It has been recently shown that the progressive breakdown (BD) is the dominant BD mode under oper... more It has been recently shown that the progressive breakdown (BD) is the dominant BD mode under operating conditions in CMOS circuits. Progressive BD at operating voltages can be very slow,with respect to the conditions of standard accelerated tests. It may take many years to reach gate leak-age levels large enough to appreciably disturb the circuit operation. The disturb level depends on the post-BD gate oxide conductance. For example, in an SRAM cell, to lose the noise margin for correct operation, BD spot resistances below about 50 kΩ have to be reached. To correctly predict the behavior of a circuit with one or more transistors in BD, it is important to have an accurate model of the conduction through the BD spot during progressive BD. For a correct modeling it is, in turn, necessary to determine the physical structure of the BD spot. We have focused our study on trying to determine this structure during progressive BD. Based on this analysis we propose a quantitative model of post-BD conductance. hi this paper we present the main results of this activity.
High-fidelity quantum computation and quantum state transfer are possible in short spin chains. W... more High-fidelity quantum computation and quantum state transfer are possible in short spin chains. We exploit a system based on a dispersive qubit-boson interaction to mimic XY coupling. In this model, the usually assumed nearest-neighbors coupling is no more valid: all the qubits are mutually coupled. We analyze the performances of our model for quantum state transfer showing how pre-engineered coupling rates allow for nearly optimal state transfer. We address a setup of superconducting qubits coupled to a microstrip cavity in which our analysis may be applied.
We analyze the crossing of a quantum critical point based on exact results for the transverse XY ... more We analyze the crossing of a quantum critical point based on exact results for the transverse XY model. In dependence of the change rate of the driving field, the evolution of the ground state is studied while the transverse magnetic field is tuned through the critical point with a linear ramping. The excitation probability is obtained exactly and is compared to previous studies and to the Landau-Zener formula, a long time solution for non-adiabatic transitions in two-level systems. The exact time dependence of the excitations density in the system allows to identify the adiabatic and diabatic regions during the sweep and to study the mesoscopic fluctuations of the excitations. The effect of white noise is investigated, where the critical point transmutes into a non-hermitian ``degenerate region''. Besides an overall increase of the excitations during and at the end of the sweep, the most destructive effect of the noise is the decay of the state purity that is enhanced by the passage through the degenerate region.
We will show how geometric phases, i.e. phases which appear when a system undergoes a cyclic adia... more We will show how geometric phases, i.e. phases which appear when a system undergoes a cyclic adiabatic evolution, can find useful application in fault tolerant quantum computation. A particular setup based on coupled superconducting nanostructures will be described
In this Letter we discuss the entanglement near a quantum phase transition by analyzing the prope... more In this Letter we discuss the entanglement near a quantum phase transition by analyzing the properties of the concurrence for a class of exactly solvable models in one dimension. We find that entanglement can be classified in the framework of scaling theory. Further, we reveal a profound difference between classical correlations and the non-local quantum correlation, entanglement: the correlation length diverges at the phase transition, whereas entanglement in general remains short ranged.
We discuss the phase diagram for a metal tunnel junction with quasiparticle dissipation. We prese... more We discuss the phase diagram for a metal tunnel junction with quasiparticle dissipation. We present some evidences of aT=0 phase transition induced by dissipation, by means of Monte Carlo simulation and studying the problem by means of a selfconsistent harmonic approximation. When the nominal conductance of the junctioin is large the predictions of the spin wave theory turn out to be correct and the algebraic decay of correlations implies quasi-long range order (phase with infinite susceptibility), this situation corresponds to the absence of a Coulomb blockade voltage threshold. The critical value of the nominal junction resistance is estimated to beR t ≈0.6 kΩ.
By means of the Lanczos method we analyze superconducting correlations in ultrasmall grains at fi... more By means of the Lanczos method we analyze superconducting correlations in ultrasmall grains at fixed particle number. We compute the ground state properties and the excitation gap of the pairing Hamiltonian as a function of the level spacing $\delta$. Both quantities turn out to be parity dependent and universal functions of the ratio $\delta/\Delta$ ($\Delta$ is the BCS gap). We then characterize superconductivity in the canonical ensemble from the scaling behavior of correlation functions in energy space.
We consider the transfer of quantum information down a single-mode quantum transmission line. Suc... more We consider the transfer of quantum information down a single-mode quantum transmission line. Such quantum channel is modeled as a damped harmonic oscillator, the interaction between the information carriers -a train of N qubits- and the oscillator being of the Jaynes-Cummings kind. Memory effects appear if the state of the oscillator is not reset after each channel use. We show that the setup without resetting is convenient in order to increase the transmission rates, both for the transfer of quantum and classical private information. Our results can be applied to the micromaser.
