arXiv subjects

V. Corato

Publications and source records attributed to V. Corato.

6 recordsLinked to original sources

arXiv2008

Direct measurement of inter-filament resistance in Nb3Sn multi-filamentary strands

In modeling the properties of superconducting multi-filamentary strands, the filament twist pitch and the inter-filament transverse resistivity play a crucial role in the definition of the current transfer length. This affects the AC losses of NbTi strands, or the transport properties of Nb3Sn wires subject to a bending strain. Although the effect of filament twist pitch on the critical current of the strand has been studied, the value of the inter-filament resistance in Nb3Sn strands is still undefined. We have performed a direct measurement of the inter-filament transverse resistance within the cross-section of a Nb3Sn multi-filamentary strand from room temperature to 4.2K. Results have been compared to the inter-filament resistance of a sample on which the outer copper stabilization layer was removed by chemical etching, obtaining interesting indications on the preferential current paths within the wire cross section.

cond-mat.supr-con↗

arXiv2007

Simulation of some quantum gates, with decoherence

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 Schroedinger 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 one, 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.

cond-mat.supr-con↗

arXiv2006

Resonance phenomena in Macroscopic Quantum Tunneling: the small viscosity limit

We present a new theoretical approach to describe the quantum behavior of a macroscopic system interacting with an external irradiation field, close to the resonant condition. Here we consider the extremely underdamped regime for a system described by a double well potential. The theory includes both: transitions from one well to the other and relaxation processes. We simulate resonant phenomena in a rf-SQUID, whose parameters lie in the range typically used in the experiments. The dependence of the transition probability W on the external drive of the system $ϕ_x$ shows three resonance peaks. One peak is connected with the resonant tunneling and the two others with the resonant pumping. The relative position of the two peaks correlated to the resonant pumping depends on the pumping frequency $ν$ and on the system parameters. The preliminary measurements on our devices show a low dissipation level and assure that they are good candidates in order to realize new experiments on the resonant phenomena in the presence of an external microwave irradiation of proper frequency.

quant-ph↗

arXiv2006

Topology-Induced Critical Current Enhancement in Josephson Networks

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.

cond-mat.supr-con↗

arXiv2003

Adiabatic evolution of a coupled-qubit Hamiltonian

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.

cond-mat↗

arXiv2002

Design of Adiabatic Logic for a Quantum CNOT Gate

We examine the realization of a quantum CNOT gate by adiabatic operations.The principles of such systems and their analysis are briefly discussed and a model consisting of two weakly coupled double- potential well qubits is studied numerically. Regions of the parameter space with suitable well-defined sets of wavefunctions are found, in which then an adiabatic sweep of an external bias produces the switching behavior of CNOT. Results are presented on the adiabatic condition and the identification with the parameters of a flux-coupled two-SQUID system is given.For typical parameters adiabatic times in the nanosecond regime are obtained.

cond-mat↗