Physicists examine weird, subatomic world to create tiny supercomputers

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 eavesdropper.

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, scientists say.

"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 laser beams.

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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 matter.

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 classical computer.

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.

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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 sea.

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 with tweezers.

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.

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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, said Monroe.

"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 their integrity.

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 time.

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."

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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 degree.

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QUANTUM BASICS

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 apply.

Who's studying it: Scientists around the world, including some in Texas.

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 problems.

Resources The NIST Quantum Information Program: qubit.nist.gov

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/ world/17/4/7

A Shortcut Through Time: The Path to the Quantum Computer, by George Johnson (Knopf, 2003).

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