24,413 research outputs found
Experimentally reducing the quantum measurement back-action in work distributions by a collective measurement
In quantum thermodynamics, the standard approach to estimate work
fluctuations in unitary processes is based on two projective measurements, one
performed at the beginning of the process and one at the end. The first
measurement destroys any initial coherence in the energy basis, thus preventing
later interference effects. In order to decrease this back-action, a scheme
based on collective measurements has been proposed in~[PRL 118, 070601 (2017)].
Here, we report its experimental implementation in an optical system. The
experiment consists of a deterministic collective measurement on identically
prepared two qubits, encoded in the polarisation and path degree of a single
photon. The standard two projective measurement approach is also experimentally
realized for comparison. Our results show the potential of collective schemes
to decrease the back-action of projective measurements, and capture subtle
effects arising from quantum coherence.Comment: 9 pages, 4 figure
Phantom Friedmann Cosmologies and Higher-Order Characteristics of Expansion
We discuss a more general class of phantom () cosmologies with
various forms of both phantom () matter. We
show that many types of evolution which include both Big-Bang and Big-Rip
singularities are admitted and give explicit examples. Among some interesting
models, there exist non-singular oscillating (or "bounce") cosmologies, which
appear due to a competition between positive and negative pressure of variety
of matter content. From the point of view of the current observations the most
interesting cosmologies are the ones which start with a Big-Bang and terminate
at a Big-Rip. A related consequence of having a possibility of two types of
singularities is that there exists an unstable static universe approached by
the two asymptotic models - one of them reaches Big-Bang, and another reaches
Big-Rip. We also give explicit relations between density parameters
and the dynamical characteristics for these generalized phantom models,
including higher-order observational characteristics such as jerk and "kerk".
Finally, we discuss the observational quantities such as luminosity distance,
angular diameter, and source counts, both in series expansion and explicitly,
for phantom models. Our series expansion formulas for the luminosity distance
and the apparent magnitude go as far as to the fourth-order in redshift
term, which includes explicitly not only the jerk, but also the "kerk" (or
"snap") which may serve as an indicator of the curvature of the universe.Comment: REVTEX 4, 23 pages, references updated, to appear in Annals of
Physics (N.Y.
Spatio-temporal variation in leaf area index in the Yan Mountains over the past 40 years and its relationship to hydrothermal conditions
Changes in hydrothermal conditions have significant effects on vegetation, but there is still a lack of understanding of how vegetation responds to land surface (surface temperature and soil moisture) and meteorological (temperature and precipitation) conditions in mountain regions. This study examined the trends of leaf area index (LAI) in the Yan Mountains over the last four decades using Global Land Surface Satellite (GLASS) data. The results showed a persistent increase of LAI (greening) over 20 % to 80 % of the study area in growing season, spring, summer and autumn. Anthropogenic activities caused the greening trend by crop management before 2000 and afforestation after 2000. The increasing rate of LAI varied with elevation, and the most significant increase occurred in areas between 300 and 900 m, and the lowest increase occurred in areas below 300 m. Moreover, we found that LAI was negatively correlated with land surface temperature and soil moisture, but positively correlated with precipitation and air temperature. The time-lag effect was found between hydro thermal factors and LAI in the past four decades. There was a time lag of 2-3 months between LAI changes and temperature/precipitation during the early and late stages of the growing season, and a time lag of 0-1 month during the middle stage. Specifically, there was no time lag in vegetation response to surface soil moisture, and a time lag of 2-3 months in vegetation response to land surface temperature from July to October. Our findings provide insights into how vegetation adapts to land surface and climatic hydrothermal conditions in mountain regions and can be used by governments to develop policies for ecological protection
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