29,297 research outputs found
Analysis of the High-Altitude Cooling of the Ranger SGV-770 D-4 Engine in the Bell XP-77 Airplane
No abstract availabl
Excitation of plasma waves by unstable photoelectron and thermal electron populations on closed magnetic field lines in the Martian ionosphere
It is argued that anisotropic electron pitch angle distributions in the closed magnetic field regions of the Martian ionosphere gives rise to excitation of plasma instabilities. We discuss two types of instabilities that are excited by two different populations of electrons. First, the generation of Langmuir waves by photoelectrons with energies of the order of 10eV is investigated. It is predicted that the measured anisotropy of their pitch angle distribution at the heights <i>z</i>&asymp;400km causes excitation of waves with frequencies <i>f</i>~30kHz and wavelengths &lambda;~30m. Near the terminators the instability of the electrostatic waves with frequencies of the order of or less than the electron gyrofrequency exited by thermal electrons is predicted. The typical frequencies of these waves depend on the local magnitude of the magnetic field and can achieve values <i>f</i>~3-5kHz above strong crustal magnetic fields
Dirac-K\"ahler approach connected to quantum mechanics in Grassmann space
We compare the way one of us got spinors out of fields, which are a priori
antisymmetric tensor fields, to the Dirac-K\"ahler rewriting. Since using our
Grassmann formulation is simple it may be useful in describing the
Dirac-K\"ahler formulation of spinors and in generalizing it to vector internal
degrees of freedom and to charges. The ``cheat'' concerning the Lorentz
transformations for spinors is the same in both cases and is put clearly
forward in the Grassmann formulation. Also the generalizations are clearly
pointed out. The discrete symmetries are discussed, in particular the
appearance of two kinds of the time-reversal operators as well as the
unavoidability of four families.Comment: 36 page
Analysis of a convenient information bound for general quantum channels
Open questions from Sarovar and Milburn (2006 J.Phys. A: Math. Gen. 39 8487)
are answered. Sarovar and Milburn derived a convenient upper bound for the
Fisher information of a one-parameter quantum channel. They showed that for
quasi-classical models their bound is achievable and they gave a necessary and
sufficient condition for positive operator-valued measures (POVMs) attaining
this bound. They asked (i) whether their bound is attainable more generally,
(ii) whether explicit expressions for optimal POVMs can be derived from the
attainability condition. We show that the symmetric logarithmic derivative
(SLD) quantum information is less than or equal to the SM bound, i.e.\
and we find conditions for equality. As
the Fisher information is less than or equal to the SLD quantum information,
i.e. , we can deduce when equality holds in
. Equality does not hold for all
channels. As a consequence, the attainability condition cannot be used to test
for optimal POVMs for all channels. These results are extended to
multi-parameter channels.Comment: 16 pages. Published version. Some of the lemmas have been corrected.
New resuts have been added. Proofs are more rigorou
Resources Required for Topological Quantum Factoring
We consider a hypothetical topological quantum computer where the qubits are
comprised of either Ising or Fibonacci anyons. For each case, we calculate the
time and number of qubits (space) necessary to execute the most computationally
expensive step of Shor's algorithm, modular exponentiation. For Ising anyons,
we apply Bravyi's distillation method [S. Bravyi, Phys. Rev. A 73, 042313
(2006)] which combines topological and non-topological operations to allow for
universal quantum computation. With reasonable restrictions on the physical
parameters we find that factoring a 128 bit number requires approximately 10^3
Fibonacci anyons versus at least 3 x 10^9 Ising anyons. Other distillation
algorithms could reduce the resources for Ising anyons substantially.Comment: 4+epsilon pages, 4 figure
Universal Quantum Computation through Control of Spin-Orbit Coupling
We propose a method for quantum computation which uses control of spin-orbit
coupling in a linear array of single electron quantum dots. Quantum gates are
carried out by pulsing the exchange interaction between neighboring electron
spins, including the anisotropic corrections due to spin-orbit coupling.
Control over these corrections, even if limited, is sufficient for universal
quantum computation over qubits encoded into pairs of electron spins. The
number of voltage pulses required to carry out either single qubit rotations or
controlled-Not gates scales as the inverse of a dimensionless measure of the
degree of control of spin-orbit coupling.Comment: 4 pages, 3 figures (minor revision, references added
Hidden parameters in open-system evolution unveiled by geometric phase
We find a class of open-system models in which individual quantum
trajectories may depend on parameters that are undetermined by the full
open-system evolution. This dependence is imprinted in the geometric phase
associated with such trajectories and persists after averaging. Our findings
indicate a potential source of ambiguity in the quantum trajectory approach to
open quantum systems.Comment: QSD analysis added; several stylistic changes; journal reference
adde
Bulk-Edge correspondence of entanglement spectrum in 2D spin ground states
General local spin ground states, described by a Valence Bond Solid (VBS)
on a two dimensional lattice are studied. The norm of these ground states is
mapped to a classical O(3) model on the same lattice. Using this
quantum-to-classical mapping we obtain the partial density matrix
associated with a subsystem of the original ground state. We show that
the entanglement spectrum of in a translation invariant lattice
is given by the spectrum of a quantum spin chain at the boundary of region ,
with local Heisenberg type interactions between spin 1/2 particles.Comment: 8 pages, 4 figures, one section and references adde
Optimal estimation of one parameter quantum channels
We explore the task of optimal quantum channel identification, and in
particular the estimation of a general one parameter quantum process. We derive
new characterizations of optimality and apply the results to several examples
including the qubit depolarizing channel and the harmonic oscillator damping
channel. We also discuss the geometry of the problem and illustrate the
usefulness of using entanglement in process estimation.Comment: 23 pages, 4 figures. Published versio
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