1,390 research outputs found
Decoherence as attenuation of mesoscopic echoes in a spin-chain channel
An initial local excitation in a confined quantum system evolves exploring
the whole system, returning to the initial position as a mesoscopic echo at the
Heisenberg time. We consider a two weakly coupled spin chains, a spin ladder,
where one is a quantum channel while the other represents an environment. We
quantify decoherence in the quantum channel through the attenuation of the
mesoscopic echoes. We evaluate decoherence rates for different ratios between
sources of amplitude fluctuation and dephasing in the inter-chain interaction
Hamiltonian. The many-body dynamics is seen as a one-body evolution with a
decoherence rate given by the Fermi golden rule.Comment: 12 pages, 7 figure
Quantum parallelism as a tool for ensemble spin dynamics calculations
Efficient simulations of quantum evolutions of spin-1/2 systems are relevant
for ensemble quantum computation as well as in typical NMR experiments. We
propose an efficient method to calculate the dynamics of an observable provided
that the initial excitation is "local". It resorts a single entangled pure
initial state built as a superposition, with random phases, of the pure
elements that compose the mixture. This ensures self-averaging of any
observable, drastically reducing the calculation time. The procedure is tested
for two representative systems: a spin star (cluster with random long range
interactions) and a spin ladder.Comment: 5 pages, 3 figures, improved version of the manuscrip
Quantum dynamics under coherent and incoherent effects of a spin bath in the Keldysh formalism: application to a spin swapping operation
We develop the Keldysh formalism for the polarization dynamics of an open
spin system. We apply it to the swapping between two qubit states in a model
describing an NMR cross-polarization experiment. The environment is a set of
interacting spins. For fast fluctuations in the environment, the analytical
solution shows effects missed by the secular approximation of the Quantum
Master Equation for the density matrix: a frequency decrease depending on the
system-environment escape rate and the quantum quadratic short time behavior.
Considering full memory of the bath correlations yields a progressive change of
the swapping frequency.Comment: 16 pages, 3 figures, final for
Perfect state transfers by selective quantum interferences within complex spin networks
We present a method that implement directional, perfect state transfers
within a branched spin network by exploiting quantum interferences in the
time-domain. That provides a tool to isolate subsystems from a large and
complex one. Directionality is achieved by interrupting the spin-spin coupled
evolution with periods of free Zeeman evolutions, whose timing is tuned to be
commensurate with the relative phases accrued by specific spin pairs. This
leads to a resonant transfer between the chosen qubits, and to a detuning of
all remaining pathways in the network, using only global manipulations. As the
transfer is perfect when the selected pathway is mediated by 2 or 3 spins,
distant state transfers over complex networks can be achieved by successive
recouplings among specific pairs/triads of spins. These effects are illustrated
with a quantum simulator involving 13C NMR on Leucine's backbone; a six-spin
network.Comment: 5 pages, 3 figure
Quantum engineering for compactly localized states in disordered Lieb lattices
Blending ordering within an uncorrelated disorder potential in families of 3D
Lieb lattices preserves the macroscopic degeneracy of compact localized states
and yields unconventional combinations of localized and delocalized phases --
as shown in Phys.Rev.B 106, 214204 (2022). We proceed to reintroduce
translation invariance in the system by further ordering the disorder, and
discuss the spectral structure and eigenstates features of the resulting
perturbed lattices. We restore order in steps by first (i) rendering the
disorder binary -- i.e. yielding a randomized checkerboard potential, then (ii)
reordering the randomized checkerboard into an ordered one, and at last (iii)
realigning all the checkerboard values yielding a constant potential shift, but
only on a sub-lattice. Along this path, we test the influence of additional
random impurities on the order restoration. We find that in each of these
steps, sub-families of states are projected upon the location of the degenerate
compact states, while the complementary ones are localized in the perturbed
sites with energy determined by the strength of checkerboard. This strategy,
herewith implemented in the 3D Lieb, highlights order restoration as
experimental pathway to engineer spectral and states features in disordered
lattice structures in the pursuit of quantum storage and memory applications
Dynamiques de Diffusion des Téchnologies de la Communication en Afrique. Le Cas du Bénin, de l'Indépendance à la République Populaire
Environmentally induced Quantum Dynamical Phase Transition in the spin swapping operation
Quantum Information Processing relies on coherent quantum dynamics for a
precise control of its basic operations. A swapping gate in a two-spin system
exchanges the degenerate states |+,-> and |-,+>. In NMR, this is achieved
turning on and off the spin-spin interaction b=\Delta E that splits the energy
levels and induces an oscillation with a natural frequency \Delta E/\hbar.
