Physics: "Putting The Weirdness To Work"
The world of the quantum stretches the limits of human imagination. Who could ever believe, for instance, that atoms -- the building blocks of our seemingly solid landscape -- are able to exist in different places at one time? That they can be "entangled" together such that an action on one atom or particle will affect another across considerable distances? Or that they are irrevocably altered simply by the act of being observed?
Yet that is what quantum laws tell us. Einstein himself was famously troubled by the implication that reality was actually just a collection of probabilities, where God not only played dice with the universe but also hid the dice. "To common sense, quantum mechanics is nonsensical," says Nobel prize-winning physicist William D. Phillips of the National Institute of Standards & Technology (NIST).
Nevertheless, developing quantum theory was "the crowning intellectual achievement of the last century," says California Institute of Technology physicist John Preskill. It's the underlying principle for many of today's devices, from lasers to magnetic resonance imaging machines. And these may prove to be just the low-hanging fruit. Many scientists foresee revolutionary technologies based on the truly strange properties of the quantum world.
For instance, there's a state of matter that scientists created less than a decade ago called the Bose-Einstein condensate, in which each of many millions of atoms act identically and are everywhere in the sample at once. Dozens of research groups around the world are experimenting with these condensates, whose properties portend a future we can barely glimpse. "Physicists relish the weirdness, but now we're starting to ask if we can put the weirdness to work," says Preskill.
Some of the theoretical possibilities boggle the mind. For example: the elusive but intensely desired quantum computer. The mathematical challenge of factoring a 400-digit number -- which would take 10 billion years on today's supercomputers -- might be cracked by a quantum computer in 30 seconds. While there are a number of approaches to building such a device, recent experiments with the Bose-Einstein condensates are opening up clever new paths.
Quantum weirdness also enables communications to be sent in unbreakable code. New companies, such as New York City's MagiQ Technologies and id Quantique of Geneva, are already turning these ideas into commercial products. At the same time, the exploration of quantum domains may shed more light on abiding scientific mysteries, such as how some substances conduct electricity with zero resistance -- a phenomenon called superconductivity. That could lead to the transmission of electricity across great distances with no loss. And a forthcoming paper from IBM researchers will show how quantum phenomena can be exploited to see molecules more clearly.
These uses may just scratch the surface of the possible. No one has ever been able to foresee transformations wrought by any revolutionary science. And the quantum world is no different. "We have not yet begun to figure out what the applications are," says NIST physicist Carl J. Williams. "But the risk is underestimating the impact."
Quantum computers and most other applications are decades away, if indeed they can be built at all. Still, the enormous potential has led to programs at companies like IBM (IBM ) and Hewlett-Packard Co. (HPQ ). The Pentagon's Defense Advanced Research Projects Agency is now beginning a major effort to construct a working quantum information processor. In all these efforts, "the goal is the control of quantum matter," says Immanuel Bloch of the Johannes Gutenberg University of Mainz. "It's a great challenge, but there are great rewards."
For a glimpse of this endeavor, drop by the lab of William Phillips and his team in Gaithersburg, Md. Sprawling over a giant lab bench is a maze of precision mirrors and lasers, all converging on a small glass vacuum chamber where the quantum world is being probed. Phillips won his Nobel in 1997 for a technique known as laser cooling, in which beams are used to slow atoms down. That chills the atoms until they are a fraction of a degree above absolute zero. Now, using rubidium atoms, Phillips is making them even colder by letting the warmer ones "evaporate."
PEAKS AND TROUGHS. Inside the glass chamber, he is creating the fragile Bose-Einstein condensate. The clump of atoms can be huge -- big enough to be visible to the naked eye. At that scale, you would expect the stolid laws of Newtonian physics to rule. Instead, the atoms obey the Heisenberg uncertainty principle, which specifies that an electron or atom can't be pinned down to any one location. Even though the clump is a tenth of a millimeter across and contains a million atoms, "every atom is everywhere -- that's what makes it so wonderful," says Williams.
