516 research outputs found
Invariant quantum discord in qubit-qutrit systems under local dephasing
We investigate the dynamics of quantum discord and entanglement for a
class of mixed qubit-qutrit states assuming that only the qutrit is under the action of a dephasing channel. We demonstrate that even though the entanglement in the qubit-qutrit state disappears in a finite time interval, partial coherence left in the system enables quantum discord to remain invariant throughout the whole time evolution
Quantum correlations in a few-atom spin-1 Bose-Hubbard model
We study the thermal quantum correlations and entanglement in spin-1 Bose-Hubbard model with two and three particles. While we use negativity to calculate entanglement, more general non-classical correlations are quantified using a new measure based on a necessary and sufficient condition for zero-discord state. We demonstrate that the energy level crossings in the ground state of the system are signalled by both the behavior of thermal quantum correlations and entanglement
Decoherence on a two-dimensional quantum walk using four- and two-state particle
We study the decoherence effects originating from state flipping and
depolarization for two-dimensional discrete-time quantum walks using four-state
and two-state particles. By quantifying the quantum correlations between the
particle and position degree of freedom and between the two spatial ()
degrees of freedom using measurement induced disturbance (MID), we show that
the two schemes using a two-state particle are more robust against decoherence
than the Grover walk, which uses a four-state particle. We also show that the
symmetries which hold for two-state quantum walks breakdown for the Grover
walk, adding to the various other advantages of using two-state particles over
four-state particles.Comment: 12 pages, 16 figures, In Press, J. Phys. A: Math. Theor. (2013
Dynamics of Atom-Atom Correlations in the Fermi problem
We present a detailed perturbative study of the dynamics of several types of
atom-atom correlations in the famous Fermi problem. This is an archetypal model
to study micro-causality in the quantum domain where two atoms, the first
initially excited and the second prepared in its ground state, interact with
the vacuum electromagnetic field. The excitation can be transferred to the
second atom via a flying photon and various kinds of quantum correlations
between the two are generated during this process. Among these, prominent
examples are given by entanglement, quantum discord and nonlocal correlations.
It is the aim of this paper to analyze the role of the light cone in the
emergence of such correlations.Comment: 14 pages, 7 figure
Bipartite Entanglement in Continuous-Variable Cluster States
We present a study of the entanglement properties of Gaussian cluster states,
proposed as a universal resource for continuous-variable quantum computing. A
central aim is to compare mathematically-idealized cluster states defined using
quadrature eigenstates, which have infinite squeezing and cannot exist in
nature, with Gaussian approximations which are experimentally accessible.
Adopting widely-used definitions, we first review the key concepts, by
analysing a process of teleportation along a continuous-variable quantum wire
in the language of matrix product states. Next we consider the bipartite
entanglement properties of the wire, providing analytic results. We proceed to
grid cluster states, which are universal for the qubit case. To extend our
analysis of the bipartite entanglement, we adopt the entropic-entanglement
width, a specialized entanglement measure introduced recently by Van den Nest M
et al., Phys. Rev. Lett. 97 150504 (2006), adapting their definition to the
continuous-variable context. Finally we add the effects of photonic loss,
extending our arguments to mixed states. Cumulatively our results point to key
differences in the properties of idealized and Gaussian cluster states. Even
modest loss rates are found to strongly limit the amount of entanglement. We
discuss the implications for the potential of continuous-variable analogues of
measurement-based quantum computation.Comment: 22 page
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A synthesis dataset of permafrost-affected soil thermal conditions for Alaska, USA
Recent observations of near-surface soil temperatures over the circumpolar
Arctic show accelerated warming of permafrost-affected soils. The
availability of a comprehensive near-surface permafrost and active layer
dataset is critical to better understanding climate impacts and to
constraining permafrost thermal conditions and its spatial distribution in
land system models. We compiled a soil temperature dataset from 72 monitoring
stations in Alaska using data collected by the U.S. Geological Survey, the
National Park Service, and the University of Alaska Fairbanks permafrost
