arXiv subjects

R. Floreanini

Publications and source records attributed to R. Floreanini.

At least 55 records · Page 3Linked to original sources

arXiv2009

Entangling two unequal atoms through a common bath

The evolution of two, non-interacting two-level atoms immersed in a weakly coupled bath can be described by a refined, time coarse grained Markovian evolution, still preserving complete positivity. We find that this improved reduced dynamics is able to entangle the two atoms even when their internal frequencies are unequal, an effect which appears impossible in the standard weak coupling limit approach. We study in detail this phenomenon for an environment made of quantum fields.

quant-ph↗

arXiv2009

Environment induced entanglement in a refined weak-coupling limit

Two non-directly interacting qubits with equal frequencies can become entangled via a Markovian, dissipative dynamics through the action of a weakly coupled Ohmic heat bath. In the standard weak-coupling limit derivation, this purely dissipative effect disappears if the frequencies are different because of the "ergodic average" used by this approach. However, there are physical situations where this technique is too rough to capture all the relevant aspects of the dissipative dynamics. In these cases, in order to better describe the physical behavior of the open system, it is necessary to go beyond the "ergodic average". We show that, in this more refined framework, the entanglement capability of the environment persists also in the case of different frequencies.

quant-ph↗

arXiv2009

Quantum measures for density correlations in optical lattices

The density-density correlation profiles obtained superimposing absorption images from atomic clouds freely expanding after the release of the confining optical lattice can be theoretically described in terms of a generalized quantum measure based on coherent-like states. We show that the corresponding density patterns differ in a testable way from those computed using standard many-body mean values, usually adopted in fitting experimental data.

quant-ph↗

arXiv2008

Noise induced interference fringes in trapped ultracold bosonic gases

We study the dynamics of ultracold atoms trapped in optical double-well potentials in presence of noise generated by an external environment. When prepared in a Fock number state, the system shows phase coherence in the averaged density profile obtained using standard absorption image techniques, that disappears in absence of noise. This effect indicates that also in many-body systems an external environment may enhance quantum coherence, instead of destroying it.

quant-ph↗

arXiv2008

Quantum measuring processes for trapped ultracold bosonic gases

The standard experimental techniques usually adopted in the study of the behaviour of ultracold atoms in optical lattices involve extracting the atom density profile from absorption images of the atomic sample after trap release. Quantum mechanically this procedure is described by a generalized measure (POVM); interference patterns found in absorption images suggest a generalized measure based on fixed-phase, coherent-like states. We show that this leads to an average atomic density which differs from the usually adopted one, obtained as the expectation value of the atom density operator in the many-body state.

quant-ph↗

arXiv2008

Cooper Pair Boxes Weakly Coupled to External Environments

We study the behaviour of charge oscillations in Superconducting Cooper Pair Boxes weakly interacting with an environment. We found that, due to the noise and dissipation induced by the environment, the stability properties of these nanodevices differ according to whether the charge oscillations are interpreted as an effect of macroscopic quantum coherence, or semiclassically in terms of the Gross-Pitaevskii equation. More specifically, occupation number states, used in the quantum interpretation of the oscillations, are found to be much more unstable than coherent ones, typical of the semiclassical explanation.

quant-ph↗

arXiv2008

Noise induced current in a double-well trap

We study the behavior of cold atoms trapped in optical double well potentials in presence of noise, either generated by an external environment or by the trap itself. We show that quite in general the noise can induce a current between the two wells even when tunneling between the two wells is highly suppressed (Mott insulator phase). An engineered environment could provide a test ground for the behavior of such a current.

quant-ph↗

arXiv2007

Non-Positive Semigroup Dynamics in Continuous Variable Models

Non-positive, Markovian semigroups are sometimes used to describe the time evolution of subsystems immersed in an external environment. A widely adopted prescription to avoid the appearance of negative probabilities is to eliminate from the admissible initial conditions those density matrices that would not remain positive by the action of the semigroup dynamics. Using a continuous variable model, we show that this procedure leads to physical inconsistencies when two subsystems are considered and their initial state is entangled.

quant-ph↗

arXiv2007

Redfield reduced dynamics and entanglement

In phenomenological applications the time evolution of subsystems immersed in an external environment are sometimes described by Markovian semigroups of Redfield type that result non-positive: the appearence of negative probabilities is avoided by restricting the admissible initial conditions to those states that remain positive under the action of the dynamics. We show that this often adopted procedure may lead to physical inconsistencies in presence of entanglement.

