2,237 research outputs found
Bounds on the entanglability of thermal states in liquid-state nuclear magnetic resonance
The role of mixed state entanglement in liquid-state nuclear magnetic
resonance (NMR) quantum computation is not yet well-understood. In particular,
despite the success of quantum information processing with NMR, recent work has
shown that quantum states used in most of those experiments were not entangled.
This is because these states, derived by unitary transforms from the thermal
equilibrium state, were too close to the maximally mixed state. We are thus
motivated to determine whether a given NMR state is entanglable - that is, does
there exist a unitary transform that entangles the state? The boundary between
entanglable and nonentanglable thermal states is a function of the spin system
size and its temperature . We provide new bounds on the location of this
boundary using analytical and numerical methods; our tightest bound scales as
, giving a lower bound requiring at least proton
spins to realize an entanglable thermal state at typical laboratory NMR
magnetic fields. These bounds are tighter than known bounds on the
entanglability of effective pure states.Comment: REVTeX4, 15 pages, 4 figures (one large figure: 414 K
Probability distributions consistent with a mixed state
A density matrix may be represented in many different ways as a
mixture of pure states, \rho = \sum_i p_i |\psi_i\ra \la \psi_i|. This paper
characterizes the class of probability distributions that may appear in
such a decomposition, for a fixed density matrix . Several illustrative
applications of this result to quantum mechanics and quantum information theory
are given.Comment: 6 pages, submitted to Physical Review
Concurrence of mixed bipartite quantum states in arbitrary dimensions
We derive a lower bound for the concurrence of mixed bipartite quantum
states, valid in arbitrary dimensions. As a corollary, a weaker, purely
algebraic estimate is found, which detects mixed entangled states with positive
partial transpose.Comment: accepted py PR
Investigation into Energy Efficiency of Outdated Cutting Machine Tools and Identification of Improvement Potentials to Promote Sustainability
AbstractCutting machine tools have a significant impact on manufacturing and sustainability. There exist a large number of outdated cutting machine tools especially in developing and emerging countries which are still taking a considerable share in global value creation. Furthermore, an increasing trend in field of reuse, retrofitting and upgrading can be observed. For Life-Cycle-Assessment and analyses of end-of-life behavior of such machine tools in context of sustainability, reliable values for energy consumption and machining efficiency under realistic machining conditions are indispensable. In the present paper the energy consumption and machining efficiency of an exemplary outdated milling machine have been measured and analyzed under consideration of different influences such as process parameter, machining material and ratio of prim time to secondary time. Additionally a comparison between a newer and the outdated milling machine has been carried out in order to identify and quantify possible improvement potentials of outdated machine tool concerning energy consumption and machining efficiency. Based on obtained results more accurate and realistic decision can be made by enterprises who aim to promote sustainable manufacturing
Uhlmann's geometric phase in presence of isotropic decoherence
Uhlmann's mixed state geometric phase [Rep. Math. Phys. {\bf 24}, 229 (1986)]
is analyzed in the case of a qubit affected by isotropic decoherence treated in
the Markovian approximation. It is demonstrated that this phase decreases
rapidly with increasing decoherence rate and that it is most fragile to weak
decoherence for pure or nearly pure initial states. In the unitary case, we
compare Uhlmann's geometric phase for mixed states with that occurring in
standard Mach-Zehnder interferometry [Phys. Rev. Lett. {\bf 85}, 2845 (2000)]
and show that the latter is more robust to reduction in the length of the Bloch
vector. We also describe how Uhlmann's geometric phase in the present case
could in principle be realized experimentally.Comment: New ref added, refs updated, journal ref adde
Fidelity and Concurrence of conjugated states
We prove some new properties of fidelity (transition probability) and
concurrence, the latter defined by straightforward extension of Wootters
notation. Choose a conjugation and consider the dependence of fidelity or of
concurrence on conjugated pairs of density operators. These functions turn out
to be concave or convex roofs. Optimal decompositions are constructed. Some
applications to two- and tripartite systems illustrate the general theorem.Comment: 10 pages, RevTex, Correction: Enlarged, reorganized version. More
explanation
Improvement of Surface Accuracy and Shop Floor Feed Rate Smoothing Through Open CNC Monitoring System and Cutting Simulation
AbstractIn the milling process of complex workpiece shapes the feed rate normally becomes instable due to the high degree of surface curvature that requires high acceleration and deceleration of the interpolated axes. This condition impacts on process time and on the surface accuracy regarding the manufactured part form and texture. The challenge to simulate the real machine and control behavior requires accurate models with a set of experiments to tune and dimension the model to the respective machine tool. The aim is to improve the HSC milling process of complex surfaces before removing any material. In this paper experiments show that the surface form accuracy and texture can be optimized through an automatic feed rate smoothing of the finishing operation directly on the machine tool. The axis positions and spindle speeds monitored through the open CNC are used as input for a geometric cutting simulation, thus enabling to predict and optimize the surface quality
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