Universal decoherence in solids

Symmetry implications for the decoherence of quantum oscillations of a two-state system in a solid are studied. When the oscillation frequency is small compared to the Debye frequency, the universal lower bound on the decoherence due to the atomic environment is derived in terms of the macroscopic parameters of the solid, with no unknown interaction constants.     

Quantum decoherence without damping

An exactly soluble model leading to decoherence is studied. The effect of the environment (subohmic, ohmic and supraohmic) and of the temperature (zero or finite) is presented. The accuracy of the Born-Markov approximation and of the semigroup evolution equation is discussed.     

Feynman’s decoherence

Gell-Mann’s quarks are coherent particles confined within a hadron at rest, but Feynman’s partons are incoherent particles which constitute a hadron moving with a velocity close to that of light. It is widely believed that the quark model and the parton model are two different manifestations of the same covariant entity. If this is the case, the question arises whether the Lorentz boost destroys coherence. It is pointed out that this is not the case, and it is possible to resolve this puzzle without inventing new physics. It is shown that this decoherence is due to the measurement processes which are less than complete.     

Energy basis via decoherence

The question of the emergence of a preferred basis which is generally understood as that basis in which the reduced density matrix is driven to a diagonal (classically interpretable) form via environment induced decoherence is addressed. The exact solutions of the Caldeira-Leggett Master Equation are analyzed for a free particle and a harmonic oscillator system. In both cases, we see that the reduced density matrix is driven diagonal in the energy basis, which is momentum for the free particle and the number states for the harmonic oscillator. This seems to single out the energy basis as the preferred basis which is contrary to the general notion that it is the position basis which is selected since the coupling to the environment is via the position coordinates     

Quantum Zeno control of decoherence

We demonstrate the possibility of controlling the quantum to classical transition by means of the quantum Zeno and anti-Zeno effects. We consider a quantum harmonic oscillator initially prepared in a Schrödinger cat state and interacting with an engineered amplitude reservoir. We show that the environment induced decoherence transforming the cat state into a statistical mixture can be strongly inhibited by means of either appropriate sequences of measurements or shuttered reservoirs.     

Quantum decoherence of single-photon counters

The interaction of a quantum system with the environment leads to the so-called quantum decoherence. Beyond its fundamental significance, the understanding and the possible control of this dynamics in various scenarios is a key element for mastering quantum information processing. Here we report the quantitative probing of what can be called the quantum decoherence of detectors, a process reminiscent of the decoherence of quantum states in the presence of coupling with a reservoir. We demonstrate how the quantum features of two single-photon counters vanish under the influence of a noisy environment. We thereby experimentally witness the transition between the full-quantum operation of the measurement device to the "semi-classical regime", described by a positive Wigner function. The exact border between these two regimes is explicitely determined and measured experimentally.     

Gravitational decoherence of atomic interferometers

We study the decoherence of atomic interferometers due to the scattering of stochastic gravitational waves. We evaluate the “direct” gravitational effect registered by the phase of the matter waves as well as the “indirect” effect registered by the light waves used as beam-splitters and mirrors for the matter waves. Considering as an example the space project HYPER, we show that both effects are negligible for the presently studied interferometers. Received 15 February 2002 / Received in final form 12 April 2002 Published online 19 July 2002     

Environment-induced decoherence II. Effect of decoherence on Bell''s inequality for an EPR pair

According to Bell's theorem, the degree of correlation between spatially separated measurements on a quantum system is limited by certain inequalities if one assumes the condition of locality. Quantum mechanics predicts that this limit can be exceeded, making it nonlocal. We analyse the effect of an environment modelled by a fluctuating magnetic field on the quantum correlations in an EPR singlet as seen in the Bell inequality. We show that in an EPR setup, the system goes from the usual ‘violation’ of Bell's inequality to a ‘non-violation’ for times larger than a characteristic time scale which is related to the parameters of the fluctuating field. We also look at these inequalities as a function of the spatial separation between the EPR pair.     

Universality in fragmentation

Fragmentation of a two-dimensional brittle solid by impact and "explosion," and a fluid by "explosion" are all shown to become critical. The critical points appear at a nonzero impact velocity, and at infinite explosion duration, respectively. Within the critical regimes, the fragment-size distributions satisfy a scaling form qualitatively similar to that of the cluster-size distribution of percolation, but they belong to another universality class. Energy balance arguments give a correlation length exponent that is exactly one-half of its percolation value. A single crack dominates fragmentation in the slow-fracture limit, as expected.     

