15,479 research outputs found
Codariocalyx Gyroides- A New Forage Option for the Humid Tropics
The Southeast Asian shrub legume, Codariocalyx gyroides is adapted to acid, low-fertility soils with poor drainage. Data from 19 RIEPT (International Network for Evaluation of Tropical Pastures) trial sites conducted in the American humid and subhumid tropics were used to define the range of adaptation of C. gyroides. The results show that C. gyroides performs better under high rainfall conditions of the humid tropics with hardly any dry season, and it responds to soil fertility (Ca, P). Future research needs are identified. The release of a commercial cultivar in Colombia is being prepared
Qubit-induced phonon blockade as a signature of quantum behavior in nanomechanical resonators
The observation of quantized nanomechanical oscillations by detecting
femtometer-scale displacements is a significant challenge for experimentalists.
We propose that phonon blockade can serve as a signature of quantum behavior in
nanomechanical resonators. In analogy to photon blockade and Coulomb blockade
for electrons, the main idea for phonon blockade is that the second phonon
cannot be excited when there is one phonon in the nonlinear oscillator. To
realize phonon blockade, a superconducting quantum two-level system is coupled
to the nanomechanical resonator and is used to induce the phonon
self-interaction. Using Monte Carlo simulations, the dynamics of the induced
nonlinear oscillator is studied via the Cahill-Glauber -parametrized
quasiprobability distributions. We show how the oscillation of the resonator
can occur in the quantum regime and demonstrate how the phonon blockade can be
observed with currently accessible experimental parameters
Entanglement dynamics of two independent qubits in environments with and without memory
A procedure to obtain the dynamics of independent qudits (-level
systems) each interacting with its own reservoir, for any arbitrary initial
state, is presented. This is then applied to study the dynamics of the
entanglement of two qubits, initially in an extended Werner-like mixed state
with each of them in a zero temperature non-Markovian environment. The
dependence of the entanglement dynamics on the purity and degree of
entanglement of the initial states and on the amount of non-Markovianity is
also given. This extends the previous work about non-Markovian effects on the
two-qubit entanglement dynamics for initial Bell-like states [B. Bellomo
\textit{et al.}, Phys. Rev. Lett. \textbf{99}, 160502 (2007)]. The effect of
temperature on the two-qubit entanglement dynamics in a Markovian environment
is finally obtained.Comment: 10 pages, 6 figure
Confidence and Backaction in the Quantum Filter Equation
We study the confidence and backaction of state reconstruction based on a
continuous weak measurement and the quantum filter equation. As a physical
example we use the traditional model of a double quantum dot being continuously
monitored by a quantum point contact. We examine the confidence of the estimate
of a state constructed from the measurement record, and the effect of
backaction of that measurement on that state. Finally, in the case of general
measurements we show that using the relative entropy as a measure of confidence
allows us to define the lower bound on the confidence as a type of quantum
discord.Comment: 9 pages, 6 figure
Large temperature dependence of the Casimir force at the metal-insulator transition
The dependence of the Casimir force on material properties is important for
both future applications and to gain further insight on its fundamental
aspects. Here we derive a general theory of the Casimir force for
low-conducting compounds, or poor metals. For distances in the micrometer
range, a large variety of such materials is described by universal equations
containing a few parameters: the effective plasma frequency, dissipation rate
of the free carriers, and electric permittivity in the infrared range. This
theory can also describe inhomogeneous composite materials containing small
regions with different conductivity. The Casimir force for mechanical systems
involving samples made with compounds that have a metal-insulator transition
shows an abrupt large temperature dependence of the Casimir force within the
transition region, where metallic and dielectric phases coexist.Comment: 23 pages, 9 figure
Spin-Wave Theory of the Multiple-Spin Exchange Model on a Triangular Lattice in a Magnetic Field : 3-Sublattice Structures
We study the spin wave in the S=1/2 multiple-spin exchange model on a
triangular lattice in a magnetic field within the linear spin-wave theory. We
take only two-, three- and four-spin exchange interactions into account and
restrict ourselves to the region where a coplanar three-sublattice state is the
mean-field ground state. We found that the Y-shape ground state survives
quantum fluctuations and the phase transition to a phase with a 6-sublattice
structure occurs with softening of the spin wave. We estimated the quantum
corrections to the ground state sublattice magnetizations due to zero-point
spin-wave fluctuations.Comment: 8 pages, 20 figure
A path integral approach to the dynamics of a random chain with rigid constraints
In this work the dynamics of a freely jointed random chain which fluctuates
at constant temperature in some viscous medium is studied. The chain is
regarded as a system of small particles which perform a brownian motion and are
subjected to rigid constraints which forbid the breaking of the chain. For
simplicity, all interactions among the particles have been switched off and the
number of dimensions has been limited to two. The problem of describing the
fluctuations of the chain in the limit in which it becomes a continuous system
is solved using a path integral approach, in which the constraints are imposed
with the insertion in the path integral of suitable Dirac delta functions. It
is shown that the probability distribution of the possible conformations in
which the fluctuating chain can be found during its evolution in time coincides
with the partition function of a field theory which is a generalization of the
nonlinear sigma model in two dimensions. Both the probability distribution and
the generating functional of the correlation functions of the positions of the
beads are computed explicitly in a semiclassical approximation for a
ring-shaped chain.Comment: 36 pages, 2 figures, LaTeX + REVTeX4 + graphicx, minor changes in the
text, reference adde
Toric AdS4/CFT3 duals and M-theory Crystals
We study the recently proposed crystal model for three dimensional
superconformal field theories arising from M2-branes probing toric Calabi-Yau
four-fold singularities. We explain the algorithms mapping a toric Calabi-Yau
to a crystal and vice versa, and show how the spectrum of BPS meson states fits
into the crystal model.Comment: 24 pages, 24 figure
Quantum metamaterials: Electromagnetic waves in a Josephson qubit line
We consider the propagation of a classical electromagnetic wave through a
transmission line, formed by identical superconducting charge qubits inside a
superconducting resonator. Since the qubits can be in a coherent superposition
of quantum states, we show that such a system demonstrates interesting new
effects, such as a ``breathing'' photonic crystal with an oscillating bandgap,
and a ``quantum Archimedean screw'' that transports, at an arbitrary controlled
velocity, Josephson plasma waves through the transmission line. The key
ingredient of these effects is that the optical properties of the Josephson
transmission line are controlled by the quantum coherent state of the qubits.Comment: References adde
The Physical Basis for Long-lived Electronic Coherence in Photosynthetic Light Harvesting Systems
The physical basis for observed long-lived electronic coherence in
photosynthetic light-harvesting systems is identified using an analytically
soluble model. Three physical features are found to be responsible for their
long coherence lifetimes: i) the small energy gap between excitonic states, ii)
the small ratio of the energy gap to the coupling between excitonic states, and
iii) the fact that the molecular characteristics place the system in an
effective low temperature regime, even at ambient conditions. Using this
approach, we obtain decoherence times for a dimer model with FMO parameters of
160 fs at 77 K and 80 fs at 277 K. As such, significant
oscillations are found to persist for 600 fs and 300 fs, respectively, in
accord with the experiment and with previous computations. Similar good
agreement is found for PC645 at room temperature, with oscillations persisting
for 400 fs. The analytic expressions obtained provide direct insight into the
parameter dependence of the decoherence time scales.Comment: 5 figures; J. Phys. Chem. Lett. (2011
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