2,870 research outputs found
Voltage probe model of spin decay in a chaotic quantum dot, with applications to spin-flip noise and entanglement production
The voltage probe model is a model of incoherent scattering in quantum
transport. Here we use this model to study the effect of spin-flip scattering
on electrical conduction through a quantum dot with chaotic dynamics. The spin
decay rate gamma is quantified by the correlation of spin-up and spin-down
current fluctuations (spin-flip noise). The resulting decoherence reduces the
ability of the quantum dot to produce spin-entangled electron-hole pairs. For
gamma greater than a critical value gamma_c, the entanglement production rate
vanishes identically. The statistical distribution P(gamma_c) of the critical
decay rate in an ensemble of chaotic quantum dots is calculated using the
methods of random-matrix theory. For small gamma_c this distribution is
proportional to gamma_c^(-1+beta/2), depending on the presence (beta=1) or
absence (beta=2) of time-reversal symmetry. To make contact with experimental
observables, we derive a one-to-one relationship between the entanglement
production rate and the spin-resolved shot noise, under the assumption that the
density matrix is isotropic in the spin degrees of freedom. Unlike the Bell
inequality, this relationship holds for both pure and mixed states. In the
tunneling regime, the electron-hole pairs are entangled if and only if the
correlator of parallel spin currents is at least twice larger than the
correlator of antiparallel spin currents.Comment: version 3: corrected a factor of two in Eq. (3.16), affecting the
final result
Counting statistics of coherent population trapping in quantum dots
Destructive interference of single-electron tunneling between three quantum
dots can trap an electron in a coherent superposition of charge on two of the
dots. Coupling to external charges causes decoherence of this superposition,
and in the presence of a large bias voltage each decoherence event transfers a
certain number of electrons through the device. We calculate the counting
statistics of the transferred charges, finding a crossover from sub-Poissonian
to super-Poissonian statistics with increasing ratio of tunnel and decoherence
rates.Comment: 4 pages, 2 figure
Interference through quantum dots
We discuss the effect of quantum interference on transport through a quantum
dot system. We introduce an indirect coherent coupling parameter alpha, which
provides constructive/destructive interference in the transport current
depending on its phase and the magnetic flux. We estimate the current through
the quantum dot system using the non-equilibrium Green's function method as
well as the master equation method in the sequential tunneling regime. The
visibility of the Aharonov-Bohm oscillation is evaluated. For a large inter-dot
Coulomb interaction, the current is strongly suppressed by the quantum
interference effect, while the current is restored by applying an oscillating
resonance field with the frequency of twice the inter-dot tunneling energy.Comment: 10 pages, 3 figure
A variety of Levitrons: A review
After a brief history and critique of some older instruments, several new
Levitron geometries are described. As a result of their greater stability these
devices can be used as analogues of a number of phenomena and applications,
including magnetic resonance techniques, atom traps and accelerator rings. In
particular, the notion of the spinning magnet (or spignet) in a linear trap is
similar to the mechanism underpinning the confinement of antihydrogen in a
magnetic minimum trap, as achieved in experiments at CERN
Toward an ecological aesthetics: music as emergence
In this article we intend to suggest some ecological based principles
to support the possibility of develop an ecological aesthetics. We consider that
an ecological aesthetics is founded in concepts as “direct perception”,
“acquisition of affordances and invariants”, “embodied embedded
perception” and so on. Here we will purpose that can be possible explain
especially soundscape music perception in terms of direct perception, working
with perception of first hand (in a Gibsonian sense). We will present notions
as embedded sound, detection of sonic affordances and invariants, and at the
end we purpose an experience with perception/action paradigm to make
soundscape music as emergence of a self-organized system
Crop Yields: Stripper Header Technology vs. Conventional Header Technology
The purpose of this paper is to compare crop yields on acres where stripper header technology was used to those where conventional header technology was used in a dryland cropping system. The results are based on experimental data collected at the USDA-ARS Central Great Plains Research Station, and include information on Northeastern Colorado wheat, millet, and sorghum yields. This analysis is important to demonstrate the benefits of using stripper header technology in a dryland cropping system. In the next section, the approaches and major findings of the previous literature are reviewed
Stub model for dephasing in a quantum dot
As an alternative to Buttiker's dephasing lead model, we examine a dephasing
stub. Both models are phenomenological ways to introduce decoherence in chaotic
scattering by a quantum dot. The difference is that the dephasing lead opens up
the quantum dot by connecting it to an electron reservoir, while the dephasing
stub is closed at one end. Voltage fluctuations in the stub take over the
dephasing role from the reservoir. Because the quantum dot with dephasing lead
is an open system, only expectation values of the current can be forced to
vanish at low frequencies, while the outcome of an individual measurement is
not so constrained. The quantum dot with dephasing stub, in contrast, remains a
closed system with a vanishing low-frequency current at each and every
measurement. This difference is a crucial one in the context of quantum
algorithms, which are based on the outcome of individual measurements rather
than on expectation values. We demonstrate that the dephasing stub model has a
parameter range in which the voltage fluctuations are sufficiently strong to
suppress quantum interference effects, while still being sufficiently weak that
classical current fluctuations can be neglected relative to the nonequilibrium
shot noise.Comment: 8 pages with 1 figure; contribution for the special issue of J.Phys.A
on "Trends in Quantum Chaotic Scattering
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