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Penn State Researchers Collaborate on Work Designed
to Control Spin of Electrons
The research, featured on the cover and discussed in the "Perspectives"
section of the journal, suggests an ability to quickly manipulate electron
spins by loosening the stringent requirements of coherence times. While previous research has been aimed at increasing the coherence time
of electron spin in semiconductor quantum structures, the experiments
reported in the Science article present an alternate approach. In effect,
the researchers have discovered a technique that can potentially circumvent
the otherwise stringent constraint of the electron spin coherence time
in solid-state materials. The use of ultra-fast laser pulses to manipulate
spins would represent a speed-up of the process by 100,000 times when
compared with conventional methods, and opens new directions for research
into solid-state implementation of quantum computers. The experiments are the result of a long-standing collaboration between David Awschalom at UCSB and Nitin Samarth at Penn State. A complete release follows. Contact: Jacquelyn Savani (UCSB) Steve Sampsell (Penn State) FULL PRESS RELEASE:
University of California at Santa Barbara (UCSB) physicist David Awschalom
heads the research effort that conducted the experiments reported in Science.
He is director of the UCSB Center for Spintronics and Quantum Computation,
a central component of the new California NanoSystems Institute (CNSI)
located jointly at UCSB and UCLA. Awschalom's long-time collaborator is Penn State materials physicist
Nitin Samarth. Together with their graduate students, the pair
had recently published related experiments in the June 14 issue of Nature.
By hooking a battery up to a semiconductor structure, tunable electric
fields were generated that move spin-aligned electrons from one semiconductor
material to another. Just like the charge currents that flow in ordinary electronics, this
spin current may form the foundation for a new type of "spintronics"
that researchers hope will improve speed in devices for information processing
including hard disk drives and nonvolatile RAM. In the Science paper Awschalom, Samarth, and graduate students
Jay Gupta and Robert Knobel focus not on moving spin-aligned
electrons, but on controllably rotating the axis of electron spins. They
do so by using ultra fast pulses of laser light to "tip" the
spin alignment of the electrons. "Ultra fast" in this case means
10-13 seconds-a trillion such pulses would fit in the time it takes to
blink an eye. Some elementary particles such as electrons spin or rotate. Classically,
physicists describe spin (e.g. of a top) by specifying the direction of
the axis of rotation and the rate of rotation (i.e., angular momentum).
But in the more fundamental framework of quantum mechanics, a measurement
of an electron's spin can only have discrete values. Thus the spin-state
of the electron along a particular axis can be visualized as either clockwise
or counterclockwise with a basic discrete unit of angular momentum. These
states are referred to as spin up or spin down along the chosen axis of
rotation. The binary bit of conventional computing-either 0 or 1-corresponds in
the spin quantum-computing paradigm to a quantum bit consisting of a particle
spin that is either up or down. What is different and what makes quantum
computation a potentially richer computational approach is that the electron
spin can be in a superposition of spin up and spin down. This feature
of quantum mechanics has intrigued physicists since the 1920s, and means
that a bit can encode not just one piece of information (for instance
whether a light is on or off), but much more, like the light's color,
intensity, etc. As Awschalom puts it, "Conventional computer bits consist of miniature
electronic switches that are either off or on (0 or 1). Quantum-bits can
be any combination thereof, for example 41% on, 59% off." This property,
he says, "enables computational algorithms with exponentially improved
speed and fundamentally different functionality." Other researchers have explored quantum-bit models based on nuclear spins,
atoms, or trapped ions. That work, says Awschalom, "has provided
some of the founding principles of the rapidly emerging field of quantum
computation. But a limitation of these model systems is their potential
scalability to the large number of quantum bits desirable for real computational
problems." Awschalom and Samarth are looking at solid-state systems to solve the
scalability problem because semiconductor microprocessing technology is
already so advanced. However, a tradeoff with this approach is a shorter
"coherence time" which is the length of the time that the electron
spins stay aligned before they succumb to environmental influences that
cause the spin alignment to randomize. As Awschalom says, "The coherence time sets the duration over which
quantum-bits maintain a well-defined state, for example, the state '41%
on, 59 off' versus some other random combination." The coherence
time is one of the critical parameters in quantum computation proposals.
In essence, the coherence time must be much greater than the "clock
speed" of the quantum computer for the device to work, because a
large number of operations on the quantum-bits (analogous to flipping
the switch back and forth) must be performed in order to produce a computation. While previous research has been aimed at increasing the coherence time
of electron spin in semiconductor quantum structures (which is typically
less than 1 microsecond), the experiments reported in the Science article
present an alternate approach. In effect, the researchers have discovered
a technique that can potentially circumvent the otherwise stringent constraint
of the electron spin coherence time in solid-state materials. The use
of ultra fast laser pulses to manipulate spins would represent a speed-up
of the process by 100,000 times when compared with conventional methods,
and opens new directions for research into solid-state implementation
of quantum computers. As Awschalom explains, "These rotations are made possible by an
effective magnetic field that arises when very intense light of a certain
energy interacts with the electron spins in a semiconductor. Although
the degree of rotation is currently about half of what is needed to perform
a full operation, many avenues exist for further optimization." The Penn State researchers engineered the materials used in the UCSB
experiments through molecular beam epitaxy (MBE), a materials synthesis
technique that allows the custom-design of complex materials with atomic
monolayer control. "The goal of developing a quantum computer provides an exciting
opportunity for combining cutting edge materials synthesis with sophisticated
physical measurements. Tailoring the material to this experiment was a
real challenge," said Samarth. "In this case, we had to meet
several constraints imposed by both the physics and technology of the
experimental measurements. Since MBE allows us to act as 'atomic scale
architects,' we met the tight conditions needed for the measurements by
building 'digital' quantum structures, one atomic layer at a time." Awschalom said, "It is hard to separate materials from ideas and
experiment. This work is truly interdisciplinary. We could never do these
experiments without the Penn State materials science expertise and creativity." DARPA (Defense Advanced Research Project Agency) funds the collaborative
research between UCSB and Penn State through two programs, SPINS and QUIST. [Note: Professor Awschalom is in Europe where he is presenting these findings to the NATO Advanced Research Workshop on Quantum Transport in Semiconductors in Maratea, Italy, as well as at two scientific conferences. He can be reached by e-mail awsch@physics.ucsb.edu or through UCSB contact Jacquelyn Savani (805) 893-4301. Professor Samarth is at 814-863-0136. For a high resolution version of a graphical representation of both classical and quantum mechanical approaches to spin, go to ftp://kk.engr.ucsb.edu/Press_Releases/spin-qc/ .]
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| This page is maintained by Barbara K. Kennedy: science@psu.edu, (814) 863-4682 and Leta A. Krumrine: LAK15@psu.edu, (814) 863-8453 Eberly College of Science, Office of Public Information, 427 Thomas Building, University Park, PA 16802-2112 This page was last updated on 28 June 2001 If you would like
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28
June 2001-- Researchers from the University of California at Santa
Barbara (UCSB) and Penn State report in the 29 June 2001 issue of the
journal Science an ability to quickly manipulate electron spins
that could positively impact the development of items such as quantum
computers.
Santa
Barbara, Calif. -- "Ultrafast Manipulation of Electron Spin
Coherence," published in the June 29 issue of Science, reveals
a new way to manipulate optically quantum spin states on ultrafast time
scales (femtoseconds). The authors suggest that the ability to quickly
manipulate electron spins could pave the way for all-optical quantum computation
in solids by loosening the stringent requirements on coherence times.
Featured on the Science cover, the research findings are also discussed
in the "Perspectives" section of the journal.