16,291 research outputs found
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
Classical Correlations and Entanglement in Quantum Measurements
We analyze a quantum measurement where the apparatus is initially in a mixed
state. We show that the amount of information gained in a measurement is not
equal to the amount of entanglement between the system and the apparatus, but
is instead equal to the degree of classical correlations between the two. As a
consequence, we derive an uncertainty-like expression relating the information
gain in the measurement and the initial mixedness of the apparatus. Final
entanglement between the environment and the apparatus is also shown to be
relevant for the efficiency of the measurement.Comment: to appear in Physical Review Letter
Acoustic and aerodynamic performance of a 6-foot-diameter fan for turbofan engines. 1 - Design of facility and QF-1 fan
Design of test facility and prototype fan for turbofan acoustic researc
Quasi-bound states in continuum
We report the prediction of quasi-bound states (resonant states with very
long lifetimes) that occur in the eigenvalue continuum of propagating states
for a wide region of parameter space. These quasi-bound states are generated in
a quantum wire with two channels and an adatom, when the energy bands of the
two channels overlap. A would-be bound state that lays just below the upper
energy band is slightly destabilized by the lower energy band and thereby
becomes a resonant state with a very long lifetime (a second QBIC lays above
the lower energy band).Comment: 4 pages, 4figures, 1 tabl
A Theory of Errors in Quantum Measurement
It is common to model random errors in a classical measurement by the normal
(Gaussian) distribution, because of the central limit theorem. In the quantum
theory, the analogous hypothesis is that the matrix elements of the error in an
observable are distributed normally. We obtain the probability distribution
this implies for the outcome of a measurement, exactly for the case of 2x2
matrices and in the steepest descent approximation in general. Due to the
phenomenon of `level repulsion', the probability distributions obtained are
quite different from the Gaussian.Comment: Based on talk at "Spacetime and Fundamental Interactions: Quantum
Aspects" A conference to honor A. P. Balachandran's 65th Birthda
Different sensing mechanisms in single wire and mat carbon nanotubes chemical sensors
Chemical sensing properties of single wire and mat form sensor structures
fabricated from the same carbon nanotube (CNT) materials have been compared.
Sensing properties of CNT sensors were evaluated upon electrical response in
the presence of five vapours as acetone, acetic acid, ethanol, toluene, and
water. Diverse behaviour of single wire CNT sensors was found, while the mat
structures showed similar response for all the applied vapours. This indicates
that the sensing mechanism of random CNT networks cannot be interpreted as a
simple summation of the constituting individual CNT effects, but is associated
to another robust phenomenon, localized presumably at CNT-CNT junctions, must
be supposed.Comment: 12 pages, 5 figures,Applied Physics A: Materials Science and
Processing 201
How state preparation can affect a quantum experiment: Quantum process tomography for open systems
We study the effects of preparation of input states in a quantum tomography
experiment. We show that maps arising from a quantum process tomography
experiment (called process maps) differ from the well know dynamical maps. The
difference between the two is due to the preparation procedure that is
necessary for any quantum experiment. We study two preparation procedures,
stochastic preparation and preparation by measurements. The stochastic
preparation procedure yields process maps that are linear, while the
preparations using von Neumann measurements lead to non-linear processes, and
can only be consistently described by a bi-linear process map. A new process
tomography recipe is derived for preparation by measurement for qubits. The
difference between the two methods is analyzed in terms of a quantum process
tomography experiment. A verification protocol is proposed to differentiate
between linear processes and bi-linear processes. We also emphasize the
preparation procedure will have a non-trivial effect for any quantum experiment
in which the system of interest interacts with its environment.Comment: 13 pages, no figures, submitted to Phys. Rev.
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
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