13,439 research outputs found
Efficient quantum key distribution secure against no-signalling eavesdroppers
By carrying out measurements on entangled states, two parties can generate a
secret key which is secure not only against an eavesdropper bound by the laws
of quantum mechanics, but also against a hypothetical "post-quantum"
eavesdroppers limited by the no-signalling principle only. We introduce a
family of quantum key distribution protocols of this type, which are more
efficient than previous ones, both in terms of key rate and noise resistance.
Interestingly, the best protocols involve large number of measurements. We show
that in the absence of noise, these protocols can yield one secret bit per
entanglement bit, implying that the key rates in the no-signalling post-quantum
scenario are comparable to the key rates in usual quantum key distribution.Comment: 11 pages, 2 color figures. v2: minor modifications, added references,
added note on the relation to quant-ph/060604
Micromagnetic Simulations of Ferromagnetic Rings
Thin nanomagnetic rings have generated interest for fundamental studies of
magnetization reversal and also for their potential in various applications,
particularly as magnetic memories. They are a rare example of a geometry in
which an analytical solution for the rate of thermally induced magnetic
reversal has been determined, in an approximation whose errors can be estimated
and bounded. In this work, numerical simulations of soft ferromagnetic rings
are used to explore aspects of the analytical solution. The evolution of the
energy near the transition states confirms that, consistent with analytical
predictions, thermally induced magnetization reversal can have one of two
intermediate states: either constant or soliton-like saddle configurations,
depending on ring size and externally applied magnetic field. The results
confirm analytical predictions of a transition in thermally activated reversal
behavior as magnetic field is varied at constant ring size. Simulations also
show that the analytic one dimensional model continues to hold even for wide
rings
A theoretical study of the conversion of gas phase methanediol to formaldehyde
Methanediol, or methylene glycol, is a product of the liquid phase reaction of water and formaldehyde and is a predicted interstellar grain surface species. Detection of this molecule in a hot core environment would advance the understanding of complex organic chemistry in the interstellar medium, but its laboratory spectroscopic characterization is a prerequisite for such observational searches. This theoretical study investigates the unimolecular decomposition of methanediol, specifically the thermodynamic and kinetic stability of the molecule under typical laboratory and interstellar conditions. Methanediol was found to be thermodynamically stable at temperatures of <100 K, which is the characteristic temperature range for interstellar grain mantles. The infinite-pressure RRKM unimolecular decomposition rate was found to be <10^(−18) s^(−1) at 300 K, indicating gas phase kinetic stability for typical laboratory and hot core temperatures. Therefore, both laboratory studies of and observational searches for this molecule should be feasible
Ferromagnetic resonance study of polycrystalline Fe_{1-x}V_x alloy thin films
Ferromagnetic resonance has been used to study the magnetic properties and
magnetization dynamics of polycrystalline FeV alloy films with
. Films were produced by co-sputtering from separate Fe and V
targets, leading to a composition gradient across a Si substrate. FMR studies
were conducted at room temperature with a broadband coplanar waveguide at
frequencies up to 50 GHz using the flip-chip method. The effective
demagnetization field and the Gilbert damping
parameter have been determined as a function of V concentration. The
results are compared to those of epitaxial FeV films
Thermal Stability of the Magnetization in Perpendicularly Magnetized Thin Film Nanomagnets
Understanding the stability of thin film nanomagnets with perpendicular
magnetic anisotropy (PMA) against thermally induced magnetization reversal is
important when designing perpendicularly magnetized patterned media and
magnetic random access memories. The leading-order dependence of magnetization
reversal rates are governed by the energy barrier the system needs to surmount
in order for reversal to proceed. In this paper we study the reversal dynamics
of these systems and compute the relevant barriers using the string method of
E, Vanden-Eijnden, and Ren. We find the reversal to be often spatially
incoherent; that is, rather than the magnetization flipping as a rigid unit,
reversal proceeds instead through a soliton-like domain wall sweeping through
the system. We show that for square nanomagnetic elements the energy barrier
increases with element size up to a critical length scale, beyond which the
energy barrier is constant. For circular elements the energy barrier continues
to increase indefinitely, albeit more slowly beyond a critical size. In both
cases the energy barriers are smaller than those expected for coherent
magnetization reversal.Comment: 5 pages, 4 Figure
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