DALLAS - (KRT) - Today's most powerful
computers owe their speed to millions of transistors etched on a
sliver of silicon. Tomorrow's best machines may need just an
artificial crystal of laser light.
By zapping laser beams across each other, scientists can fashion
a three-dimensional structure like a miniature egg carton. Each
dimple in the "carton" traps atoms like eggs. And each atom could
serve as its own little computing bit, transforming the entire
crystal into a blazingly fast computer processor.
Physicists dream that the atom-trapping crystals made of laser
light - known as optical lattices - will help harness new "quantum
technologies," based on the bizarre behaviors common in the
microworld of atoms and molecules.
Most of these technologies remain in the realm of imagination.
But researchers have already reported many laboratory successes in
the quest to turn quantum dreams into quantum reality.
Besides progress in perfecting optical lattices, physicists are
learning how to go "quantum fishing," yanking individual atoms out
of an atomic reservoir with a pair of "quantum tweezers."
Physicists also study the bizarre phenomenon of "quantum
entanglement," in which particles separated at birth maintain a
spooky long-distance link with each other. And secret messages can
be transmitted with the foolproof protection provided by "quantum
cryptography," in which spinning particles flag the presence of an
Most of these quantum adventures have the same distant goal - to
enable the construction of a superfast quantum computer. For some
kinds of problems, such a computer could far outperform all of
today's supercomputers combined.
Large-scale quantum computers are decades off, but the quest to
build one should lead to many other discoveries along the way,
"There's a lot of reasons to go for this, even if in the end we
don't have one," said Christopher Monroe, a physicist at the
University of Michigan.
In fact, quantum research has already produced several new
technologies, most involving precise control of ultracold atoms and
Optical lattices are one of the hottest new applications of cold
atomic physics. For years, scientists have been able to confine
single atoms in "microtraps" created with lasers; light beams
hitting an atom from opposite directions can suspend it in place. In
the newest work, scientists can use multiple beams to create a grid,
or lattice, of numerous traps, with the goal of controlling the
number, spacing and behavior of many atoms.
Laser beams can form optical lattices because of light's wave
properties. Where the laser beams cross, the waves interfere with
each other to create alternating bright and dark spots of light.
Each bright spot marks a "well" where an atom likes to sit, similar
to the way atoms occupy specific locations in a crystal made of
So far, scientists have been able to fashion optical lattices
containing about 100 wells - but are still struggling with how to
control the atoms inside, said Mark Raizen, a physicist at the
University of Texas at Austin.
Ideally, researchers would like to shuffle atoms around in an
optical lattice as they might peas in a shell game. And to process
information, scientists want to read and write information onto an
individual atom - for instance, by switching the direction in which
it spins to represent an "on" or "off" state, like 1s and 0s in a
To make the atoms perform computations for a quantum computer,
physicists need to be able to put one atom in each well, like one
pea in each shell, and switch it on or off at will, said Immanuel
Bloch, a physicist at Johannes Gutenberg University in Mainz,
Germany. Hundreds of trapped atoms, each working individually, would
then perform in concert as a massive computer.
"This is really a whole new approach to quantum computing," Bloch
said in February during a meeting of the American Association for
the Advancement of Science.
But it's not easy. In the past, scientists could make a lattice
that trapped several atoms in each well, but not just one per well.
Or they could try to switch individual atoms off and on, but all the
atoms in the lattice would turn on and off simultaneously.
Optical lattices work best using very cold atoms, at temperatures
just fractions of a degree above absolute zero. Many physicists use
an exotic form of matter called a Bose-Einstein condensate, in which
atoms are cooled down so much that they begin behaving as a single
collective - a sort of Borg of the atomic world.
In new experiments, Bloch's group put a Bose-Einstein condensate
into an optical lattice and coaxed the atoms to drop into individual
wells. Just by changing the intensity of the six lasers that formed
the lattice, the scientists got the atoms in the condensate to
separate out, one to each well.
By switching back and forth between arrangements, the physicists
can control the lattice in ways that might one day lead to quantum
computing applications, said Bloch.
Another way to get a specific number of atoms, other than using
an optical lattice, is to fish them out one by one from an atomic
In Austin, Texas, Raizen is working to develop a "quantum
tweezer" made of laser light, which could pluck individual atoms
from within a Bose-Einstein condensate.
"It is sort of like going fishing where you cast your net in the
water," he said. "Here the net is our optical tweezer and the lake
is our Bose condensate."
Actually, Raizen's team has two working condensates - one made of
rubidium atoms and one made of sodium atoms. Within each, the
scientists arrange several hundred atoms in a line, then reach in
The tweezers' light contains a tiny well into which atoms can
fall. Leaving the tweezers inside the condensate for a while allows
lots of atoms to fall into the well. Yanking the tweezers out too
quickly prevents any atoms from falling in.
