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IEEE Transactions on Magnetics

LC resonances in Nb baselayer dc SQUIDs

Gianfranco Paternò, Anna Maria Cucolo, G. Modestino

Published Mar 1, 1985• 4 citations
Publisher

Abstract

LC resonances in planar interferometers with two Nb-NbOx-Pb Josephson junctions have been investigated. The junction areas are defined by two "windows" in an SiO insulating layer. Both symmetric and asymmetric configurations with equal and different junctions size respectively have been considered. The magnetic field and the voltage dependence of the resonance amplitudes have been compared with the theory reported in the literature.

DOI
10.1109/tmag.1985.1063846

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LC resonances in planar interferometers with two Nb-NbOx-Pb Josephson junctions have been investigated. The junction areas are defined by two "windows" in an SiO insulating layer. Both symmetric and asymmetric configurations with equal and different junctions size respectively have been considered. The magnetic field and the voltage dependence of the resonance amplitudes have been compared with the theory reported in the literature.
Resonances in the current-voltage characteristics of interferometers with two Nb-NbOx-Pb window junctions have been experimentally investigated. The data are compared with the theoretical curves computed both for the voltage and for the magnetic field dependence of the resonance amplitudes. Very good agreement is found in all the range of values of the damping parameter Γ=I0 R/Vr, where Vr is the resonant voltage and I0 and R are the maximum Josephson current and the parallel resistance of each junction. New features that cannot be explained in terms of the existing theory have been observed in devices with larger values of the parameter β=2πLI0/Φ0, where L is the loop inductance, and Φ0 is the flux quantum.
Planar interferometers with two Nb-NbOx-Pbln functions have been fabricated. Because of the peculiar configuration adopted, both a symmetric and an asymmetric current bias are possible on the same sample. The loop area is on a plane perpendicular to the cross section of the functions, and, therefore, the flux coupled to them is negligible. ThreshoM curves have been measured for different values of the characteristic parameter fl = LI1/qbo, where 1i and L are the maximum Josephson current and the inductance of the interferometer and C~o is the flux quantum. The dependence of the current modulation depth upon fl has been also investigated. A comparison of the experimental data with the theory shows quantitative agreement.
The LC\mathrm{LC}LC (inductance-capacitance) resonance phenomenon in the dc SQUID (superconducting quantum interference device) creates a current peak in the $I\ensuremath{-}V$ characteristics at a voltage determined by the resonance frequency. These $I\ensuremath{-}V$ characteristics have been analyzed with the resistively shunted Josephson-junction model in the low-damping and small-oscillation regime. The resonance $I\ensuremath{-}V$ characteristics contain sufficient information to determine all circuit parameters for the dc SQUID's. Specifically, we can apply this analysis to symmetric, tunnel-junction dc SQUID's to deduce the pair-quasiparticle interference conductance ($cos\ensuremath{\theta}$ term).
This thesis examines the behavior of dc SQUID phase qubits in terms of their proposed use in a quantum computer. In a phase qubit, the two lowest energy states (n=0 and n=1) of a current-biased Josephson junction form the qubit states, with the gauge invariant phase difference across the junction being relatively well defined. In a dc SQUID phase qubit, the Josephson junction is isolated from the environment using an inductive isolation network and Josephson junction, which are connected across the phase qubit junction to form a dc SQUID. Five dc SQUID phase qubits were examined at temperatures down to 25 mK. Three of the devices had qubit junctions that were Nb/AlOx/Nb junctions with critical currents of roughly 30 microamps. The other two had Al/AlOx/Al junctions with critical currents of roughly 1.3 microamps. The device that had the best performance was an Al/AlOx/Al device with a relaxation time of 30 ns and a coherence time of 24 ns. The devices were characterized using microwave spectroscopy, Rabi oscillations, relaxation and Ramsey fringe measurements. I was also able to see coupling between two Nb/AlOx/Nb dc SQUID phase qubits and perform Rabi oscillations with them. The Nb/AlOx/Nb devices had a relaxation time and coherence time that were half that of the Al/AlOx/Al device. One of the goals of this work was to understand the nature of parasitic quantum systems (TLSs) that interact with the qubit. Coupling between a TLS and a qubit causes an avoided level crossing in the transition spectrum of the qubit. In the Al/AlOx/Al devices unintentional avoided level crossings were visible with sizes up to 240 MHz, although most visible splittings were of order ~20 MHz. The measured spectra were compared to a model of the avoided level crossing based on the TLSs coupling to the junction, through either the critical current or the voltage across the junction.