quant-ph↗

arXiv2006

Slipped non-Positive Reduced Dynamics and Entanglement

Non-positive Markov approximations are sometimes used to describe the dynamics of qubits in weak interaction with suitable environments; the appearance of negative probabilities is avoided by assuming that the transient regime eliminates from the possible initial conditions those qubit states which would otherwise be mapped out of the Bloch sphere by the subsequent Markovian time-evolution. By means of a simple model, we discuss some physical inconsistencies of this approach in relation to entanglement; in particular, we show that slipped non-positive reduced dynamics might create entanglement through a purely local action.

quant-ph↗

arXiv2006

Tests of Complete Positivity in Fiber Optics

We consider the propagation of polarized photons in optical fibers under the action of randomly generated noise. In such situation, the change in time of the photon polarization can be described by a quantum dynamical semigroup. We show that the hierarchy among the decay constants of the polarization density matrix elements as prescribed by complete positivity can be experimentally probed using standard laboratory set-ups.

quant-ph↗

arXiv2006

Asymptotic Entanglement of Two Independent Systems in a Common Bath

Two, non-interacting systems immersed in a common bath and evolving with a Markovian, completely positive dynamics can become initially entangled via a purely noisy mechanism. Remarkably, for certain, phenomenologically relevant environments, the quantum correlations can persist even in the asymptotic long-time regime.

quant-ph↗

arXiv2006

Entangling oscillators through environment noise

We consider two independent bosonic oscillators immersed in a common bath, evolving in time with a completely positive, markovian, quasi-free (Gaussian) reduced dynamics. We show that an initially separated Gaussian state can become entangled as a result of a purely noisy mechanism. In certain cases, the dissipative dynamics allows the persistence of these bath induced quantum correlations even in the asymptotic equilibrium state.

quant-ph↗

arXiv2005

Controlling Entanglement Generation in External Quantum Fields

Two, non-interacting two-level atoms immersed in a common bath can become mutually entangled when evolving with a Markovian, completely positive dynamics. For an environment made of external quantum fields, this phenomenon can be studied in detail: one finds that entanglement production can be controlled by varying the bath temperature and the distance between the atoms. Remarkably, in certain circumstances, the quantum correlations can persist in the asymptotic long-time regime.

quant-ph↗

arXiv2005

Open Quantum Dynamics: Complete Positivity and Entanglement

We review the standard treatment of open quantum systems in relation to quantum entanglement, analyzing, in particular, the behaviour of bipartite systems immersed in a same environment. We first focus upon the notion of complete positivity, a physically motivated algebraic constraint on the quantum dynamics, in relation to quantum entanglement, i.e. the existence of statistical correlations which can not be accounted for by classical probability. We then study the entanglement power of heat baths versus their decohering properties, a topic of increasing importance in the framework of the fast developing fields of quantum information, communication and computation. The presentation is self contained and, through several examples, it offers a detailed survey of the physics and of the most relevant and used techniques relative to both quantum open system dynamics and quantum entanglement.

quant-ph↗

arXiv2004

Dissipative neutrino oscillations in randomly fluctuating matter

The generalized dynamics describing the propagation of neutrinos in randomly fluctuating media is analyzed: it takes into account matter-induced, decoherence phenomena that go beyond the standard MSW effect. A widely adopted density fluctuation pattern is found to be physically untenable: a more general model needs to be instead considered, leading to flavor changing effective neutrino-matter interactions. They induce new, dissipative effects that modify the neutrino oscillation pattern in a way amenable to a direct experimental analysis.

hep-ph↗

arXiv2004

Entanglement generation in the Unruh effect

The master equation describing the completely positive time evolution of a uniformly accelerated two-level system in weak interaction with a scalar field in the Minkowski vacuum is derived and explicitly solved. It encodes the well known Unruh effect, leading to a purely thermal equilibrium state. This asymptotic state turns out to be entangled when the uniformly accelerating system is composed by two, independent two-level atoms.

quant-ph↗

arXiv2004

Quantum dynamical semigroups and non-decomposable positive maps

We study dynamical semigroups of positive, but not completely positive maps on finite-dimensional bipartite systems and analyze properties of their generators in relation to non-decomposability and bound-entanglement. An example of non-decomposable semigroup leading to a 4x4-dimensional bound-entangled density matrix is explicitly obtained.

quant-ph↗