From: Gina Miller (echoz@hotmail.com)
Date: Fri Apr 30 1999 - 19:53:17 MDT
QUANTUM SCULPTING
Feedback enables researchers to control an
atom's wave function
by Graham P. Collins
The development of quantum mechanics, the underlying laws that govern matter
and energy on the scale of atoms and electrons, has not only revolutionized
our understanding of the universe but also has given us such technologies as
the transistor, the laser and magnetic resonance imaging. Now Philip H.
Bucksbaum and his co-workers at the University of Michigan have combined
several recently developed techniques with a feedback system to control the
very essence of quantum particles: their wave functions. The Bucksbaum
experiment "is true quantum engineering," says physicist Michael G. Raymer
of the University of Oregon. "It should open up many new possibilities, most
of which we cannot even imagine now."
A wave function defines the physical state of a quantum object. Wave
functions are slippery characters, tied to probabilities, not certainties.
They obey the famous Heisenberg uncertainty principle: if one characteristic
is well defined, a related feature must be highly uncertain. For instance,
an electron with a very precise position must have a wide range of possible
velocities. Nevertheless, during the past decade a number of research groups
have assembled techniques for manipulating and analyzing complete wave
functions in detail.
Bucksbaum and his graduate students Thomas C. Weinacht and Jaewook Ahn apply
their technique to a type of quantum state known as a Rydberg state, which
occurs when an electron in an atom is excited to such a high energy level
that it barely remains bound to the atom. "Rydberg states are a great
laboratory to test new ideas," Bucksbaum explains. An electron with such
high energy can occupy a very large number of quantum states. Combining
those states in different proportions (that is, placing them in
superposition) sculpts the shape of the electron's wave function. In one
combination, for example, the electron is smeared out in a ring around the
atom; in another, it is localized and orbits the atom much like a planet
orbiting the sun.
The basic tool for such wave function sculpting is a strong, ultrashort
laser pulse, which excites the electron from a lower energy level. Through a
design developed by Warren S. Warren of acousto-optic modulator--a crystal
whose optical properties are governed by precisely shaped sound waves. How
the laser's intensity and phase vary over the 150-femtosecond pulse
determines how the available excited states combine to produce the
electron's sculpted wave function.
But what shape of laser pulse is needed to generate a specific sculpted
electron wave function? In principle, this shape can be predicted by
computations, but in practice one must contend with nonideal equipment and
incomplete understanding of the physical system being controlled.
Bucksbaum's new trick, described in the January 21 issue of Nature, is to
use feedback to modify the shaping pulse. His group works with a gas of
cesium atoms in batches of about a million atoms. An approximate pulse
excites the atoms, and the researchers map the shape of the resulting wave
function with quantum holography, a technique they demonstrated a year ago.
In optical holography, the three-dimensional shape of an object is
reconstructed from its hologram, a special two-dimensional interference
pattern. In quantum holography, measurements produce data loosely analogous
to a hologram from which the complete wave function of the object can be
reconstructed. In accordance with the uncertainty principle, however, each
measurement disturbs the quantum "object," so the "hologram" must be built
up one pixel at a time over many experimental runs, with thousands of
identically prepared atoms measured on each run.
Once the physicists have mapped the resulting wave function, they look at
the difference between that one and the desired one. This information is
then used to adjust the detailed shape of the laser pulse used on subsequent
batches of atoms. Bucksbaum found that after only two or three iterations
this feedback zeroed in on the desired wave function.
Quantum control has applications in the burgeoning field of quantum
computing, in which the encoding of data onto individual quantum states may
allow the development of computers that function on quantum principles.
Another application is control of chemical reactions. Shaped optical pulses
that induce just the right excitations at specific bonds in a molecule can
enhance or suppress alternate reaction pathways. Some groups have
independently used feedback for this type of control, but the feedback has
not been based on detailed mapping of a wave function.
Quantum physicist Carlos R. Stroud of the University of Rochester cautions
that further research is needed to see if Bucksbaum's method is applicable
to a wider range of quantum systems. Still, he says, "they have expanded the
quantum mechanics toolbox."
The Author
GRAHAM P. COLLINS, based in College Park, Md., has written articles for New
Zealand Science Monthly and Physics Today.
Gina "Nanogirl" Miller
Nanotechnology Industries
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http://www.nanoindustries.com
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