perform high-precision measurements of the condensation temperature of a
harmonically-trapped atomic Bose gas with widely-tuneable interactions. For
weak interactions we observe a negative shift of the critical temperature in
excellent agreement with mean-field theory. However for sufficiently strong
interactions we clearly observe an additional positive shift, characteristic of
beyond-mean-field critical correlations. We also discuss non-equilibrium
effects on the apparent critical temperature for both very weak and very strong
interactions.Comment: 4 pages, 4 figure
Phase-locking transition in a chirped superconducting Josephson resonator
By coupling a harmonic oscillator to a quantum system it is possible to
perform a dispersive measurement that is quantum non-demolition (QND), with
minimal backaction. A non-linear oscillator has the advantage of measurement
gain, but what is the backaction? Experiments on superconducting quantum bits
(qubits) coupled to a non-linear Josephson oscillator have thus far utilized
the switching of the oscillator near a dynamical bifurcation for sensitivity,
and have demonstrated partial QND measurement. The detailed backaction
associated with the switching process is complex, and may ultimately limit the
degree to which such a measurement can be QND. Here we demonstrate a new
dynamical effect in Josephson oscillators by which the bifurcation can be
accessed without switching. When energized with a frequency chirped drive with
an amplitude close to a sharp, phase-locking threshold, the oscillator evolves
smoothly in one of two diverging trajectories - a pointer for the state of a
qubit. The observed critical behavior agrees well with theory and suggests a
new modality for quantum state measurement.Comment: 5 pages, 4 figure
Millikelvin thermal and electrical performance of lossy transmission line filters
We report on the scattering parameters and Johnson noise emission of low-pass
stripline filters employing a magnetically loaded silicone dielectric down to
25 mK. The transmission characteristic of a device with =1.3 GHz
remains essentially unchanged upon cooling. Another device with =0.4
GHz, measured in its stopband, exhibits a steady state noise power emission
consistent with a temperature difference of a few mK relative to a
well-anchored cryogenic microwave attenuator at temperatures down to 25 mK,
thus presenting a matched thermal load.Comment: 4 pages, 4 figure
Collective effects in charge transfer within a hybrid organic-inorganic system
A collective electron transfer (ET) process was discovered by studying the
current noise in a field effect transistor with light-sensitive gate formed by
nanocrystals linked by organic molecules to its surface. Fluctuations in the ET
through the organic linker are reflected in the fluctuations of the transistor
conductivity. The current noise has an avalanche character. Critical exponents
obtained from the noise power spectra, avalanche distributions, and the
dependence of the average avalanche size on avalanche duration are consistent
with each other. A plausible model is proposed for this phenomenonComment: 15 pages 4 figures. Accepted for publication in Physical Review
Letter
Efficient Production of Large 39K Bose-Einstein Condensates
We describe an experimental setup and the cooling procedure for producing 39K
Bose-Einstein condensates of over 4x10^5 atoms. Condensation is achieved via a
combination of sympathetic cooling with 87Rb in a
quadrupole-Ioffe-configuration (QUIC) magnetic trap, and direct evaporation in
a large volume crossed optical dipole trap, where we exploit the broad Feshbach
resonance at 402 G to tune the 39K interactions from weak and attractive to
strong and repulsive. In the same apparatus we create quasi-pure 87Rb
condensates of over 8x10^5 atoms.Comment: 7 pages, 5 figures; figure font compatibility improve
Field Investigation of Damage to Islamic Monuments Caused by the Egyptian Earthquake of October 12, 1992
https://deepblue.lib.umich.edu/bitstream/2027.42/154130/1/39015094007583.pd
Poisson transition rates from time-domain measurements with finite bandwidth
In time-domain measurements of a Poisson two-level system, the observed
transition rates are always smaller than those of the actual system, a general
consequence of finite measurement bandwidth in an experiment. This
underestimation of the rates is significant even when the measurement and
detection apparatus is ten times faster than the process under study. We derive
here a quantitative form for this correction using a straightforward
state-transition model that includes the detection apparatus, and provide a
method for determining a system's actual transition rates from
bandwidth-limited measurements. We support our results with computer
simulations and experimental data from time-domain measurements of
quasiparticle tunneling in a single-Cooper-pair transistor.Comment: 4 pages, 5 figure
Can a Bose gas be saturated?
Bose-Einstein condensation is unique among phase transitions between
different states of matter in the sense that it occurs even in the absence of
interactions between particles. In Einstein's textbook picture of an ideal gas,
purely statistical arguments set an upper bound on the number of particles
occupying the excited states of the system, and condensation is driven by this
saturation of the quantum vapour. Dilute ultracold atomic gases are celebrated
as a realisation of Bose-Einstein condensation in close to its purely
statistical form. Here we scrutinise this point of view using an ultracold gas
of potassium (39K) atoms, in which the strength of interactions can be tuned
via a Feshbach scattering resonance. We first show that under typical
experi-mental conditions a partially condensed atomic gas strongly deviates
from the textbook concept of a saturated vapour. We then use measurements at a
range of interaction strengths and temperatures to extrapolate to the
non-interacting limit, and prove that in this limit the behaviour of a Bose gas
is consistent with the saturation picture. Finally, we provide evidence for the
universality of our observations through additional measurements with a
different atomic species, 87Rb. Our results suggest a new way of characterising
condensation phenomena in different physical systems.Comment: 6 pages, 5 figure
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