528 research outputs found
Generalized constraints on quantum amplification
We derive quantum constraints on the minimal amount of noise added in linear
amplification involving input or output signals whose component operators do
not necessarily have c-number commutators, as is the case for fermion currents.
This is a generalization of constraints derived for the amplification of
bosonic fields whose components posses c-number commutators.Comment: 4 pages, 1 figure, submitted to Physical Review Letter
AC-Conductance through an Interacting Quantum Dot
We investigate the linear ac-conductance for tunneling through an arbitrary
interacting quantum dot in the presence of a finite dc-bias. In analogy to the
well-known Meir-Wingreen formula for the dc case, we are able to derive a
general formula for the ac-conductance. It can be expressed entirely in terms
of local correlations on the quantum dot, in the form of a Keldysh block
diagram with four external legs. We illustrate the use of this formula as a
starting point for diagrammatic calculations by considering the ac-conductance
of the noninteracting resonant level model and deriving the result for the
lowest order of electron-phonon coupling. We show how known results are
recovered in the appropriate limits.Comment: 4+ pages, 4 figure
Emission and absorption noise in the fractional quantum Hall effect
We compute the high-frequency emission and absorption noise in a fractional
quantum Hall effect (FQHE) sample at arbitrary temperature. We model the edges
of the FQHE as chiral Luttinger liquids (LL) and we use the non-equilibrium
perturbative Keldysh formalism. We find that the non-symmetrized high frequency
noise contains important signatures of the electron-electron interactions that
can be used to test the Luttinger liquid physics, not only in FQHE edge states,
but possibly also in other one-dimensional systems such as carbon nanotubes. In
particular we find that the emission and absorption components of the excess
noise (defined as the difference between the noise at finite voltage and at
zero voltage) are different in an interacting system, as opposed to the
non-interacting case when they are identical. We study the resonance features
which appear in the noise at the Josephson frequency (proportional to the
applied voltage), and we also analyze the effect of the distance between the
measurement point and the backscattering site. Most of our analysis is
performed in the weak backscattering limit, but we also compute and discuss
briefly the high-frequency noise in the tunneling regime.Comment: 26 pages, 11 figure
Effective temperature of a dissipative driven mesoscopic system
We study the nonequilibrium dynamics of a mesoscopic metallic ring threaded
by a time-dependent magnetic field and coupled to an electronic reservoir. We
analyze the relation between the (non-stationary) real-time Keldysh and
retarded Green functions and we find that, in the linear response regime with
weak heat transfer to the environment, an effective temperature accounts for
the modification of the equilibrium fluctuation-dissipation relation. We
discuss possible extensions of this analysis.Comment: 4 pages, 4 figures, RevTe
Full counting statistics of strongly non-Ohmic transport through single molecules
We study analytically the full counting statistics of charge transport
through single molecules, strongly coupled to a weakly damped vibrational mode.
The specifics of transport in this regime - a hierarchical sequence of
avalanches of transferred charges, interrupted by "quiet" periods - make the
counting statistics strongly non-Gaussian. We support our findings for the
counting statistics as well as for the frequency-dependent noise power by
numerical simulations, finding excellent agreement.Comment: 4+ pages, 2 figures; minor changes, version published in Phys. Rev.
Let
DEEP: a provenance-aware executable document system
The concept of executable documents is attracting growing interest from both academics and publishers since it is a promising technology for the dissemination of scientific results. Provenance is a kind of metadata that provides a rich description of the derivation history of data products starting from their original sources. It has been used in many different e-Science domains and has shown great potential in enabling reproducibility of scientific results. However, while both executable documents and provenance are aimed at enhancing the dissemination of scientific results, little has been done to explore the integration of both techniques. In this paper, we introduce the design and development of DEEP, an executable document environment that generates scientific results dynamically and interactively, and also records the provenance for these results in the document. In this system, provenance is exposed to users via an interface that provides them with an alternative way of navigating the executable document. In addition, we make use of the provenance to offer a document rollback facility to users and help to manage the system's dynamic resources
ScreeNOT: Exact MSE-Optimal Singular Value Thresholding in Correlated Noise
We derive a formula for optimal hard thresholding of the singular value
decomposition in the presence of correlated additive noise; although it
nominally involves unobservables, we show how to apply it even where the noise
covariance structure is not a-priori known or is not independently estimable.
The proposed method, which we call ScreeNOT, is a mathematically solid
alternative to Cattell's ever-popular but vague Scree Plot heuristic from 1966.
ScreeNOT has a surprising oracle property: it typically achieves exactly, in
large finite samples, the lowest possible MSE for matrix recovery, on each
given problem instance - i.e. the specific threshold it selects gives exactly
the smallest achievable MSE loss among all possible threshold choices for that
noisy dataset and that unknown underlying true low rank model. The method is
computationally efficient and robust against perturbations of the underlying
covariance structure.
Our results depend on the assumption that the singular values of the noise
have a limiting empirical distribution of compact support; this model, which is
standard in random matrix theory, is satisfied by many models exhibiting either
cross-row correlation structure or cross-column correlation structure, and also
by many situations where there is inter-element correlation structure.
Simulations demonstrate the effectiveness of the method even at moderate matrix
sizes. The paper is supplemented by ready-to-use software packages implementing
the proposed algorithm
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