21,402 research outputs found
Zeno effect and ergodicity in finite-time quantum measurements
We demonstrate that an attempt to measure a non-local in time quantity, such
as the time average \la A\ra_T of a dynamical variable , by separating
Feynman paths into ever narrower exclusive classes traps the system in
eigensubspaces of the corresponding operator \a. Conversely, in a long
measurement of \la A\ra_T to a finite accuracy, the system explores its
Hilbert space and is driven to a universal steady state in which von Neumann
ensemble average of \a coincides with \la A\ra_T. Both effects are
conveniently analysed in terms of singularities and critical points of the
corresponding amplitude distribution and the Zeno-like behaviour is shown to be
a consequence of conservation of probability
Correlated Equilibria of Classical Strategic Games with Quantum Signals
Correlated equilibria are sometimes more efficient than the Nash equilibria
of a game without signals. We investigate whether the availability of quantum
signals in the context of a classical strategic game may allow the players to
achieve even better efficiency than in any correlated equilibrium with
classical signals, and find the answer to be positive.Comment: 8 pages, LaTe
Is MS1054-03 an exceptional cluster? A new investigation of ROSAT/HRI X-ray data
We reanalyzed the ROSAT/HRI observation of MS1054-03, optimizing the channel
HRI selection and including a new exposure of 68 ksec. From a wavelet analysis
of the HRI image we identify the main cluster component and find evidence for
substructure in the west, which might either be a group of galaxies falling
onto the cluster or a foreground source. Our 1-D and 2-D analysis of the data
show that the cluster can be fitted well by a classical betamodel centered only
20arcsec away from the central cD galaxy. The core radius and beta values
derived from the spherical model(beta = 0.96_-0.22^+0.48) and the elliptical
model (beta = 0.73+/-0.18) are consistent. We derived the gas mass and total
mass of the cluster from the betamodel fit and the previously published ASCA
temperature (12.3^{+3.1}_{-2.2} keV). The gas mass fraction at the virial
radius is fgas = (14[-3,+2.5]+/-3)% for Omega_0=1, where the errors in brackets
come from the uncertainty on the temperature and the remaining errors from the
HRI imaging data. The gas mass fraction computed for the best fit ASCA
temperature is significantly lower than found for nearby hot clusters,
fgas=20.1pm 1.6%. This local value can be matched if the actual virial
temperature of MS1054-032 were close to the lower ASCA limit (~10keV) with an
even lower value of 8 keV giving the best agreement. Such a bias between the
virial and measured temperature could be due to the presence of shock waves in
the intracluster medium stemming from recent mergers. Another possibility, that
reconciles a high temperature with the local gas mass fraction, is the
existence of a non zero cosmological constant.Comment: 12 pages, 5 figures, accepted for publication in Ap
Bubble statistics and coarsening dynamics for quasi-two dimensional foams with increasing liquid content
We report on the statistics of bubble size, topology, and shape and on their
role in the coarsening dynamics for foams consisting of bubbles compressed
between two parallel plates. The design of the sample cell permits control of
the liquid content, through a constant pressure condition set by the height of
the foam above a liquid reservoir. We find that in the scaling state, all
bubble distributions are independent not only of time but also of liquid
content. For coarsening, the average rate decreases with liquid content due to
the blocking of gas diffusion by Plateau borders inflated with liquid. By
observing the growth rate of individual bubbles, we find that von Neumann's law
becomes progressively violated with increasing wetness and with decreasing
bubble size. We successfully model this behavior by explicitly incorporating
the border blocking effect into the von Neumann argument. Two dimensionless
bubble shape parameters naturally arise, one of which is primarily responsible
for the violation of von Neumann's law for foams that are not perfectly dry
Measurement of the total energy of an isolated system by an internal observer
We consider the situation in which an observer internal to an isolated system
wants to measure the total energy of the isolated system (this includes his own
energy, that of the measuring device and clocks used, etc...). We show that he
can do this in an arbitrarily short time, as measured by his own clock. This
measurement is not subjected to a time-energy uncertainty relation. The
properties of such measurements are discussed in detail with particular
emphasis on the relation between the duration of the measurement as measured by
internal clocks versus external clocks.Comment: 7 pages, 1 figur
Coarsening of Two Dimensional Foam on a Dome
In this paper we report on bubble growth rates and on the statistics of
bubble topology for the coarsening of a dry foam contained in the narrow gap
between two hemispheres. By contrast with coarsening in flat space, where
six-sided bubbles neither grow nor shrink, we observe that six sided bubbles
grow with time at a rate that depends on their size. This result agrees with
the modification to von Neumann's law predicted by J.E. Avron and D. Levine.
For bubbles with a different number of sides, except possibly seven, there is
too much noise in the growth rate data to demonstrate a difference with
coarsening in flat space. In terms of the statistics of bubble topology, we
find fewer 3, 4, and 5 sided bubbles, and more 6 and greater sided bubbles, in
comparison with the stationary distribution for coarsening in flat space. We
also find good general agreement with the Aboav-Weaire law for the average
number of sides of the neighbors of an n-sided bubble
No-cloning theorem in thermofield dynamics
We discuss the relation between the no-cloning theorem from quantum
information and the doubling procedure used in the formalism of thermofield
dynamics (TFD). We also discuss how to apply the no-cloning theorem in the
context of thermofield states defined in TFD. Consequences associated to mixed
states, von Neumann entropy and thermofield vacuum are also addressed.Comment: 16 pages, 3 figure
The Hartman effect and weak measurements "which are not really weak"
We show that in wavepacket tunnelling localisation of the transmitted
particle amounts to a quantum measurement of the delay it experiences in the
barrier. With no external degree of freedom involved, the envelope of the
wavepacket plays the role of the initial pointer state. Under tunnelling
conditions such 'self measurement' is necessarily weak, and the Hartman effect
just reflects the general tendency of weak values to diverge, as post-selection
in the final state becomes improbable. We also demonstrate that it is a good
precision, or 'not really weak' quantum measurement: no matter how wide the
barrier d, it is possible to transmit a wavepacket with a width {\sigma} small
compared to the observed advancement. As is the case with all weak
measurements, the probability of transmission rapidly decreases with the ratio
{\sigma}/d.Comment: 6 pages, 1 figur
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