7,321 research outputs found
How quickly do cloud droplets form on atmospheric particles?
International audienceThe influence of aerosols on cloud properties is an important modulator of the climate system. Traditional Köhler theory predicts the equilibrium concentration of cloud condensation nuclei (CCN); however, it is not known to what extent particles exist in the atmosphere that may be prevented from acting as CCN by kinetic limitations. We measured the rate of cloud droplet formation on atmospheric particles sampled at four sites across the United States during the summer of 2006: Great Smoky Mountain National Park, TN; Bondville, IL; Houston, TX; and the Atmospheric Radiation Measurement Program Southern Great Plains site near Lamont, OK. We express droplet growth rates with the mass accommodation coefficient (?), and report values of ? measured in the field normalized to the mean ? measured for lab-generated ammonium sulfate (AS) particles (i.e., ?'=?/?AS). Overall, 61% of ambient CCN grew at a rate similar to AS. We report the fraction of CCN that were "low-?'" (?'?0.33). Of the 16 days during which these measurements were made, 7 had relatively few low-?'CCN (77% during at least one ~30 min period). Day to day variability was greatest in Tennessee and Illinois, and low-?' CCN were most prevalent on days when back trajectories suggested that air was arriving from aloft. The highest fractions of low-?' CCN in Houston and Illinois occurred around local noon, and decreased later in the day. These results suggest that for some air masses, accurate quantification of CCN concentrations may need to account for kinetic limitations
On clocks and clouds
Cumulus clouds exhibit a life cycle that consists of (a) the growth phase
(increasing size, most notably in the vertical direction); (b) the mature
phase (growth ceases; any precipitation that develops is strongest during
this period); and (c) the dissipation phase (cloud dissipates because of
precipitation and/or entrainment; no more dynamical support). Although radar
can track clouds over time and give some sense of the age of a cloud, most
aircraft in situ measurements lack temporal context. We use large eddy
simulations of trade wind cumulus cloud fields from cases during the Barbados
Oceanographic and Meteorological Experiment (BOMEX) and Rain In Cumulus over
the Ocean (RICO) campaigns to demonstrate a potential cumulus cloud
"clock." We find that the volume-averaged total water mixing ratio rt
is a useful cloud clock for the 12 clouds studied. A cloud's initial rt
is set by the subcloud mixed-layer mean rt and decreases monotonically
from the initial value due primarily to entrainment. The clock is insensitive
to aerosol loading, environmental sounding and extrinsic cloud properties
such as lifetime and volume. In some cases (more commonly for larger clouds),
multiple pulses of buoyancy occur, which complicate the cumulus clock by
replenishing rt. The clock is most effectively used to classify clouds
by life phase
Quantum Bit Regeneration
Decoherence and loss will limit the practicality of quantum cryptography and
computing unless successful error correction techniques are developed. To this
end, we have discovered a new scheme for perfectly detecting and rejecting the
error caused by loss (amplitude damping to a reservoir at T=0), based on using
a dual-rail representation of a quantum bit. This is possible because (1)
balanced loss does not perform a ``which-path'' measurement in an
interferometer, and (2) balanced quantum nondemolition measurement of the
``total'' photon number can be used to detect loss-induced quantum jumps
without disturbing the quantum coherence essential to the quantum bit. Our
results are immediately applicable to optical quantum computers using single
photonics devices.Comment: 4 pages, postscript only, figures available at
http://feynman.stanford.edu/qcom
Thermal and Athermal Swarms of Self-Propelled Particles
Swarms of self-propelled particles exhibit complex behavior that can arise
from simple models, with large changes in swarm behavior resulting from small
changes in model parameters. We investigate the steady-state swarms formed by
self-propelled Morse particles in three dimensions using molecular dynamics
simulations optimized for GPUs. We find a variety of swarms of different
overall shape assemble spontaneously and that for certain Morse potential
parameters coexisting structures are observed. We report a rich "phase diagram"
of athermal swarm structures observed across a broad range of interaction
parameters. Unlike the structures formed in equilibrium self-assembly, we find
that the probability of forming a self-propelled swarm can be biased by the
choice of initial conditions. We investigate how thermal noise influences swarm
formation and demonstrate ways it can be exploited to reconfigure one swarm
into another. Our findings validate and extend previous observations of
self-propelled Morse swarms and highlight open questions for predictive
theories of nonequilibrium self-assembly.Comment: 21 pages, 7 figure
Ferromagnetic Enhancement of CE-type Spin Ordering in (Pr,Ca)MnO
We present resonant soft X-ray scattering (RSXS) results from small band
width manganites (Pr,Ca)MnO, which show that the CE-type spin ordering (SO)
at the phase boundary is stabilized only below the canted antiferromagnetic
transition temperature and enhanced by ferromagnetism in the macroscopically
insulating state (FM-I). Our results reveal the fragility of the CE-type
ordering that underpins the colossal magnetoresistance (CMR) effect in this
system, as well as an unexpected cooperative interplay between FM-I and CE-type
SO which is in contrast to the competitive interplay between the ferromagnetic
metallic (FM-M) state and CE-type ordering.Comment: Accepted for publication in Phys. Rev. Let
The Influence of Superpositional Wave Function Oscillations on Shor's Quantum Algorithm
We investigate the influence of superpositional wave function oscillations on
the performance of Shor's quantum algorithm for factorization of integers. It
is shown that the wave function oscillations can destroy the required quantum
interference. This undesirable effect can be routinely eliminated using a
resonant pulse implementation of quantum computation, but requires special
analysis for non-resonant implementations.Comment: 4 pages, NO figures, revte
Limitations of Quantum Simulation Examined by Simulating a Pairing Hamiltonian using Nuclear Magnetic Resonance
Quantum simulation uses a well-known quantum system to predict the behavior
of another quantum system. Certain limitations in this technique arise,
however, when applied to specific problems, as we demonstrate with a
theoretical and experimental study of an algorithm to find the low-lying
spectrum of a Hamiltonian. While the number of elementary quantum gates does
scale polynomially with the size of the system, it increases inversely to the
desired error bound . Making such simulations robust to decoherence
using fault-tolerance constructs requires an additional factor of
gates. These constraints are illustrated by using a three qubit nuclear
magnetic resonance system to simulate a pairing Hamiltonian, following the
algorithm proposed by Wu, Byrd, and Lidar.Comment: 6 pages, 2 eps figure
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