7,160 research outputs found
Density Matrix Renormalization Group for Dummies
We describe the Density Matrix Renormalization Group algorithms for time
dependent and time independent Hamiltonians. This paper is a brief but
comprehensive introduction to the subject for anyone willing to enter in the
field or write the program source code from scratch.Comment: 29 pages, 9 figures. Published version. An open source version of the
code can be found at http://qti.sns.it/dmrg/phome.htm
Equilibrium and Disorder-induced behavior in Quantum Light-Matter Systems
We analyze equilibrium properties of coupled-doped cavities described by the
Jaynes-Cummings- Hubbard Hamiltonian. In particular, we characterize the
entanglement of the system in relation to the insulating-superfluid phase
transition. We point out the existence of a crossover inside the superfluid
phase of the system when the excitations change from polaritonic to purely
photonic. Using an ensemble statistical approach for small systems and
stochastic-mean-field theory for large systems we analyze static disorder of
the characteristic parameters of the system and explore the ground state
induced statistics. We report on a variety of glassy phases deriving from the
hybrid statistics of the system. On-site strong disorder induces insulating
behavior through two different mechanisms. For disorder in the light-matter
detuning, low energy cavities dominate the statistics allowing the excitations
to localize and bunch in such cavities. In the case of disorder in the light-
matter coupling, sites with strong coupling between light and matter become
very significant, which enhances the Mott-like insulating behavior. Inter-site
(hopping) disorder induces fluidity and the dominant sites are strongly coupled
to each other.Comment: about 10 pages, 12 figure
Effective thermal dynamics following a quantum quench in a spin chain
We study the nonequilibrium dynamics of the Quantum Ising Model following an
abrupt quench of the transverse field. We focus on the on-site autocorrelation
function of the order parameter, and extract the phase coherence time
from its asymptotic behavior. We show that the initial state
determines only through an effective temperature set by its
energy and the final Hamiltonian. Moreover, we observe that the dependence of
on the effective temperature fairly agrees with that obtained
in thermal equilibrium as a function of the equilibrium temperature.Comment: 4 pages, 4 figures. Published versio
Optimal correlations in many-body quantum systems
Information and correlations in a quantum system are closely related through
the process of measurement. We explore such relation in a many-body quantum
setting, effectively bridging between quantum metrology and condensed matter
physics. To this aim we adopt the information-theory view of correlations, and
study the amount of correlations after certain classes of
Positive-Operator-Valued Measurements are locally performed. As many-body
system we consider a one-dimensional array of interacting two-level systems (a
spin chain) at zero temperature, where quantum effects are most pronounced. We
demonstrate how the optimal strategy to extract the correlations depends on the
quantum phase through a subtle interplay between local interactions and
coherence.Comment: 5 pages, 5 figures + supplementary material. To be published in PR
Long time dynamics following a quench in an integrable quantum spin chain: local versus non-local operators and effective thermal behavior
We study the dynamics of the quantum Ising chain following a zero-temperature
quench of the transverse field strength. Focusing on the behavior of two-point
spin correlation functions, we show that the correlators of the order parameter
display an effective asymptotic thermal behavior, i.e., they decay
exponentially to zero, with a phase coherence rate and a correlation length
dictated by the equilibrium law with an effective temperature set by the energy
of the initial state. On the contrary, the two-point correlation functions of
the transverse magnetization or the density-of-kinks operator decay as a
power-law and do not exhibit thermal behavior. We argue that the different
behavior is linked to the locality of the corresponding operator with respect
to the quasi-particles of the model: non-local operators, such as the order
parameter, behave thermally, while local ones do not. We study which features
of the two-point correlators are a consequence of the integrability of the
model by analizing their robustness with respect to a sufficiently strong
integrability-breaking term.Comment: 18 pages, 11 figures, published version. Extensive changes, one
author adde
Decoding Information From Neural Signals Recorded Using Intraneural Electrodes: Toward the Development of a Neurocontrolled Hand Prosthesis
The possibility of controlling dexterous hand prostheses by using a direct connection with the nervous system is particularly interesting for the significant improvement of the quality of life of patients, which can derive from this achievement. Among the various approaches, peripheral nerve based intrafascicular electrodes are excellent neural interface candidates, representing an excellent compromise between high selectivity and relatively low invasiveness. Moreover, this approach has undergone preliminary testing in human volunteers and has shown promise. In this paper, we investigate whether the use of intrafascicular electrodes can be used to decode multiple sensory and motor information channels with the aim to develop a finite state algorithm that may be employed to control neuroprostheses and neurocontrolled hand prostheses. The results achieved both in animal and human experiments show that the combination of multiple sites recordings and advanced signal processing techniques (such as wavelet denoising and spike sorting algorithms) can be used to identify both sensory stimuli (in animal models) and motor commands (in a human volunteer). These findings have interesting implications, which should be investigated in future experiments. © 2006 IEEE
RBF-Based Partition of Unity Methods for Elliptic PDEs: Adaptivity and Stability Issues Via Variably Scaled Kernels
We investigate adaptivity issues for the approximation of Poisson equations via radial basis
function-based partition of unity collocation. The adaptive residual subsampling approach
is performed with quasi-uniform node sequences leading to a flexible tool which however
might suffer from numerical instability due to ill-conditioning of the collocation matrices.
We thus develop a hybrid method which makes use of the so-called variably scaled kernels.
The proposed algorithm numerically ensures the convergence of the adaptive procedure
Photon transfer in ultrastrongly coupled three-cavity arrays
We study the photon transfer along a linear array of three coupled cavities
where the central one contains an interacting two-level system in the strong
and ultrastrong coupling regimes. We find that an inhomogeneously coupled array
forbids a complete single-photon transfer between the external cavities when
the central one performs a Jaynes-Cummings dynamics. This is not the case in
the ultrastrong coupling regime, where the system exhibits singularities in the
photon transfer time as a function of the cavity-qubit coupling strength. Our
model can be implemented within the state-of-the-art circuit quantum
electrodynamics technology and it represents a building block for studying
photon state transfer through scalable cavity arrays.Comment: 5 pages, 5 figures, supplemental materia
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