740 research outputs found
Quantum cryptography: a practical information security perspective
Quantum Key Exchange (QKE, also known as Quantum Key Distribution or QKD)
allows communicating parties to securely establish cryptographic keys. It is a
well-established fact that all QKE protocols require that the parties have
access to an authentic channel. Without this authenticated link, QKE is
vulnerable to man-in-the-middle attacks. Overlooking this fact results in
exaggerated claims and/or false expectations about the potential impact of QKE.
In this paper we present a systematic comparison of QKE with traditional key
establishment protocols in realistic secure communication systems.Comment: 5 pages, new title, published version, minor changes onl
The angular momentum transport by unstable toroidal magnetic fields
We demonstrate with a nonlinear MHD code that angular momentum can be
transported due to the magnetic instability of toroidal fields under the
influence of differential rotation, and that the resulting effective viscosity
may be high enough to explain the almost rigid-body rotation observed in
radiative stellar cores. Only stationary current-free fields and only those
combinations of rotation rates and magnetic field amplitudes which provide
maximal numerical values of the viscosity are considered. We find that the
dimensionless ratio of the effective over molecular viscosity, ;,
linearly grows with the Reynolds number of the rotating fluid multiplied with
the square-root of the magnetic Prandtl number - which is of order unity for
the considered red sub-giant KIC 7341231.
For the considered interval of magnetic Reynolds numbers - which is
restricted by numerical constraints of the nonlinear MHD code - there is a
remarkable influence of the magnetic Prandtl number on the relative importance
of the contributions of the Reynolds stress and the Maxwell stress to the total
viscosity, which is magnetically dominated only for Pm 0.5. We also
find that the magnetized plasma behaves as a non-Newtonian fluid, i.e. the
resulting effective viscosity depends on the shear in the rotation law. The
decay time of the differential rotation thus depends on its shear and becomes
longer and longer during the spin-down of a stellar core.Comment: Revised version. 7 pages, 9 figures; accepted for publication in A&
The eddy heat-flux in rotating turbulent convection
The three components of the heat-flux vector F_\thetaF_r
which for \emph{free} turbulence is also believed to peak at the poles (see Eq.
(19) below). As we can show, however, the consequences of this unexpected
result (also obtained by Kaepylae, Korpi and Tuominen 2004) for the theory of
differential rotation are small as mainly the F_\thetaF_\phiF_\phi$ is closely related to the radial
\Lambda-effect which is known to be also negative in stratified and rapidly
rotating convection zones.Comment: 8 pages, 9 figures, Astron. Astrophys. (subm.
- …