7,330 research outputs found
Hybrid solid state qubits: the powerful role of electron spins
We review progress on the use of electron spins to store and process quantum
information, with particular focus on the ability of the electron spin to
interact with multiple quantum degrees of freedom. We examine the benefits of
hybrid quantum bits (qubits) in the solid state that are based on coupling
electron spins to nuclear spin, electron charge, optical photons, and
superconducting qubits. These benefits include the coherent storage of qubits
for times exceeding seconds, fast qubit manipulation, single qubit measurement,
and scalable methods for entangling spatially separated matter-based qubits. In
this way, the key strengths of different physical qubit implementations are
brought together, laying the foundation for practical solid-state quantum
technologies.Comment: 54 pages, 7 figure
Implementation of Quantum Gates via Optimal Control
Starting with the basic control system model often employed in NMR pulse
design, we derive more realistic control system models taking into account
effects such as off-resonant excitation for systems with fixed inter-qubit
coupling controlled by globally applied electromagnetic fields, as well as for
systems controlled by a combination of a global fields and local control
electrodes. For both models optimal control is used to find controls that
implement a set of two- and three-qubit gates with fidelity greater than
99.99%. While in some cases the optimal pulses obtained appear to be
surprisingly simple and experimentally realistic, the results also show that
the "optimal" pulses obtained in other cases are experimentally infeasible, and
more sophisticated parametrization of the control fields and numerical
algorithms are needed.Comment: 10 pages, 4 figure
Block-block entanglement and quantum phase transitions in one-dimensional extended Hubbard model
In this paper, we study block-block entanglement in the ground state of
one-dimensional extended Hubbard model. Our results show that the phase diagram
derived from the block-block entanglement manifests richer structure than that
of the local (single site) entanglement because it comprises nonlocal
correlation. Besides phases characterized by the charge-density-wave, the
spin-density-wave, and phase-separation, which can be sketched out by the local
entanglement, singlet superconductivity phase could be identified on the
contour map of the block-block entanglement. Scaling analysis shows that behavior of the block-block entanglement may exist in both
non-critical and the critical regions, while some local extremum are induced by
the finite-size effect. We also study the block-block entanglement defined in
the momentum space and discuss its relation to the phase transition from
singlet superconducting state to the charge-density-wave state.Comment: 8 pages, 9 figure
Conformally rescaled spacetimes and Hawking radiation
We study various derivations of Hawking radiation in conformally rescaled
metrics. We focus on two important properties, the location of the horizon
under a conformal transformation and its associated temperature. We find that
the production of Hawking radiation cannot be associated in all cases to the
trapping horizon because its location is not invariant under a conformal
transformation. We also find evidence that the temperature of the Hawking
radiation should transform simply under a conformal transformation, being
invariant for asymptotic observers in the limit that the conformal
transformation factor is unity at their location.Comment: 22 pages, version submitted to journa
Entanglement and quantum phase transition in the extended Hubbard model
We study quantum entanglement in one-dimensional correlated fermionic system.
Our results show, for the first time, that entanglement can be used to identify
quantum phase transitions in fermionic systems.Comment: 5 pages, 4 figure
A multi-domain Chebyshev collocation method for predicting ultrasonic field parameters in complex material geometries
The use of ultrasound to measure elastic field parameters as well as to detect cracks in solid materials has received much attention, and new important applications have been developed recently, e.g., the use of laser generated ultrasound in non-destructive evaluation (NDE). To model such applications requires a realistic calculation of field parameters in complex geometries with discontinuous, layered materials. In this paper we present an approach for solving the elastic wave equation in complex geometries with discontinuous layered materials. The approach is based on a pseudospectral elastodynamic formulation, giving a direct solution of the time-domain elastodynamic equations. A typical calculation is performed by decomposing the global computational domain into a number of subdomains. Every subdomain is then mapped on a unit square using transfinite blending functions and spatial derivatives are calculated efficiently by a Chebyshev collocation scheme. This enables that the elastodynamic equations can be solved within spectral accuracy, and furthermore, complex interfaces can be approximated smoothly, hence avoiding staircasing. A global solution is constructed from the local solutions by means of characteristic variables. Finally, the global solution is advanced in time using a fourth order Runge-Kutta scheme. Examples of field prediction in discontinuous solids with complex geometries are given and related to ultrasonic NDE. (C) 2002 Elsevier Science B.V. All rights reserved
Targeted temperature management after out-of-hospital cardiac arrest: certainties and uncertainties
Special Values of Generalized Polylogarithms
We study values of generalized polylogarithms at various points and
relationships among them. Polylogarithms of small weight at the points 1/2 and
-1 are completely investigated. We formulate a conjecture about the structure
of the linear space generated by values of generalized polylogarithms.Comment: 32 page
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