A. Napoli

RWTH Aachen University, Aachen, North Rhine-Westphalia, Germany

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Publications (87)113.51 Total impact

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    ABSTRACT: The connection between the concepts underlying Interaction-Free Evolutions (IFE) and Decoherence-Free Subspaces (DFS) is explored, bringing to light similarities and differences. It is shown that IFE states and DFS can exist independently, in spite of the fact that both classes of states are connected with the idea of an interaction which is not felt. Inspired by a generalization of DFS already known, we introduce the notion of generalized IFE and provide some very simple examples that allow to better understand the relevant idea.
    Full-text · Article · Jun 2015
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    ABSTRACT: The fluxonium qubit has arisen as one of the most promising candidate devices for implementing quantum information in superconducting devices, since it is both insensitive to charge noise (like flux qubits) and insensitive to flux noise (like charge qubits). Here, we investigate the stability of the quantum information to quasiparticle tunneling through a Josephson junction. Microscopically, this dephasing is due to the dependence of the quasiparticle transmission probability on the qubit state. We argue that on a phenomenological level the dephasing mechanism can be understood as originating from heat currents, which are flowing in the device due to possible effective temperature gradients, and their sensitivity to the qubit state. The emerging dephasing time is found to be insensitive to the number of junctions with which the superinductance of the fluxonium qubit is realised. Furthermore, we find that the dephasing time increases quadratically with the shunt-inductance of the circuit which highlights the stability of the device to this dephasing mechanism.
    Preview · Article · Mar 2015 · New Journal of Physics
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    ABSTRACT: The generalization of the concept of interaction-free evolutions (IFE) [A. Napoli, {\it et al.}, Phys. Rev. A {\bf 89}, 062104 (2014)] to the case of time-dependent Hamiltonians is discussed. It turns out that the time-dependent case allows for much more rich structures of interaction free states and interaction free subspaces. The general condition for the occurrence of IFE is found and exploited to analyze specific situations. Several examples are presented, each one associated to a class of Hamiltonians with specific features.
    Full-text · Article · Feb 2015 · Physical Review A
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    ABSTRACT: An interaction free evolving state of a closed bipartite system composed of two interacting subsystems is a generally mixed state evolving as if the interaction were a c-number. In this paper we find the characteristic equation of states possessing similar properties for a bipartite systems governed by a linear dynamical equation whose generator is sum of a free term and an interaction term. In particular in the case of a small system coupled to its environment, we deduce the characteristic equation of decoherence free states namely mixed states evolving as if the interaction term were effectively inactive. Several examples illustrate the applicability of our theory in different physical contexts.
    Full-text · Article · Feb 2015 · Physica Scripta
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    ABSTRACT: By quickly modifying the shape of the effective potential of a double SQUID flux qubit from a single-well to a double-well condition, we experimentally observe an anomalous behavior, namely, an alternation of resonance peaks, in the probability to find the qubit in a given flux state. The occurrence of Landau-Zener transitions as well as resonant tunneling between degenerate levels in the two wells may be invoked to partially justify the experimental results. A quantum simulation of the time evolution of the system indeed suggests that the observed anomalous behavior can be imputable to quantum coherence effects. The interplay among all these mechanisms has a practical implication for quantum computing purposes, giving a direct measurement of the limits on the sweeping rates possible for a correct manipulation of the qubit state by means of fast flux pulses, avoiding transitions to noncomputational states.
    Full-text · Article · Apr 2014 · Physical Review B
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    A. Napoli · M. Guccione · A. Messina · Chruściński
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    ABSTRACT: We show that two interacting physical systems may admit entangled pure or non separable mixed states evolving in time as if the mutual interaction hamiltonian were absent. In this paper we define these states Interaction Free Evolving (IFE) states and characterize their existence for a generic binary system described by a time independent Hamiltonian. A comparison between IFE subspace and the decoherence free subspace is reported. The set of all pure IFE states is explicitly constructed for a non homogeneous spin star system model.
