P. Silvestrini

Second University of Naples, Napoli, Campania, Italy

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Publications (85)139.59 Total impact

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    ABSTRACT: We report experiments on energy level quantization in Nb/AlOx/Nb Josephson junctions while measurements on SQUIDs are in progress. Data on the presence of energy level quantization are shown at temperatures above the classical-quantum regime crossover temperature. This has been possible by extending the measurements of the rate of the escape from the zero-voltage state at higher sweep bias rate in order to induce non-stationary conditions in the energy potential describing the junction dynamics. Data are compatible with extremely low "effective" dissipation in our system. Theoretical predictions on quantum effects related to the presence of non-stationary conditions in the potential associated to a rf SQUID are reported in view of new possible experiments.
    International Journal of Modern Physics B 01/2012; 13(09n10). · 0.46 Impact Factor
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    ABSTRACT: We present our recent experiments on macroscopic tunneling in the quantum regime for underdamped Josephson junctions with different classical-quantum crossover temperatures. These experiments are performed towards the observation of macroscopic quantum coherence in Josephson systems. A new method to measure coherence in a two-level system based on the adiabatic inversion in an rf squid, is presented. This approach could open new perspectives in view of realization of an elementary quantum bit.
    International Journal of Modern Physics B 01/2012; 14(25n27). · 0.46 Impact Factor
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    ABSTRACT: We report on the design, the fabrication and the performance of an integrated magnetic nano-sensor based on niobium dc-SQUID (Superconducting QUantum Interference Device) for nanoscale applications is presented. The nano-sensors are based on nanometric niobium constrictions (Dayem bridges) inserted in a square loop having a side length of 200 nm. Measurements of voltage-flux characteristic, flux to voltage transfer factor and noise performances are reported. In small signal mode, the sensors have shown a magnetic flux noise spectral density of 1.5 μΦ0/Hz1/2 corresponding to a spin sensitivity in unit of Bohr magneton of 60 spin/Hz1/2. Supercurrent decay measurements of these devices are also reported. Such measurements provide useful information for applications which employ the SQUID as a trigger where the sensor works on the zero voltage state. The experimental data, have shown an intrinsic current fluctuation less than 0.2% of the critical current at liquid helium temperature, corresponding to an intrinsic sensor magnetic flux resolution of a few mΦ0. In view of the nano-SQUID employments in the detection of small spin populations, the authors calculated the spin sensitivity and the magnetic response relative to the single spin, as a function of its position within the SQUID hole. The results show that the SQUID response depends strongly on the spin position.
    Journal of Physics Conference Series 07/2010; 234(4):042010.
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    ABSTRACT: We present a theoretical study of the resonant quantum behavior for a macroscopic superconducting system interacting with an external microwave at proper frequency. Here we consider a system described by a double well potential, an rf-SQUID, in the extremely underdamped regime. Numerical simulations for resonant phenomena have been performed for this system, whose parameters belong to the range typically used in the experiments. The dependence of the transition probability W on the external drive of the system, φx, can show three resonance peaks, in a small microwave frequency range. One peak is connected with the anticrossing and the other two with the external microwave frequency ν. The relative position and the height of the two lateral peaks depend on the microwave frequency. This behavior is studied here for the first time.
    Physics Letters A 02/2008; · 1.77 Impact Factor
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    ABSTRACT: We present the design and the performances of a vertical Josephson interferometer, which can be inserted within superconducting loops in order to obtain controllable Josephson devices. The most interesting feature of the vertical interferometer is that the Josephson current is precisely controlled by a small transversal magnetic field parallel to the superconducting loop plane and can be modulated from a maximum to zero with high precision. This system can be used to realize a switchable flux transformer to couple quantum bits when desired as well as to vary the critical current of a flux qubit in situ. In the latter application, the vertical interferometer is controlled by a magnetic field orthogonal to the one applied to tilt the qubit potential, thus minimizing the cross talking of the two magnetic fields.
    IEEE Transactions on Applied Superconductivity 07/2007; · 1.20 Impact Factor
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    ABSTRACT: We study the resonant quantum behaviour of a macroscopic superconducting system interacting with an external field. First we consider the resonant tunneling assisted by microwave frequency in the moderate underdamped quantum regime. Then we consider the resonant process in the extreme underdamped limit. In both regimes we have investigated a small frequency range, where resonant phenomena take place. The transition probability W from the left to the right potential well has been numerically calculated for an rf-SQUID as function of the external flux φ x for both regimes. Results indicate that, in a proper frequency and φ x range, the transition probability W can present two or three resonant peaks, depending on the damping regime.
    Open Systems & Information Dynamics 05/2007; 14(2):209-216. · 0.90 Impact Factor
