7,877 research outputs found
Symmetric Extension of Two-Qubit States
Quantum key distribution uses public discussion protocols to establish shared
secret keys. In the exploration of ultimate limits to such protocols, the
property of symmetric extendibility of underlying bipartite states
plays an important role. A bipartite state is symmetric extendible
if there exits a tripartite state , such that the marginal
state is identical to the marginal state, i.e. .
For a symmetric extendible state , the first task of the public
discussion protocol is to break this symmetric extendibility. Therefore to
characterize all bi-partite quantum states that possess symmetric extensions is
of vital importance. We prove a simple analytical formula that a two-qubit
state admits a symmetric extension if and only if
\tr(\rho_B^2)\geq \tr(\rho_{AB}^2)-4\sqrt{\det{\rho_{AB}}}. Given the
intimate relationship between the symmetric extension problem and the quantum
marginal problem, our result also provides the first analytical necessary and
sufficient condition for the quantum marginal problem with overlapping
marginals.Comment: 10 pages, no figure. comments are welcome. Version 2: introduction
rewritte
Microstructure, magneto-transport and magnetic properties of Gd-doped magnetron-sputtered amorphous carbon
The magnetic rare earth element gadolinium (Gd) was doped into thin films of
amorphous carbon (hydrogenated \textit{a}-C:H, or hydrogen-free \textit{a}-C)
using magnetron co-sputtering. The Gd acted as a magnetic as well as an
electrical dopant, resulting in an enormous negative magnetoresistance below a
temperature (). Hydrogen was introduced to control the amorphous carbon
bonding structure. High-resolution electron microscopy, ion-beam analysis and
Raman spectroscopy were used to characterize the influence of Gd doping on the
\textit{a-}GdC(:H) film morphology, composition, density and
bonding. The films were largely amorphous and homogeneous up to =22.0 at.%.
As the Gd doping increased, the -bonded carbon atoms evolved from
carbon chains to 6-member graphitic rings. Incorporation of H opened up the
graphitic rings and stabilized a -rich carbon-chain random network. The
transport properties not only depended on Gd doping, but were also very
sensitive to the ordering. Magnetic properties, such as the spin-glass
freezing temperature and susceptibility, scaled with the Gd concentration.Comment: 9 figure
Minimum Entangling Power is Close to Its Maximum
Given a quantum gate acting on a bipartite quantum system, its maximum
(average, minimum) entangling power is the maximum (average, minimum)
entanglement generation with respect to certain entanglement measure when the
inputs are restricted to be product states. In this paper, we mainly focus on
the 'weakest' one, i.e., the minimum entangling power, among all these
entangling powers. We show that, by choosing von Neumann entropy of reduced
density operator or Schmidt rank as entanglement measure, even the 'weakest'
entangling power is generically very close to its maximal possible entanglement
generation. In other words, maximum, average and minimum entangling powers are
generically close. We then study minimum entangling power with respect to other
Lipschitiz-continuous entanglement measures and generalize our results to
multipartite quantum systems.
As a straightforward application, a random quantum gate will almost surely be
an intrinsically fault-tolerant entangling device that will always transform
every low-entangled state to near-maximally entangled state.Comment: 26 pages, subsection III.A.2 revised, authors list updated, comments
are welcom
Project Icebreaker: Offshore Wind Project in Lake Erie
Wind energy is one of the most promising renewable energy resources. The Great Lakes region in the US has huge potential for offshore wind energy development. However, ice loading in winter brings a unique challenge to the foundations for wind turbines. Model tests and numerical simulation have been conducted to investigate different types of foundations and techniques to reduce the ice loading. The ultimate goal is to design a safe and economical foundation for future large-scale wind farms in the Great Lakes
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