Universality in bibliometrics

Many discussions have enlarged the literature in Bibliometrics since the Hirsch proposal, the so called h-index. Ranking papers according to their citations, this index quantifies a researcher only by its greatest possible number of papers that are cited at least h times. A closed formula for h-index distribution that can be applied for distinct databases is not yet known. In fact, to obtain such distribution, the knowledge of citation distribution of the authors and its specificities are required. Instead of dealing with researchers randomly chosen, here we address different groups based on distinct databases. The first group is composed of physicists and biologists, with data extracted from Institute of Scientific Information (ISI). The second group is composed of computer scientists, in which data were extracted from Google-Scholar system. In this paper, we obtain a general formula for the h-index probability density function (pdf) for groups of authors by using generalized exponentials in the context of escort probability. Our analysis includes the use of several statistical methods to estimate the necessary parameters. Also an exhaustive comparison among the possible candidate distributions are used to describe the way the citations are distributed among authors. The h-index pdf should be used to classify groups of researchers from a quantitative point of view, which is meaningfully interesting to eliminate obscure qualitative methods.     

Universality of electroweak couplings

The present experimental tests of the electroweak universality in the leptonic sector are reviewed for both charged- and neutral-current electroweak interactions. To cite this article: A. Rougé, R. Tanaka, C. R. Physique 3 (2002) 1165–1172.     

Universality of striped morphologies

We present a method for predicting the low-temperature behavior of spherical and Ising spin models with isotropic potentials. For the spherical model the characteristic length scales of the ground states are exactly determined but the morphology is shown to be degenerate with checkerboard patterns, stripes and more complex morphologies having identical energy. For the Ising models we show that the discretization breaks the degeneracy causing striped morphologies to be energetically favored and therefore they arise universally as ground states to potentials whose Hankel transforms have nontrivial minima.     

On decoherence in quantum gravity

It was previously argued that the phenomenon of quantum gravitational decoherence described by the Wheeler‐DeWitt equation is responsible for the emergence of the arrow of time. Here we show that the characteristic spatio‐temporal scales of quantum gravitational decoherence are typically logarithmically larger than a characteristic curvature radius of the background space‐time. This largeness is a direct consequence of the fact that gravity is a non‐renormalizable theory, and the corresponding effective field theory is nearly decoupled from matter degrees of freedom in the physical limit . Therefore, as such, quantum gravitational decoherence is too ineffective to guarantee the emergence of the arrow of time and the “quantum‐to‐classical” transition to happen at scales of physical interest. We argue that the emergence of the arrow of time is directly related to the nature and properties of physical observer.

     

Multipartite entanglement dynamics and decoherence

We study the dynamics of multipartite entanglement under decoherence induced by the environment consisting of a fermionic bath. Based on the algebraic measure of entanglement-negativity, we analyze the time evolution of entanglement of both pure states and mixed ones, and find that entanglement evolution depends on both bath temperature and the number of qubits of the system. A linear space S_LDF which is dynamically decoupled from the environment is identified in the sense of linear entropy to symbolize the environment effect.     

A quantum trajectory description of decoherence

A complete theoretical treatment in many problems relevant to physics, chemistry, and biology requires considering the action of the environment over the system of interest. Usually the environment involves a relatively large number of degrees of freedom, this making the problem numerically intractable from a purely quantum-mechanical point of view. To overcome this drawback, a new class of quantum trajectories is proposed. These trajectories, based on the same grounds as Bohmian ones, are solely associated to the system reduced density matrix, since the evolution of the environment degrees of freedom is not considered explicitly. Within this approach, environment effects come into play through a time-dependent damping factor that appears in the system equations of motion. Apart from their evident computational advantage, this type of trajectories also results very insightful to understand the system decoherence. In particular, here we show the usefulness of these trajectories analyzing decoherence effects in interference phenomena, taking as a working model the well-known double-slit experiment.     

Controlled decoherence of floating flux qubits

In Born-Markov approximation,this paper calculates the energy relaxation time T 1 and the decoherence time T 2 of a floating flux qubit by solving the set of Bloch-Redfield equations.It shows that there are two main factors influencing the floating flux qubits:coupling capacitor in the circuit and the environment resistor.It also discusses how to improve the quantum coherence time of a qubit.Through shunt connecting/series connecting inductive elements,an inductive environment resistor is obtained and further the reactance component of the environment resistor is improved,which is beneficial to the enhancement of decoherence time of floating flux qubits.