We study decoherence due to low frequency noise in Josephson qubits. Non-Markovian classical nois... more We study decoherence due to low frequency noise in Josephson qubits. Non-Markovian classical noise due to switching impurities determines inhomogeneous broadening of the signal. The theory is extended to include effects of high-frequency quantum noise, due to impurities or to the electromagnetic environment. The interplay of slow noise with intrinsically non-gaussian noise sources may explain the rich physics observed in the spectroscopy and in the dynamics of charge based devices.
When a quantum mechanical system undergoes an adiabatic cyclic evolution it acquires a geometrica... more When a quantum mechanical system undergoes an adiabatic cyclic evolution it acquires a geometrical phase factor in addition to the dynamical one. This effect has been demonstrated in a variety of microscopic systems. Advances in nanotechnologies should enable the laws of quantum dynamics to be tested at the macroscopic level, by providing controllable artificial two-level systems (for example, in quantum dots and superconducting devices). Here we propose an experimental method to detect geometric phases in a superconducting device. The setup is a Josephson junction nanocircuit consisting of a superconducting electron box. We discuss how interferometry based on geometrical phases may be realized, and show how the effect may applied to the design of gates for quantum computation.
Journal of Physics A-mathematical and General, 2001
We study the connection between the BCS pairing model and the inhomogeneous vertex model. The two... more We study the connection between the BCS pairing model and the inhomogeneous vertex model. The two spectral problems coincide in the quasi-classical limit of the off-shell Bethe Ansatz of the disordered six vertex model. The latter problem is transformed into an auxiliary spectral problem which corresponds to the diagonalization of the integrals of motion of the BCS model. A generating functional whose quasi classical expansion leads to the constants of motion of the BCS model and in particular the Hamiltonian, is identified.
We show that the amount of coherent quantum information that can be reliably transmitted down a d... more We show that the amount of coherent quantum information that can be reliably transmitted down a dephasing channel with memory is maximized by separable input states. In particular, we model the channel as a Markov chain or a multimode environment of oscillators. While in the first model the maximization is achieved for the maximally mixed input state, in the latter it is convenient to exploit the presence of a decoherence-protected subspace generated by memory effects. We explicitly compute the quantum channel capacity for the first model while numerical simulations suggest a lower bound for the latter. In both cases memory effects enhance the coherent information. We present results valid for arbitrary size of the input.
It has been recently shown that the progressive breakdown (BD) is the dominant BD mode under oper... more It has been recently shown that the progressive breakdown (BD) is the dominant BD mode under operating conditions in CMOS circuits. Progressive BD at operating voltages can be very slow,with respect to the conditions of standard accelerated tests. It may take many years to reach gate leak-age levels large enough to appreciably disturb the circuit operation. The disturb level depends on the post-BD gate oxide conductance. For example, in an SRAM cell, to lose the noise margin for correct operation, BD spot resistances below about 50 kΩ have to be reached. To correctly predict the behavior of a circuit with one or more transistors in BD, it is important to have an accurate model of the conduction through the BD spot during progressive BD. For a correct modeling it is, in turn, necessary to determine the physical structure of the BD spot. We have focused our study on trying to determine this structure during progressive BD. Based on this analysis we propose a quantitative model of post-BD conductance. hi this paper we present the main results of this activity.
High-fidelity quantum computation and quantum state transfer are possible in short spin chains. W... more High-fidelity quantum computation and quantum state transfer are possible in short spin chains. We exploit a system based on a dispersive qubit-boson interaction to mimic XY coupling. In this model, the usually assumed nearest-neighbors coupling is no more valid: all the qubits are mutually coupled. We analyze the performances of our model for quantum state transfer showing how pre-engineered coupling rates allow for nearly optimal state transfer. We address a setup of superconducting qubits coupled to a microstrip cavity in which our analysis may be applied.
We analyze the crossing of a quantum critical point based on exact results for the transverse XY ... more We analyze the crossing of a quantum critical point based on exact results for the transverse XY model. In dependence of the change rate of the driving field, the evolution of the ground state is studied while the transverse magnetic field is tuned through the critical point with a linear ramping. The excitation probability is obtained exactly and is compared to previous studies and to the Landau-Zener formula, a long time solution for non-adiabatic transitions in two-level systems. The exact time dependence of the excitations density in the system allows to identify the adiabatic and diabatic regions during the sweep and to study the mesoscopic fluctuations of the excitations. The effect of white noise is investigated, where the critical point transmutes into a non-hermitian ``degenerate region''. Besides an overall increase of the excitations during and at the end of the sweep, the most destructive effect of the noise is the decay of the state purity that is enhanced by the passage through the degenerate region.