But by timing it just right, the scientists may be able to dip
out a selected number of atoms from the Bose-Einstein pool.
The UT team hasn't landed a big one yet, but calculations suggest
that quantum fishing just might work, Raizen said.
Whatever scheme scientists devise to control individual atoms,
success in quantum computing is likely to hinge on mastering one of
quantum theory's deepest mysteries: entanglement.
Entanglement is so bizarre that even quantum physicists struggle
with it. "If people say they understand it," said Michigan's Monroe,
"don't believe them."
Particles can become entangled by interacting in some way or
sharing a common event, as when a single atom spits out two
particles of light simultaneously. Entangled particles then share an
essential characteristic, such as the way they spin. By looking at
one of the entangled particles, an observer would automatically know
the state of the other. But until such an observation is made, the
property oscillates between two possibilities.
In other words, quantum entanglement isn't like slicing a penny
and giving one side to your sister. In that case, looking at your
half of the penny would automatically tell you about hers: If you
had heads, she would have to have tails. The state of each half
would have been well-defined all along.
But things would be different if the penny halves were entangled,
"Quantum entanglement would mean my penny is sometimes heads and
sometimes tails," he said. And the choice wouldn't be set until
somebody actually observed one of the halves.
As bizarre as it sounds, entanglement is key to building a
quantum computer. Quantum computing gets its power because of the
way entangled particles can mysteriously share information.
In quantum circuits, that information is stored in quantum bits,
or "qubits," just as ordinary bits store information in ordinary
computers. But qubits (pronounced CUE-bits) are inherently much more
fragile than ordinary bits, requiring clever engineering to preserve
The best candidates for qubits are either atoms or particles of
light, called photons. Atoms have the advantage of staying put.
Photons have the advantage of carrying lots of information.
Until now, most scientists have tried to entangle atoms with
atoms, or photons with photons. But physicists in Monroe's lab
recently managed to entangle an atom with a photon for the first
Inside a laboratory device, the researchers trapped a charged
particle of cadmium. They watched a photon zoom away, and through
careful measurements they saw that the photon and the cadmium
particle were entangled with each other.
"We were able to show that in our particular system, owing to the
amount of control that we have over that atom, the atom was
entangled with the photon," said Monroe.
Inside a quantum computer, he added, the stationary cadmium
particle could serve as the qubit to perform computations, while the
moving light particle could carry the information far away.
Despite such advances, scientists can't say whether quantum
computers will ever be practical for widespread use. The kinds of
problems they can solve may be limited, or the technology may prove
just too complex to master.
"I prefer to talk about quantum control and quantum engineering,"
said Raizen of UT. "That's more honest."
Scientists are much closer to building better atomic clocks, or
more sensitive remote-sensing instruments, because of the new
breakthroughs in quantum physics. The quantum computer may just have
to wait, Dr. Raizen said.
"There are really interesting possibilities with quantum
computing," he said. "But there's enough uncertainty that you don't
want to promise too much."
Taming the wayward atom
Playing with atoms as they might with marbles, physicists coax
individual atoms (red balls) inside the dimples of an "optical
lattice." The pattern in which the atoms get trapped greatly affects
the way the gas behaves - and determines its practical applications.
Scientists begin with a form of matter called a Bose-Einstein
condensate, in which a group of atoms behave together as a wave. In
this state the atoms interfere with each other, creating the
dramatic peaks seen at right. The applications of such a gas are
limited, because there's no way to control how many atoms end up in
each dimple.By tweaking the depth of the lattice, physicists have
coaxed atoms into an arrangement with one atom per well. In this
state, the interference peaks disappear (right). But this pattern
has many more practical applications - such as in quantum computing
- because the atoms are distributed predictably within the
lattice.Like an alien landscape spread out before an explorer, this
vista of rainbow-colored mountains provides a glimpse into the inner
workings of atoms in an ultracold gas. The ridged "mountains" are
actually patterns produced as atoms inside a lattice created by
light beams interfere with one another to a greater or lesser
What it is: Quantum physics refers to the math that describes the
strange behavior of matter at atom-size and smaller scales.
How it's different from traditional physics: At the atomic level,
reality is fuzzy and ordinary rules of cause and effect don't
Who's studying it: Scientists around the world, including some in
Why it's important: Quantum technologies could provide more
powerful microscopes, more precise atomic clocks, more accurate
sensing devices, and other useful things.
The ultimate goal: To build a "quantum computer," vastly faster
than today's best supercomputers for solving certain kinds of
Resources The NIST Quantum Information Program:
The Centre for Quantum Computation at Oxford University: http://www.qubit.org/
The April issue of Physics World magazine has an article about
quantum gases in optical lattices: physicsweb.org/article/
A Shortcut Through Time: The Path to the Quantum Computer, by
George Johnson (Knopf, 2003).
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