This strange state of matter was predicted by Einstein, building on work by Indian physicist Satyendra Nath Bose, back in 1924. It was first created by Phillips' NIST colleague, Eric A. Cornell, and Carl E. Wieman of the University of Colorado, in 1995 -- a Nobel prize-winning achievement. Now, an estimated 50 groups around the world are experimenting with the strange stuff. "It can do some amazing things," says Phillips.
One of the most intriguing -- and potentially useful -- maneuvers in Phillips' lab involves putting the atoms into neat little rows. The trick is using precisely tuned laser light. Imagine dropping pebbles into a pond, sending waves across the water. Then drop pebbles at the opposite shore, dispatching waves in the other direction. Where the two groups of waves meet, they create so-called standing waves -- an unchanging collection of peaks and troughs, like a row of sand dunes in the desert.
Laser light is also a wave. So two intersecting beams similarly create peaks and valleys. Scientists call this an optical lattice. And when Phillips and other researchers shine intersecting laser beams though the Bose-Einstein clump of atoms, individual atoms almost magically go from being everywhere at once to nestling in the valleys. "It's a great gift of nature," says Phillips. "We've been lucky that things worked better than expected."
To information scientists, such a neat arrangement of atoms looks startlingly like the basis for a computer. It can be arranged that each atom is in one of two energy levels, separated by a small quantum jump. Thus, each atom could represent a 0 or a 1, like the bits in a regular computer.
But these are no ordinary bits. Because of quantum weirdness, an atom can be a 0 and a 1 at the same time. What's more, the different quantum bits, or "qubits," can be entangled with each other, even if there is no actual connection. "Because of the mystery of entanglement, the state of one atom will be dependent on the state of the other," explains Williams. "It's a much stronger relationship than marriage." As a result, for some calculations, the power of a quantum machine grows exponentially with the number of qubits -- twice the bits gives you four times the power. A 300-qubit machine could store more combinations than there are atoms in the entire universe, says Williams.
CLEVER ALTERNATIVES. Without doubt, there's a long, long path to building such a machine, and today's researchers have only begun the journey. Phillips and his team are now working on the next small step. They're trying to figure out how to get information to and from the individual qubits, by flipping the atoms from one state to the other with laser beams.
Meanwhile, other labs are pursuing clever alternatives. At the University of Mainz, Bloch is also putting Bose-Einstein condensate atoms into the valleys of an optical lattice. His special twist is creating two simultaneous lattices with two different "colors" of laser beams. He also puts his atoms in two states at the same time. Then he can move one of the landscapes so that the atom particles interact in new ways. "We can entangle hundreds of thousands of atoms and measure the state of each particle," he says. "It is a completely new way of thinking about a quantum computer."
Another tack is to use ions trapped in a magnetic field as qubits, instead of atoms in the optical lattice. Out in NIST's Boulder (Colo.) labs, David J. Wineland has built working logic gates -- a building block of computers -- using such ions. And many other groups are experimenting with tiny bits of semiconductor material, dubbed quantum dots.
The ultimate payoff, however, is expected to go far beyond computing. Since the very act of observing quantum information changes it, communications that are encrypted with quantum "keys" could be sent safely across a network. The reason: Any attempt by spies to intercept the key would immediately be obvious, so users could switch to a different one.
As exciting as these applications are, researchers are also thrilled by the basic science. Earlier this year, NIST physicist Deborah S. Jin created a state of matter called a fermionic condensate that is even rarer than the closely related Bose-Einstein materials. She managed to put atoms that don't normally like being next to each other into the same low-energy state. Her work could lead to a better understanding of superconductivity, which depends on similar pairs of quantum particles.
Scientists are often surprised by what they encounter. Not long ago, Phillips was experimenting with faint laser beams, which unexpectedly impeded the movements of atoms. "We don't know if this is interesting new physics or some stupid mistake," he says. "In learning about quantum computing, we're at the forefront of fundamen- tal physics." That's how science and technology sometimes advance -- one small quantum step at a time.
By John Carey in Gaithersburg, Md.