    Full-text · Article · Feb 2014 · Physical Review A
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    ABSTRACT: A stationary physical system satisfies peculiar balance conditions involving mean values of appropriate observables. In this paper we show how to deduce such quantitative links, named balance equations, demonstrating as well their usefulness in bringing to light physical properties of the system without solving the Schrodinger equation. The knowledge of such properties in the case of Rabi Hamiltonian is exploit to provide arguments to make easier the variational engineering of the ground state of this model.
    Preview · Article · Feb 2014 · Journal of Russian Laser Research
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    ABSTRACT: We report the experimental observation of resonance peaks in the characteristics of a double SQUID flux qubit, obtained by quickly modifying the potential shape from single well to double well cases at a base temperature of 30mK, for different sweeping rates and potential tiltings. Theoretical analysis and numerical simulations explain the result in term of a quantum phenomenon due to the interplay of Landau-Zener transitions, resonant tunneling between degenerate levels in the two wells, and coherent states evolution. This interesting mechanism has a practical implication for quantum computing purposes, giving a direct measurement of the limits on the sweeping rates possible for a correct manipulation of the qubit state by means of fast flux pulses, avoiding transitions to non-computational states.
    Full-text · Article · Oct 2013
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    ABSTRACT: Employing the stochastic wave function method, we study quantum features of stochastic entropy production in nonequilibrium processes of open systems. It is demonstrated that continuous measurements on the environment introduce an additional, nonthermal contribution to the entropy flux, which is shown to be a direct consequence of quantum fluctuations. These features lead to a quantum definition of single trajectory entropy contributions, which accounts for the difference between classical and quantum trajectories and results in a quantum correction to the standard form of the integral fluctuation theorem.
    No preview · Article · Oct 2013 · Physical Review A
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    ABSTRACT: A microscopic master equation describing the dynamics of two qubits coupled via a nonlinear mediator is constructed supposing that the two qubits, as well as the nonlinear mode, interact, each with its own independent bosonic bath. Generally speaking the master equation derived in this way represents a more appropriate tool for studying the dynamics of open quantum systems. Indeed we show that it is more complex than the phenomenological master equation, constructed simply adding ad hoc dissipative terms.
    No preview · Article · Oct 2013 · Journal of Engineering Mathematics
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    ABSTRACT: Exploiting previous results on Markovian dynamics and fluctuation theorems, we study the consequences of memory effects on single realizations of nonequilibrium processes within an open system approach. The entropy production along single trajectories for forward and backward processes is obtained with the help of a recently proposed classical-like non-Markovian stochastic unravelling, which is demonstrated to lead to a correction of the standard entropic fluctuation theorem. This correction is interpreted as resulting from the interplay between the information extracted from the system through measurements and the flow of information from the environment to the open system: Due to memory effects single realizations of a dynamical process are no longer independent, and their correlations fundamentally affect the behavior of entropy fluctuations.
    Full-text · Article · Mar 2013 · Physical Review E
  • R. Migliore · A. Messina · A. Napoli
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    ABSTRACT: We prove that a mesoscopic Josephson junction, irradiated with a quantum superposition of two 180o{180^o}-out of phase optical coherent states, exhibits an experimentally observable sensitivity to the quantum coherences of the field state. PACS. 73.23.-b Mesoscopic systems–74.50.+r Proximity effects, weak links, tunneling phenomena, and Josephson effects–42.50.Dv Nonclassical field states; squeezed, antibunched, and sub-Poissonian states; operational definitions of the phase of the field; phase measurements
    No preview · Article · Apr 2012 · Physics of Condensed Matter
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    S Spilla · R Migliore · M Scala · A Napoli
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    ABSTRACT: We analyze an entangling protocol to generate tripartite Greenberger-Horne-Zeilinger states in a system consisting of three superconducting qubits with pairwise coupling. The dynamics of the open quantum system is investigated by taking into account the interaction of each qubit with an independent bosonic bath with an ohmic spectral structure. To this end a microscopic master equation is constructed and exactly solved. We find that the protocol here discussed is stable against decoherence and dissipation due to the presence of the external baths.
