Fluctuations in dc SQUIDs: Quantum Noise Effects, Low Frequency Noise, and Single Electron Trapping

We have studied the signal and noise properties of extremely sensitive dc SQUIDs composed of submicron area tunnel junctions. At high frequencies, the flux resolution is expected to be limited by intrinsic quantum fluctuations, making these devices prime candidates for ideal quantum-limited detectors. The observed signal properties are excellent and a minimum for energy resolution of 1.6$\hbar$ was measured at 1.5K, the lowest value reported to date. At low frequencies, we observe broad, Lorentzian features superimposed on a background which is always much flatter than 1/f ($\sim {\rm f}\sp{-2/3}$). When these features are strongest, the real-time voltage noise displays discrete switching behavior which results from the trapping and untrapping of single electrons into localized defect states residing within the tunneling barrier. The weak temperature of the trap lifetimes reveals that the trapping process displays tunneling kinetics. In contrast, the voltage bias dependence of the lifetimes is consistent with a simple nonequilibrium model in which the bias strongly enhances the rate for electrons to tunnel in from one side of the barrier and exit out the other side. Some junctions show clear evidence of interactions between traps, and for certain bias conditions, the noise displays predominantly series kinetics. These observations show that the low frequency noise in this system cannot always be described by a simple parallel kinetics model composed of independent fluctuators.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2015

Author dc:creator

• Wakai, Ronald Tatsuya

• Physics, Condensed Matter

Language dc:language

eng

Identifier

(UMI)AAI8721778

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/77409