Quantum
Information Science (QIS) Seminar
(CSI 991 -
006)
Monday, October 31, 2005, 4:30 - 6:00 PM
George Mason University, Fairfax Campus
Science Showcase, George Johnson Center, Room 237
Presenter:
Professor J. Robert Anderson, University of Maryland
Title:
Quantum Computing Based on Josephson-Junction Phase Qubits
Abstract: I will introduce the subject of
quantum computing by mentioning briefly Shor’s algorithm, which is
driving the
search for a quantum computer. Then I will discuss our work on single
Josephson
junctions as phase qubits. Measurements with microwaves applied to one
and
two-qubit systems will be described. Microwave spectroscopy has
provided
evidence for the entanglement of two capacitively coupled junctions
(a.k.a.
qubits). I will also show that this two-junction system, at bias
currents
corresponding to larger energy separations than used initially, is
actually
coupled by an LC resonator, which has its own energy levels. We
have
performed microwave spectroscopy on this three-component system and
have
observed avoided level-crossings of at least three levels. These
results
will be compared with theoretical calculations. In addition, Rabi
oscillations, observed in single and coupled junctions will be
presented.
From these measurements and from measurements of escape rates as
bias
currents through junctions are ramped, we have estimated coherence
times.
These estimates are being used to compare different techniques
for
isolating junctions from external sources of decoherence. At the
present
time we are investigating inductance-junction (LJ) isolation schemes by
means
of which we have been successful in observing the Rabi oscillations
mentioned
above (If time permits, I will discuss a method for state
initialization with
LJ isolation.). Up to now we have studied no more than two
coupled
junctions, but we are beginning to investigate multi-qubit systems of
three or
more junctions. Different schemes for control of qubit coupling
will also
be compared since it is important to be able to turn the coupling on
and off.
To conclude, I will discuss the prospects for gate operations and
the
realization of a Josephson-junction quantum computer.
About the
Author: Dr.
Anderson
received his Ph.D. from Iowa State University, with an assist from Cambridge University, investigating the electronic
structure of metals. He has been a tenured professor at the University of Maryland since 1971. After a National
Academy of Sciences fellowship in Moscow during 1974-1975, he began research
on diluted magnetic semiconductors, work which is continuing to the
present.
In the late 1990s he began collaboration with colleagues from the
University of Maryland Center for Superconductivity Research on quantum
computing. Dr. Anderson has been a consultant at Sandia
Laboratories and
has worked closely with scientists at the Laboratory for Physical
Science.
In addition, he has spent sabbatical years at the Institute of Physics in Warsaw, Poland, at the Institute for Materials
Research and the Research Institute for Scientific Measurements of
Tohoku
University, Sendai, Japan, and at the National Research
Institute for Metals, Tsukuba, Japan.