8,215 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
Entanglement production by quantum error correction in the presence of correlated environment
We analyze the effect of a quantum error correcting code on the entanglement
of encoded logical qubits in the presence of a dephasing interaction with a
correlated environment. Such correlated reservoir introduces entanglement
between physical qubits. We show that for short times the quantum error
correction interprets such entanglement as errors and suppresses it. However
for longer time, although quantum error correction is no longer able to correct
errors, it enhances the rate of entanglement production due to the interaction
with the environment.Comment: 7 pages, 3 figures, published versio
Berry phase for a spin 1/2 in a classical fluctuating field
The effect of fluctuations in the classical control parameters on the Berry
phase of a spin 1/2 interacting with a adiabatically cyclically varying
magnetic field is analyzed. It is explicitly shown that in the adiabatic limit
dephasing is due to fluctuations of the dynamical phase.Comment: 4 pages, 1 figure, published versio
Adiabatic quantum dynamics of a random Ising chain across its quantum critical point
We present here our study of the adiabatic quantum dynamics of a random Ising
chain across its quantum critical point. The model investigated is an Ising
chain in a transverse field with disorder present both in the exchange coupling
and in the transverse field. The transverse field term is proportional to a
function which, as in the Kibble-Zurek mechanism, is linearly
reduced to zero in time with a rate , , starting
at from the quantum disordered phase () and ending
at in the classical ferromagnetic phase (). We first analyze
the distribution of the gaps -- occurring at the critical point --
which are relevant for breaking the adiabaticity of the dynamics. We then
present extensive numerical simulations for the residual energy
and density of defects at the end of the annealing, as a function of
the annealing inverse rate . %for different lenghts of the chain. Both
the average and are found to behave
logarithmically for large , but with different exponents, with , and
. We propose a mechanism for
-behavior of based on the Landau-Zener
tunneling theory and on a Fisher's type real-space renormalization group
analysis of the relevant gaps. The model proposed shows therefore a
paradigmatic example of how an adiabatic quantum computation can become very
slow when disorder is at play, even in absence of any source of frustration.Comment: 10 pages, 11 figures; v2: added references, published versio
Observations Outside the Light-Cone: Algorithms for Non-Equilibrium and Thermal States
We apply algorithms based on Lieb-Robinson bounds to simulate time-dependent
and thermal quantities in quantum systems. For time-dependent systems, we
modify a previous mapping to quantum circuits to significantly reduce the
computer resources required. This modification is based on a principle of
"observing" the system outside the light-cone. We apply this method to study
spin relaxation in systems started out of equilibrium with initial conditions
that give rise to very rapid entanglement growth. We also show that it is
possible to approximate time evolution under a local Hamiltonian by a quantum
circuit whose light-cone naturally matches the Lieb-Robinson velocity.
Asymptotically, these modified methods allow a doubling of the system size that
one can obtain compared to direct simulation. We then consider a different
problem of thermal properties of disordered spin chains and use quantum belief
propagation to average over different configurations. We test this algorithm on
one dimensional systems with mixed ferromagnetic and anti-ferromagnetic bonds,
where we can compare to quantum Monte Carlo, and then we apply it to the study
of disordered, frustrated spin systems.Comment: 19 pages, 12 figure
Improved Core Genes Prediction for Constructing well-supported Phylogenetic Trees in large sets of Plant Species
The way to infer well-supported phylogenetic trees that precisely reflect the
evolutionary process is a challenging task that completely depends on the way
the related core genes have been found. In previous computational biology
studies, many similarity based algorithms, mainly dependent on calculating
sequence alignment matrices, have been proposed to find them. In these kinds of
approaches, a significantly high similarity score between two coding sequences
extracted from a given annotation tool means that one has the same genes. In a
previous work article, we presented a quality test approach (QTA) that improves
the core genes quality by combining two annotation tools (namely NCBI, a
partially human-curated database, and DOGMA, an efficient annotation algorithm
for chloroplasts). This method takes the advantages from both sequence
similarity and gene features to guarantee that the core genome contains correct
and well-clustered coding sequences (\emph{i.e.}, genes). We then show in this
article how useful are such well-defined core genes for biomolecular
phylogenetic reconstructions, by investigating various subsets of core genes at
various family or genus levels, leading to subtrees with strong bootstraps that
are finally merged in a well-supported supertree.Comment: 12 pages, 7 figures, IWBBIO 2015 (3rd International Work-Conference
on Bioinformatics and Biomedical Engineering
Detection of gravitational waves from the QCD phase transition with pulsar timing arrays
If the cosmological QCD phase transition is strongly first order and lasts
sufficiently long, it generates a background of gravitational waves which may
be detected via pulsar timing experiments. We estimate the amplitude and the
spectral shape of such a background and we discuss its detectability prospects.Comment: 7 pages, 5 figs. Version accepted by PR
Magnonic Charge Pumping via Spin-Orbit Coupling
The interplay between spin, charge, and orbital degrees of freedom has led to
the development of spintronic devices like spin-torque oscillators, spin-logic
devices, and spin-transfer torque magnetic random-access memories. In this
development spin pumping, the process where pure spin-currents are generated
from magnetisation precession, has proved to be a powerful method for probing
spin physics and magnetisation dynamics. The effect originates from direct
conversion of low energy quantised spin-waves in the magnet, known as magnons,
into a flow of spins from the precessing magnet to adjacent normal metal leads.
The spin-pumping phenomenon represents a convenient way to electrically detect
magnetisation dynamics, however, precessing magnets have been limited so far to
pump pure spin currents, which require a secondary spin-charge conversion
element such as heavy metals with large spin Hall angle or multi-layer layouts
to be detectable. Here, we report the experimental observation of charge
pumping in which a precessing ferromagnet pumps a charge current, demonstrating
direct conversion of magnons into high-frequency currents via the relativistic
spin-orbit interaction. The generated electric current, differently from spin
currents generated by spin-pumping, can be directly detected without the need
of any additional spin to charge conversion mechanism and amplitude and phase
information about the relativistic current-driven magnetisation dynamics. The
charge-pumping phenomenon is generic and gives a deeper understanding of the
recently observed spin-orbit torques, of which it is the reciprocal effect and
which currently attract interest for their potential in manipulating magnetic
information. Furthermore, charge pumping provides a novel link between
magnetism and electricity and may find application in sourcing alternating
electric currents.Comment: 3 figure
Concurrence in Disordered Systems
Quantum systems exist at finite temperatures and are likely to be disordered
to some level. Since applications of quantum information often rely on
entanglement, we require methods which allow entanglement measures to be
calculated in the presence of disorder at non-zero temperatures. We demonstrate
how the disorder averaged concurrence can be calculated using thermal many-body
perturbation theory. Our technique can also be applied to other entanglement
measures. To illustrate, we find the disorder averaged concurrence of an XX
spin chain. We find that concurrence can be increased by disorder in some
parameter regimes.Comment: 14 pages, 5 figure
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