    Full-text · Article · Mar 2012 · Journal of Physics B Atomic Molecular and Optical Physics
  • B. Leggio · A. Napoli · A. Messina
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    ABSTRACT: We show that a two-atom Bose–Hubbard model exhibits three different phases in the behavior of thermal entanglement in its parameter space. These phases are demonstrated to be traceable back to the existence of level crossings in the ground state of the same system. Significant similarities between the behaviors of thermal entanglement and heat capacity in the parameter space are brought to light thus allowing to interpret the occurrence and the meaning of all these three phases.
    No preview · Article · Jan 2012 · Physics Letters A
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    ABSTRACT: In this paper, we review some results concerning the dissipative dynamics of two coupled qubits interacting with independent reservoirs. In particular, we focus on the role of counter-rotating terms in the qubit-qubit coupling, showing that their presence is the origin of stationary entanglement, which also turns out to be robust with respect to temperature. We also discuss the performances of different non-Markovian approaches in the description of the qubit-qubit dynamics, by considering a simplified exactly solvable Hamiltonian model.
    No preview · Article · Oct 2011 · Optics and Spectroscopy
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    B. Leggio · A. Napoli · A. Messina
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    ABSTRACT: We show that a two-atoms Bose-Hubbard model exhibits three different phases in the behavior of thermal entanglement in its parameter space. These phases are demonstrated to be traceable back to the existence of quantum phase transitions in the same system. Significant similarities between the behaviors of thermal entanglement and heat capacity in the parameter space are brought to light thus allowing to interpret the occurrence and the meaning of all these three phases.
    Preview · Article · Jul 2011
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    B. Leggio · A. Napoli · H. Nakazato · A. Messina
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    ABSTRACT: A simple two-qubit model showing Quantum Phase Transitions as a consequence of ground state level crossings is studied in detail. Using the Concurrence of the system as an entanglement measure and heat capacity as a marker of thermodynamical properties, an analytical expression giving the latter in terms of the former is obtained. A protocol allowing an experimental measure of entanglement is then presented and compared with a related proposal recently reported by Wie\'sniak, Vedral and Brukner
    Preview · Article · Jul 2011 · Journal of Russian Laser Research
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    R Migliore · M Scala · A Napoli · K Yuasa · H Nakazato · A Messina
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    ABSTRACT: The dynamics of an open quantum system, consisting of three superconducting qubits interacting with independent reservoirs, is investigated to elucidate the effects of the environment on a unitary generation scheme of W states (Migliore R et al 2006 Phys. Rev. B 74 104503). To this end a microscopic master equation is constructed and its exact resolution predicts the generation of a Werner-like state instead of the W state. A comparison between our model and a more intuitive phenomenological model is also considered, in order to find the limits of the latter approach in the case of structured reservoirs.
    Full-text · Article · Mar 2011 · Journal of Physics B Atomic Molecular and Optical Physics
  • E Ferraro · M Scala · R Migliore · A Napoli
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    ABSTRACT: In the framework of the dissipative dynamics of coupled qubits interacting with independent reservoirs, a comparison between non-Markovian master equation techniques and an exact solution is presented here. We study various regimes in order to find the limits of validity of the Nakajima–Zwanzig and the time-convolutionless master equations in the description of the entanglement dynamics. A comparison between the performances of the concurrence and the negativity as entanglement measures for the system under study is also presented.
    No preview · Article · Sep 2010 · Physica Scripta
  • E Ferraro · H-P Breuer · A Napoli · A Messina
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    ABSTRACT: We investigate the dynamics of a central spin that is coupled to a bath of spins through a non-uniform distribution of coupling constants. Simple analytical arguments based on master equation techniques as well as numerical simulations of the full von Neumann equation of the total system show that the short-time damping and decoherence behaviour of the central spin can be modelled accurately through an effective Hamiltonian involving a single effective coupling constant. The reduced short-time dynamics of the central spin is thus reproduced by an analytically solvable effective Hamiltonian model.
    No preview · Article · Sep 2010 · Physica Scripta