Article

# Improved critical-current-density uniformity by using anodization

Dept. of Electr. Eng., Massachusetts Inst. of Technol., Cambridge, MA, USA

IEEE Transactions on Applied Superconductivity (Impact Factor: 1.2). 07/2003; DOI: 10.1109/TASC.2003.813658 Source: IEEE Xplore

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**ABSTRACT:**Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006. This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. Includes bibliographical references (p. 211-218). Superconducting Josephson junction devices rank among the best candidates for realizing a quantum computer. While the coherent control of quantum dynamics has been demonstrated in these solid-state, macroscopic quantum systems, a major challenge has been to increase the coherence times for these qubits. With an objective to reduce the level of readout-induced decoherence, this thesis work focuses on a resonant readout scheme developed for a niobium persistent-current (PC) qubit. This non-dissipative readout approach detects the flux state of the qubit by sensing a change in the Josephson inductance of a SQUID magnetometer. By incorporating the SQUID inductor in a high-Q resonant circuit, we distinguished the flux states of the qubit as a shift in the resonant frequency at 300 mK. The nonlinearity due to the Josephson inductance has characteristic effects on the resonant behavior of the readout circuit. We observed novel manifestation of this nonlinearity given the high quality factor of the resonance. The readout circuit was characterized in the linear as well as the nonlinear regime for its potential use as a bifurcation amplifier. Numerical simulations based on Josephson-junction circuits were also performed to understand the observed nonlinearity in the resonant behavior. by Janice C. Lee. Ph.D. - [Show abstract] [Hide abstract]

**ABSTRACT:**We have designed, fabricated and operated a scalable system for applying independently programmable time-independent, and limited time-dependent flux biases to control superconducting devices in an integrated circuit. Here we report on the operation of a system designed to supply 64 flux biases to devices in a circuit designed to be a unit cell for a superconducting adiabatic quantum optimization system. The system requires six digital address lines, two power lines, and a handful of global analog lines. Comment: 14 pages, 15 figuresSuperconductor Science and Technology 04/2010; 23:065004. · 2.76 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2003. Includes bibliographical references (p. 203-208). The persistent current qubit is a superconducting ring interrupted by three Josephson junctions. Its two quantum states have circulating currents in opposite directions which can be measured by a dc SQUID magnetometer. This work examines a persistent current qubit fabricated in niobium, using Lincoln Laboratory's DPARTS process. Measurements of the niobium qubit show a promisingly high subgap resistance, demonstrate discrete energy levels, and give good estimates of the qubit parameters as fabricated. Although the variances on the qubit parameters are large, it is possible to design a qubit whose parameters are in the quantum regime. Additionally, we show how the qubit can be integrated with on-chip electronics while taking into account decoherence. A dc SQUID oscillator has been designed which can deliver an oscillating field at 10 GHz without reducing the decoherence time below 1 microsecond. The oscillator is shown to deliver power to the measurement device, although the qubit rotation has not yet been observed. by Donald S. Crankshaw. Ph.D.

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