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Office: 0367 Physics (301) 405-7649 well@squid.umd.edu

Prof. Wellstood's research involves investigations of the physics and applications of superconducting devices in two main areas:

I. Scanning SQUID microscopy

Scanning SQUID microscopy involves scanning a sample closely under a SQUID while a computer records the output of the SQUID. Since SQUIDs are extremely sensitive to magnetic field, the resulting data can be converted into a image of the magnetic field coming from the sample. Images can be taken of static and audio frequency magnetic fields, as well as rf and microwave fields to a certain extent. SQUID microscopes tend to have a very wide bandwidth and extremely high flux and field resolution, but only moderate spatial resolution. As with other types of microscopes, a SQUID microscope can be used to examine a wide variety of samples. In collaboration with others, our group has used SQUID microscopes to locate short-circuit faults in semiconductor microchips and multi-chip modules, study the pairing symmetry of the high-T c superconductors, search for new magnetic materials, and for a variety of other applications. This area is headed by Dr. John Matthews and we have maintained a long-term collaboration with Neocera, Inc ., involving the commercialization of this unique instrument for use in semiconductor failure analysis. Our recent research in this area includes understanding the ultimate limits of this technique, development of a novel Fast Fourier Transform (FFT) based filter for minimizing the effects of a finite image size in SQUID microscopy, investigating the role of position noise limiting our existing SQUID microscopes, and initial proof of principle demonstrating the capability of doing nano-second time-resolved SQUID imaging. The microscopy section has also been developing a SQUID-based NDE system for testing km long sections of wire for Medical resonance imaging (MRI) systems.

 

 

 

II. Quantum Computation using superconducting devices .

This project involves a collaboration with and Profs. J. R. Anderson , A. J. Dragt , and C. J. Lobb , postdoctoral research associate Dr. Rupert Lewis, and Professor R. Ramos at Drexel University . Our group was the first to propose using an individual current biased Josephson junction as a qubit and we and other groups have now made significant progress towards its realization. Our group's most significant recent accomplishments include:

A. Reported detailed calculations of the spectra of two-coupled qubits (prior to the experimental work) and also proposed schemes for building quantum gates in this system that can have a high-fidelity and low-leakage. The spectroscopic calculations were reported in Phys Rev. Letters.

B. Spectroscopic measurements of the quantum energy levels in two coupled Josephson junction qubits . We see an avoided energy level crossing when the energy levels of the two qubits are brought into coincidence. We interpret the results as providing evidence for the existence of quantum states of the coupled system that theory tells us are entangled. We also reported spectroscopic coherence times and evidence for escape rate broadening of the transitions. Although the measured coherence times are short, just a few ns due to low-frequency current noise, the times are not shorter than in the uncoupled devices. Another remarkable aspect of this result is that the qubits were separated by a distance of almost 1 mm. This work was published in Science.

C. Reported spectroscopic measurements of the energy levels in two junction qubits that are coupled to an LC (inductor-capacitor) resonator . In this three-body system, we see a triple avoided crossing which theory indicates involves a superposition of states of the three different qubits. Detailed calculations of the full three-body quantum system reveal good agreement with the data. The spectra also show evidence for resonant coupling between the two junctions, mediated by the LC resonator. A preprint of this work is available on CondMat .

D. Developed a robust technique for measuring the relaxation time T 1 in a junction qubit. This work was published in PRB Rapid Communications .

E. Analyzed sources and effects of de-coherence in a current-biased josephson junction qubit . This work was published in Phys. Rev. B .

F. Reported measurements of critical current flicker (1/f) noise from 4.2 K down to 0.09 K . 1/f critical current noise is a potential source of decoherence in superconducting qubits and its behavior at the low temperatures of interest is still poorly understood. This work was published in Applied Physics Letters .

 

 

 

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Center for Superconductivity Research, University of Maryland, College Park, MD 20742-4111
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