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    ABSTRACT: Methods and results for numerical simulations of one and two interacting rf-SQUID systems suitable for adiabatic quantum gates are presented. These are based on high accuracy numerical solutions to the static and time dependent Schrödinger equation for the full SQUID Hamiltonian in one and two variables. Among the points examined in the static analysis is the range of validity of the effective two-state or ``spin- (1)/(2) '' picture. A range of parameters is determined where the picture holds to good accuracy as the energy levels undergo gate manipulations. Some general points are presented concerning the relations between device parameters and ``good'' quantum mechanical state spaces. The time dependent simulations allow the examination of suitable conditions for adiabatic behavior, and permits the introduction of a random noise to simulate the effects of decoherence. A formula is derived and tested relating the random noise to the decoherence rate. Sensitivity to device and operating parameters for the logical gates NOT and CNOT are examined, with particular attention to values of the tunnel parameter beta slightly above one. It appears that with values of beta close to 1, a quantum CNOT gate is possible even with rather short decoherence times. Many of the methods and results will apply to coupled double-potential well systems in general.
    Physical review. B, Condensed matter 01/2007; 75(18). · 3.77 Impact Factor
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    ABSTRACT: We show that some of the Josephson couplings of junctions arranged to form an inhomogeneous network undergo a non-perturbative renormalization provided that the network's connectivity is pertinently chosen. As a result, the zero-voltage Josephson critical currents $I_c$ turn out to be enhanced along directions selected by the network's topology. This renormalization effect is possible only on graphs whose adjacency matrix admits an hidden spectrum (i.e. a set of localized states disappearing in the thermodynamic limit). We provide a theoretical and experimental study of this effect by comparing the superconducting behavior of a comb-shaped Josephson junction network and a linear chain made with the same junctions: we show that the Josephson critical currents of the junctions located on the comb's backbone are bigger than the ones of the junctions located on the chain. Our theoretical analysis, based on a discrete version of the Bogoliubov-de Gennes equation, leads to results which are in good quantitative agreement with experimental results.
    New Journal of Physics 10/2006; · 4.06 Impact Factor
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    ABSTRACT: In this paper we present a theoretical approach to describe the quantum behaviour of a macroscopic system interacting with an external field at frequencies close to resonant condition. Moreover we apply our results to simulate resonant phenomena in rf SQUIDs, whose parameters lie in the range typically used in the experiments.
    Journal of Physics Conference Series 07/2006; 43(1):1195.
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    ABSTRACT: We study the Hamiltonian of a system of inductively coupled flux qubits, which has been theoretically proposed for adiabatic quantum computation to handle NP problems. We study the evolution of a basic structure consisting of three coupled rf-SQUIDs upon tuning the external flux bias, and we show that the adiabatic nature of the evolution is guaranteed by the presence of the single-SQUID gap. We further propose a scheme and the first realization of an experimental device suitable for verifying the theoretical results.
    Journal of Physics Conference Series 07/2006; 43(1):1401.
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    ABSTRACT: We investigate the properties of Josephson junction networks with inhomogeneous architecture. The networks are shaped as "quare comb" planar lattices on which Josephson junctions link superconducting islands arranged in the plane to generate the pertinent topology. Compared to the behavior of reference linear arrays, the temperature dependencies of the Josephson currents of the branches of the network exhibit relevant differences. The observed phenomena evidence new and surprising behavior of superconducting Josephson arrays as well as remarkable similarities with bosonic junction arrays.
    Physics Letters A 01/2006; · 1.77 Impact Factor
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    ABSTRACT: We present a fully integrated rf SQUID-based system as flux qubit with a high control of the flux transfer function of the superconducting transformer modulating the coupling between the flux qubit and the readout system. The control of the system is possible by including into the superconducting flux transformer a vertical two-Josephson-junctions interferometer (VJI) in which the Josephson current is precisely modulated from a maximum to zero by a transversal magnetic field parallel to the flux transformer plane. The proposed system can be also used in a more general configuration to control the off-diagonal terms in the Hamiltonian of the flux qubit and to turn on and off the coupling between two or more qubits.
    Physics Letters A 01/2006; · 1.77 Impact Factor
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    ABSTRACT: We present a niobium-based Josephson device as prototype for quantum computation with flux qubits. The most interesting feature of this device is the use of a Josephson vertical interferometer to tune the flux qubit allowing the control of the off-diagonal Hamiltonian terms of the system. In the vertical interferometer, the Josephson current is precisely modulated from a maximum to zero with fine control by a small transversal magnetic field parallel to the rf superconducting loop plane.
    7TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS'05); 01/2006