We will show how geometric phases, i.e. phases which appear when a system undergoes a cyclic adia... more We will show how geometric phases, i.e. phases which appear when a system undergoes a cyclic adiabatic evolution, can find useful application in fault tolerant quantum computation. A particular setup based on coupled superconducting nanostructures will be described
In this Letter we discuss the entanglement near a quantum phase transition by analyzing the prope... more In this Letter we discuss the entanglement near a quantum phase transition by analyzing the properties of the concurrence for a class of exactly solvable models in one dimension. We find that entanglement can be classified in the framework of scaling theory. Further, we reveal a profound difference between classical correlations and the non-local quantum correlation, entanglement: the correlation length diverges at the phase transition, whereas entanglement in general remains short ranged.
We discuss the phase diagram for a metal tunnel junction with quasiparticle dissipation. We prese... more We discuss the phase diagram for a metal tunnel junction with quasiparticle dissipation. We present some evidences of aT=0 phase transition induced by dissipation, by means of Monte Carlo simulation and studying the problem by means of a selfconsistent harmonic approximation. When the nominal conductance of the junctioin is large the predictions of the spin wave theory turn out to be correct and the algebraic decay of correlations implies quasi-long range order (phase with infinite susceptibility), this situation corresponds to the absence of a Coulomb blockade voltage threshold. The critical value of the nominal junction resistance is estimated to beR t ≈0.6 kΩ.
By means of the Lanczos method we analyze superconducting correlations in ultrasmall grains at fi... more By means of the Lanczos method we analyze superconducting correlations in ultrasmall grains at fixed particle number. We compute the ground state properties and the excitation gap of the pairing Hamiltonian as a function of the level spacing $\delta$. Both quantities turn out to be parity dependent and universal functions of the ratio $\delta/\Delta$ ($\Delta$ is the BCS gap). We then characterize superconductivity in the canonical ensemble from the scaling behavior of correlation functions in energy space.
We consider the transfer of quantum information down a single-mode quantum transmission line. Suc... more We consider the transfer of quantum information down a single-mode quantum transmission line. Such quantum channel is modeled as a damped harmonic oscillator, the interaction between the information carriers -a train of N qubits- and the oscillator being of the Jaynes-Cummings kind. Memory effects appear if the state of the oscillator is not reset after each channel use. We show that the setup without resetting is convenient in order to increase the transmission rates, both for the transfer of quantum and classical private information. Our results can be applied to the micromaser.
We study decoherence due to low frequency noise in Josephson qubits. Non-Markovian classical nois... more We study decoherence due to low frequency noise in Josephson qubits. Non-Markovian classical noise due to switching impurities determines inhomogeneous broadening of the signal. The theory is extended to include effects of high-frequency quantum noise, due to impurities or to the electromagnetic environment. The interplay of slow noise with intrinsically non-gaussian noise sources may explain the rich physics observed in the spectroscopy and in the dynamics of charge based devices.
When a quantum mechanical system undergoes an adiabatic cyclic evolution it acquires a geometrica... more When a quantum mechanical system undergoes an adiabatic cyclic evolution it acquires a geometrical phase factor in addition to the dynamical one. This effect has been demonstrated in a variety of microscopic systems. Advances in nanotechnologies should enable the laws of quantum dynamics to be tested at the macroscopic level, by providing controllable artificial two-level systems (for example, in quantum dots and superconducting devices). Here we propose an experimental method to detect geometric phases in a superconducting device. The setup is a Josephson junction nanocircuit consisting of a superconducting electron box. We discuss how interferometry based on geometrical phases may be realized, and show how the effect may applied to the design of gates for quantum computation.
Journal of Physics A-mathematical and General, 2001
We study the connection between the BCS pairing model and the inhomogeneous vertex model. The two... more We study the connection between the BCS pairing model and the inhomogeneous vertex model. The two spectral problems coincide in the quasi-classical limit of the off-shell Bethe Ansatz of the disordered six vertex model. The latter problem is transformed into an auxiliary spectral problem which corresponds to the diagonalization of the integrals of motion of the BCS model. A generating functional whose quasi classical expansion leads to the constants of motion of the BCS model and in particular the Hamiltonian, is identified.
We show that the amount of coherent quantum information that can be reliably transmitted down a d... more We show that the amount of coherent quantum information that can be reliably transmitted down a dephasing channel with memory is maximized by separable input states. In particular, we model the channel as a Markov chain or a multimode environment of oscillators. While in the first model the maximization is achieved for the maximally mixed input state, in the latter it is convenient to exploit the presence of a decoherence-protected subspace generated by memory effects. We explicitly compute the quantum channel capacity for the first model while numerical simulations suggest a lower bound for the latter. In both cases memory effects enhance the coherent information. We present results valid for arbitrary size of the input.
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Papers by Giuseppe Falci