Interaction of strength \hbar/\tau_{SE}, with an environment of neighboring
spins, degrades this oscillation within a decoherence time scale \tau_{\phi}.
While the experimental frequency \omega and decoherence time \tau_{\phi} were
expected to be roughly proportional to b/\hbar and \tau_{SE} respectively, we
present here experiments that show drastic deviations in both \omega and
\tau_{\phi}. By solving the many spin dynamics, we prove that the swapping
regime is restricted to \Delta E \tau_{SE} > \hbar. Beyond a critical
interaction with the environment the swapping freezes and the decoherence rate
drops as 1/\tau_{\phi} \propto (b/\hbar)^2 \tau_{SE}. The transition between
quantum dynamical phases occurs when \omega \propto
\sqrt{(b/\hbar)^{2}-(k/\tau_{SE})^2} becomes imaginary, resembling an
overdamped classical oscillator. Here, 0<k^2<1 depends only on the anisotropy
of the system-environment interaction, being 0 for isotropic and 1 for XY
interactions. This critical onset of a phase dominated by the Quantum Zeno
effect opens up new opportunities for controlling quantum dynamics.Comment: Final version. One figure and some equations corrected, 10 pages, 4
figure
Decoherence under many-body system-environment interactions: a stroboscopic representation based on a fictitiously homogenized interaction rate
An environment interacting with portions of a system leads to
multiexponential interaction rates. Within the Keldysh formalism, we
fictitiously homogenize the system-environment interaction yielding a uniform
decay rate facilitating the evaluation of the propagators. Through an injection
procedure we neutralize the fictitious interactions. This technique justifies a
stroboscopic representation of the system-environment interaction which is
useful for numerical implementation and converges to the natural continuous
process. We apply this procedure to a fermionic two-level system and use the
Jordan-Wigner transformation to solve a two-spin swapping gate in the presence
of a spin environment.Comment: 11 pages, 3 figures, title changed, some typos change
Unconventional delocalization in a family of 3D Lieb lattices
Uncorrelated disorder in generalized 3D Lieb models gives rise to the
existence of bounded mobility edges, destroys the macroscopic degeneracy of the
flat bands and breaks their compactly-localized states. We now introduce a mix
of order and disorder such that this degeneracy remains and the
compactly-localized states are preserved. We obtain the energy-disorder phase
diagrams and identify mobility edges. Intriguingly, for large disorder the
survival of the compactly-localized states induces the existence of delocalized
eigenstates close to the original flat band energies -- yielding seemingly
divergent mobility edges. For small disorder, however, a change from extended
to localized behavior can be found upon decreasing disorder -- leading to an
unconventional ``inverse Anderson" behavior. We show that transfer matrix
methods, computing the localization lengths, as well as sparse-matrix
diagonalization, using spectral gap-ratio energy-level statistics, are in
excellent quantitative agreement. The preservation of the compactly-localized
states even in the presence of this disorder might be useful for envisaged
storage applications
A multipurpose leguminous plant for the mediterranean countries: Leucaena leucocephala as an alternative protein source: a review
SIMPLE SUMMARY: The need to address the shortage of protein ingredients linked to both territoriality and growing demand pushes research to focus attention on alternative protein sources, both vegetable and animal (insects). This review describes the characteristics, uses, strengths, and weaknesses of Leucaena leucocephala, a legume that can be used in the zootechnical field as an alternative to traditional protein sources for feed formulation. ABSTRACT: In tropical and subtropical regions, as well as in the internal and/or marginal Mediterranean areas, one of the most important problems related to animal production is represented by the inadequate nutritional supplies. The low productivity of the animals, often connected to reduced annual growth, is, in fact, not infrequently attributable to the low nitrogen content and the high fiber content of the local plant species and crop residues that constitute the base ingredients of the rations commonly adopted by farmers. The use of the supplementation with arboreal and shrub fodder, although often containing anti-nutritional factors and toxins that limit its use, could be a profitable way to alleviate the nutritional deficiencies of the basic diets. Leucaena leucocephala (Lam.) De Wit is native to Central America and widely naturalized in the majority of Latin American countries. It is a legume suitable for tropical and subtropical environments including the countries of the Mediterranean area. Moreover, its spread is desirable if we consider the multiple uses to which it is suitable, the considerable amount of biomass produced, and its role in preserving the environment. The aim of this work was to highlight the characteristics of Leucaena that can justify its wide diffusion. A structured analysis of strengths and weaknesses was performed accordingly. Being a good protein source for feeding livestock, it could be a species to be introduced in the inland areas of the Mediterranean countries as an alternative protein source; the limit represented by the presence of anti-nutritional factors could be overcome by feed processing and by launching targeted research programs
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