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    ABSTRACT: We present niobium-based Josephson devices as prototypes for applications to quantum computation with flux qubits. The most interesting feature of this device is the use of a Josephson vertical interferometer for a fine tuning of flux qubit and interquibit coupling. These devices allow a control of the off-diagonal Hamiltonian terms of flux qubit and of the flux transfer function of a superconducting transformer. In the vertical interferometer, the Josephson current is precisely modulated from a maximum to zero with fine control and precision by a small transversal magnetic field parallel to the radio-frequency superconducting loop plane and to the flux transformer.
    Applied Physics Letters 11/2005; · 3.79 Impact Factor
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    ABSTRACT: We characterize a niobium-based vertical Josephson interferometer which we propose to include in a superconducting loop for applications to quantum computation using flux qubits. The most interesting feature of this device is that the Josephson current is precisely modulated by a small transversal magnetic field parallel to superconducting loop plane from a maximum to zero, with fine control and precision. This device can be used to independently control the off-diagonal Hamiltonian terms of flux qubits and/or to control the flux transfer function of a superconducting transformer for inter-qubits coupling.
    Physics Letters A 02/2005; 336(1):71-75. · 1.77 Impact Factor
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    ABSTRACT: We consider the theoretical problem of resonant phenomena in the macroscopic quantum tunneling for an rf superconducting quantum interference device in the presence of an external irradiating field. The transition probability between different flux states is studied by varying the external parameters of the double-well potential describing the system in a way that the pumping level in the left potential well is close to one of the levels in the right potential well. The dependence of the transition probability on the external drive of the system shows two resonance peaks, the former connected with the resonant tunneling and the latter with the resonant pumping. The relative position of the peaks depends on the pumping frequency ω and on the system parameters.
    Physical Review B 01/2005; 71(2). · 3.66 Impact Factor
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    ABSTRACT: We study the Hamiltonian of a system of inductively coupled flux qubits, which has been theoretically proposed for adiabatic quantum computation to handle NP problems. We study the evolution of a basic structure consisting of three coupled rf-SQUIDs upon tuning the external flux bias, and we show that the adiabatic nature of the evolution is guaranteed by the presence of the single-SQUID gap. We further propose an experiment to study the problem of scaling up the number of coupled qubits guaranteeing the adiabatic nature of the process.
    Physics Letters A 01/2005; 345(1):224-230. · 1.77 Impact Factor
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    ABSTRACT: We present an experimental characterization of Nb/AlOx/Nb Josephson devices in the quantum regime. Josephson junctions and fully integrated Josephson devices consisting of an rf SQUID coupled to a read-out system based on a dc SQUID have been measured. Data on the decay rates from the metastable states show evidence of macroscopic quantum tunneling with a very low dissipation level and a high isolation of the devices from the external environment.
    Physical Review B 11/2004; 70(17). · 3.66 Impact Factor
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    ABSTRACT: We present the experimental observation of the effects of macroscopic quantum tunnelling in a SQUID device, consisting of a rf SQUID coupled to a readout system based on a dc SQUID sensor. Data on the decay rate from the metastable flux states of a rf SQUID are reported, both in the classical and quantum regimes. The low dissipation level and the good insulation of the probe from external noise are encouraging in view of building a macroscopic quantum coherent system.
    Superconductor Science and Technology 05/2004; 17(5, SI):S385-S388. · 2.76 Impact Factor
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    V. Corato, P. Silvestrini, L. Stodolsky, J. Wosiek
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    ABSTRACT: We present a general method for studying coupled qubits driven by adiabatically changing external parameters. Extended calculations are provided for a two-bit Hamiltonian whose eigenstates can be used as logical states for a quantum CNOT gate. From a numerical analysis of the stationary Schroedinger equation we find a set of parameters suitable for representing CNOT, while from a time-dependent study the conditions for adiabatic evolution are determined. Specializing to a concrete physical system involving SQUIDs, we determine reasonable parameters for experimental purposes. The dissipation for SQUIDs is discussed by fitting experimental data. The low dissipation obtained supports the idea that adiabatic operations could be performed on a time scale shorter than the decoherence time. Comment: 10 pages, 4 figures, to be pub.in Phys Rev B
    Physical Review B 10/2003; · 3.66 Impact Factor

Publication Stats

439 Citations
139.59 Total Impact Points

Institutions

  • 2002–2010
    • Second University of Naples
      • Dipartimento di Ingegneria Industriale e dell'Informazione
      Napoli, Campania, Italy
    • Sapienza University of Rome
      • Department of Physics
      Roma, Latium, Italy
  • 2008
    • ENEA
      Roma, Latium, Italy
  • 2006
    • Università degli Studi di Perugia
      Perugia, Umbria, Italy
  • 1985–2006
    • National Research Council
      • • Institute of Cybernetics “Eduardo Caianiello”
      • • Institute for Photonics and Nanotechnologies IFN
      Roma, Latium, Italy
    • University of Naples Federico II
      • Department of Physical Sciences
      Napoli, Campania, Italy
  • 2001
    • Max Planck Institute of Physics
      München, Bavaria, Germany
  • 1997–1999
    • INFN - Istituto Nazionale di Fisica Nucleare
      Frascati, Latium, Italy