A. Mastellone

Initial decoherence in solid state qubits.

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.

SMALL SUPERCONDUCTING GRAIN IN THE CANONICAL ENSEMBLE

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

Decoherence times of universal two-qubit gates in the presence of broad-band noise

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Optimal tuning of solid-state quantum gates: A universal two-qubit gate

. Both quantities turn out to be parity dependent and universal functions of the ratio

Thermodynamic and spectral properties of ultrasmall superconducting grains

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Broadband noise decoherence in solid-state complex architectures

(

Universal features in ensembles of small superconducting grains

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Thermodynamics in disordered metallic dots

is the BCS gap). We then characterize superconductivity in the canonical ensemble from the scaling behavior of correlation functions in energy space.

Superconducting dot in a magnetic field

The controlled generation of entangled states of two quantum bits is a fundamental step toward the implementation of a quantum information processor. In nano-devices this operation is counteracted by the solid-state environment, characterized by a broadband and non-monotonic power spectrum, often 1/f, at low frequencies. For single-qubit gates, incoherent processes due to fluctuations acting on different time scales result in peculiar short- and long-time behavior. Markovian noise gives rise to exponential decay with relaxation and decoherence times, T1 and T2, simply related to the symmetry of the qubit?environment coupling Hamiltonian. Noise with the 1/f power spectrum at low frequencies is instead responsible for defocusing processes and algebraic short-time behavior. In this paper, we identify the relevant decoherence times of an entangling operation due to the different decoherence channels originating from solid-state noise. Entanglement is quantified by concurrence, which we evaluate in an analytic form employing a multi-stage approach. The ?optimal? operating conditions of reduced sensitivity to noise sources are identified. We apply this analysis to a superconducting gate for experimental noise spectra.

Effects of low-frequency noise cross-correlations in coupled superconducting qubits

We present a general route to reduce inhomogeneous broadening in nanodevices due to