3,500 research outputs found
Optimal distinction between non-orthogonal quantum states
Given a finite set of linearly independent quantum states, an observer who
examines a single quantum system may sometimes identify its state with
certainty. However, unless these quantum states are orthogonal, there is a
finite probability of failure. A complete solution is given to the problem of
optimal distinction of three states, having arbitrary prior probabilities and
arbitrary detection values. A generalization to more than three states is
outlined.Comment: 9 pages LaTeX, one PostScript figure on separate pag
Deep ROSAT-HRI observation of the elliptical galaxy NGC 1399
We present the preliminary results of a deep (167 ks) ROSAT HRI observation
of the cD galaxy NGC1399 in the Fornax cluster. We find, in agreement with
previous observations, an extended (41 Kpc adopting a distance of 19 Mpc)
gaseous halo with a luminosity of L_X=(4.41\pm 0.04)x10^{41} erg/s. The 5
arcsec resolution of the data allows us to detect a very complex and asymmetric
structure of the halo with respect to the optical galaxy. Moreover the analysis
of the radial structure reveals the presence of a multi-component profile not
consistent with a simple King model over the whole 40 Kpc. We do not detect the
presence of a central source and pose an upper limit to the luminosity of a
possible active nucleus. Due to the length of the observation, comparable to
that of a deep survey, we detect a large number of sources within the HRI FOV,
in slight excess with respect to the estimates based on previous surveys. We
study the flux distribution of the sources, their temporal behaviour and their
spatial distribution with respect to the central galaxy.Comment: 15 pages, 10 figures (6 in color), 1 table; uses subfigure.sty,
supertabular.sty, lscape.sty, color.sty; To be published in the proceeding of
"X-ray Astronomy 2000" (Mondello (Palermo), September 2000
Information-disturbance tradeoff in quantum measurements
We present a simple information-disturbance tradeoff relation valid for any
general measurement apparatus: The disturbance between input and output states
is lower bounded by the information the apparatus provides in distinguishing
these two states.Comment: 4 Pages, 1 Figure. Published version (reference added and minor
changes performed
Power of unentangled measurements on two antiparallel spins
We consider a pair of antiparallel spins polarized in a random direction to
encode quantum information. We wish to extract as much information as possible
on the polarization direction attainable by an unentangled measurement, i.e.,
by a measurement, whose outcomes are associated with product states. We develop
analytically the upper bound 0.7935 bits to the Shannon mutual information
obtainable by an unentangled measurement, which is definitely less than the
value 0.8664 bits attained by an entangled measurement. This proves our main
result, that not every ensemble of product states can be optimally
distinguished by an unentangled measurement, if the measure of
distinguishability is defined in the sense of Shannon. We also present results
from numerical calculations and discuss briefly the case of parallel spins.Comment: Latex file, 18 pages, 1 figure; published versio
Quantum state targeting
We introduce a new primitive for quantum communication that we term "state
targeting" wherein the goal is to pass a test for a target state even though
the system upon which the test is performed is submitted prior to learning the
target state's identity. Success in state targeting can be described as having
some control over the outcome of the test. We show that increasing one's
control above a minimum amount implies an unavoidable increase in the
probability of failing the test. This is analogous to the unavoidable
disturbance to a quantum state that results from gaining information about its
identity, and can be shown to be a purely quantum effect. We provide some
applications of the results to the security analysis of cryptographic tasks
implemented between remote antagonistic parties. Although we focus on weak coin
flipping, the results are significant for other two-party protocols, such as
strong coin flipping, partially binding and concealing bit commitment, and bit
escrow. Furthermore, the results have significance not only for the traditional
notion of security in cryptography, that of restricting a cheater's ability to
bias the outcome of the protocol, but also on a novel notion of security that
arises only in the quantum context, that of cheat-sensitivity. Finally, our
analysis of state targeting leads to some interesting secondary results, for
instance, a generalization of Uhlmann's theorem and an operational
interpretation of the fidelity between two mixed states
Quantum mechanics explained
The physical motivation for the mathematical formalism of quantum mechanics
is made clear and compelling by starting from an obvious fact - essentially,
the stability of matter - and inquiring into its preconditions: what does it
take to make this fact possible?Comment: 29 pages, 5 figures. v2: revised in response to referee comment
Quantum Field Theory with Null-Fronted Metrics
There is a large class of classical null-fronted metrics in which a free
scalar field has an infinite number of conservation laws. In particular, if the
scalar field is quantized, the number of particles is conserved. However, with
more general null-fronted metrics, field quantization cannot be interpreted in
terms of particle creation and annihilation operators, and the physical meaning
of the theory becomes obscure.Comment: 11 page
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