7,113 research outputs found
Comment on "Classical interventions in quantum systems II. Relativistic invariance"
In a recent paper [Phys. Rev. A 61, 022117 (2000)], quant-ph/9906034, A.
Peres argued that quantum mechanics is consistent with special relativity by
proposing that the operators that describe time evolution do not need to
transform covariantly, although the measurable quantities need to transform
covariantly. We discuss the weaknesses of this proposal.Comment: 4 pages, to appear in Phys. Rev.
Quantum information and special relativity
Relativistic effects affect nearly all notions of quantum information theory.
The vacuum behaves as a noisy channel, even if the detectors are perfect. The
standard definition of a reduced density matrix fails for photon polarization
because the transversality condition behaves like a superselection rule. We can
however define an effective reduced density matrix which corresponds to a
restricted class of positive operator-valued measures. There are no pure photon
qubits, and no exactly orthogonal qubit states. Reduced density matrices for
the spin of massive particles are well-defined, but are not covariant under
Lorentz transformations. The spin entropy is not a relativistic scalar and has
no invariant meaning. The distinguishability of quantum signals and their
entanglement depend on the relative motion of observers.Comment: RevTex, 6 pages with one figure. Proceedings of TH-2002, Paris, 200
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
Complete light absorption in graphene-metamaterial corrugated structures
We show that surface-plasmon polaritons excited in negative permittivity
metamaterials having shallow periodic surface corrugation profiles can be
explored to push the absorption of single and continuous sheets of graphene up
to 100%. In the relaxation regime, the position of the plasmonic resonances of
the hybrid system is determined by the plasma frequency of the metamaterial,
allowing the frequency range for enhanced absorption to be set without the need
of engineering graphene.Comment: 6 pages, 4 figures; published version: text revised and references
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TRACE-derived temperature and emission measure profiles along long-lived coronal loops: the role of filamentation
In a recent letter (ApJ 517, L155) Lenz et al. have shown the evidence of
uniform temperature along steady long coronal loops observed by TRACE in two
different passbands (171 A and 195 A filters). We propose that such an evidence
can be explained by the sub-arcsecond structuring of the loops across the
magnetic field lines. In this perspective, we present a model of a bundle of
six thin parallel hydrostatic filaments with temperature stratification
dictated by detailed energy balance and with temperatures at their apex ranging
between 0.8 and 5 MK. If analyzed as a single loop, the bundle would appear
isothermal along most of its length.Comment: 9 pages, 4 figs, LaTeX text, PostScript figure
Solution of the quantum harmonic oscillator plus a delta-function potential at the origin: The oddness of its even-parity solutions
We derive the energy levels associated with the even-parity wave functions of
the harmonic oscillator with an additional delta-function potential at the
origin. Our results bring to the attention of students a non-trivial and
analytical example of a modification of the usual harmonic oscillator
potential, with emphasis on the modification of the boundary conditions at the
origin. This problem calls the attention of the students to an inaccurate
statement in quantum mechanics textbooks often found in the context of solution
of the harmonic oscillator problem.Comment: 9 pages, 3 figure
Trees and Markov convexity
We show that an infinite weighted tree admits a bi-Lipschitz embedding into
Hilbert space if and only if it does not contain arbitrarily large complete
binary trees with uniformly bounded distortion. We also introduce a new metric
invariant called Markov convexity, and show how it can be used to compute the
Euclidean distortion of any metric tree up to universal factors
Coronal loop hydrodynamics. The solar flare observedon November 12 1980 revisited: the UV line emission
We revisit a well-studied solar flare whose X-ray emission originating from a
simple loop structure was observed by most of the instruments on board SMM on
November 12 1980. The X-ray emission of this flare, as observed with the XRP,
was successfully modeled previously. Here we include a detailed modeling of the
transition region and we compare the hydrodynamic results with the UVSP
observations in two EUV lines, measured in areas smaller than the XRP rasters,
covering only some portions of the flaring loop (the top and the foot-points).
The single loop hydrodynamic model, which fits well the evolution of coronal
lines (those observed with the XRP and the \FeXXI 1354.1 \AA line observed with
the UVSP) fails to model the flux level and evolution of the \OV 1371.3 \AA
line.Comment: A&A, in press, 6 pages, 5 figure
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