monitoring networks. The array of monitoring stations spans a large range of
latitudes from 60.9 to 71.3° N and elevations from near sea level to
∼ 1300 m, comprising tundra and boreal forest regions. This dataset
consists of monthly ground temperatures at depths up to 1 m,
volumetric soil water content, snow depth, and air temperature during
1997–2016. These data have been quality controlled in collection and
processing. Meanwhile, we implemented data harmonization evaluation for the
processed dataset. The final product (PF-AK, v0.1) is available at the Arctic
Data Center (https://doi.org/10.18739/A2KG55).</p
Foundations and Measures of Quantum Non-Markovianity
The basic features of the dynamics of open quantum systems, such as the
dissipation of energy, the decay of coherences, the relaxation to an
equilibrium or non-equilibrium stationary state, and the transport of
excitations in complex structures are of central importance in many
applications of quantum mechanics. The theoretical description, analysis and
control of non-Markovian quantum processes play an important role in this
context. While in a Markovian process an open system irretrievably loses
information to its surroundings, non-Markovian processes feature a flow of
information from the environment back to the open system, which implies the
presence of memory effects and represents the key property of non-Markovian
quantum behavior. Here, we review recent ideas developing a general
mathematical definition for non-Markoviantiy in the quantum regime and a
measure for the degree of memory effects in the dynamics of open systems which
are based on the exchange of information between system and environment. We
further study the dynamical effects induced by the presence of
system-environment correlations in the total initial state and design suitable
methods to detect such correlations through local measurements on the open
system.Comment: 31 pages, to be published in the special issue: Loss of coherence and
memory effects in quantum dynamics, Journal of Physics B: Atomic, Molecular
and Optical Physic
Slowing and cooling molecules and neutral atoms by time-varying electric field gradients
A method of slowing, accelerating, cooling, and bunching molecules and
neutral atoms using time-varying electric field gradients is demonstrated with
cesium atoms in a fountain. The effects are measured and found to be in
agreement with calculation. Time-varying electric field gradient slowing and
cooling is applicable to atoms that have large dipole polarizabilities,
including atoms that are not amenable to laser slowing and cooling, to Rydberg
atoms, and to molecules, especially polar molecules with large electric dipole
moments. The possible applications of this method include slowing and cooling
thermal beams of atoms and molecules, launching cold atoms from a trap into a
fountain, and measuring atomic dipole polarizabilities.Comment: 13 pages, 10 figures. Scheduled for publication in Nov. 1 Phys. Rev.
Josephson Coupling and Fiske Dynamics in Ferromagnetic Tunnel Junctions
We report on the fabrication of Nb/AlO_x/Pd_{0.82}Ni_{0.18}/Nb
superconductor/insulator/ferromagnetic metal/superconductor (SIFS) Josephson
junctions with high critical current densities, large normal resistance times
area products, high quality factors, and very good spatial uniformity. For
these junctions a transition from 0- to \pi-coupling is observed for a
thickness d_F ~ 6 nm of the ferromagnetic Pd_{0.82}Ni_{0.18} interlayer. The
magnetic field dependence of the \pi-coupled junctions demonstrates good
spatial homogeneity of the tunneling barrier and ferromagnetic interlayer.
Magnetic characterization shows that the Pd_{0.82}Ni_{0.18} has an out-of-plane
anisotropy and large saturation magnetization, indicating negligible dead
layers at the interfaces. A careful analysis of Fiske modes provides
information on the junction quality factor and the relevant damping mechanisms
up to about 400 GHz. Whereas losses due to quasiparticle tunneling dominate at
low frequencies, the damping is dominated by the finite surface resistance of
the junction electrodes at high frequencies. High quality factors of up to 30
around 200 GHz have been achieved. Our analysis shows that the fabricated
junctions are promising for applications in superconducting quantum circuits or
quantum tunneling experiments.Comment: 15 pages, 9 figure
Geometric global quantum discord
Geometric quantum discord, proposed by Dakic, Vedral, and Brukner [Phys. Rev.
Lett. 105 (2010) 190502], is an important measure for bipartite correlations.
In this paper, we generalize it to multipartite states, we call the generalized
version geometric global quantum discord (GGQD). We characterize GGQD in
different ways, and provide some special states which allow analytical GGQD.Comment: 8 pages,no figure;added a lower bound for GGQD to version
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