Team toys with ions to simulate quantum world: manipulations could help design better materials.

If the Matrix really existed, it would probably have to be a quantum simulator. The fictional computer in the movie with the same name can create virtual worlds indistinguishable from the real one and project them into people's minds. But the real world includes quantum phenomena, something ordinary computers can't fully simulate.

Now physicists have created a rudimentary prototype of a machine that simulates quantum phenomena using quantum physics, rather than using data kept in a classical computer. While the new device can't make people fly like the Matrix does, it demonstrates a technique that could enable physicists to create, in the virtual world, materials that don't yet exist in nature and perhaps figure out how to build, in the re al world, superconductors that work at room temperature.

Tobias Schatz of the Max Planck Institute of Quantum Optics in Garching, Germany, and his collaborators built a model of one of the smallest solid objects imaginable--made of just two atoms--by suspending two ions in a vacuum. The researchers used laser light to vary the electrical repulsion of the ions to simulate the magnetic interaction of atoms, harnessing one force to simulate the other.

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In a paper published online July 27 in Nature Physics, the researchers describe how their system reproduced the magnetic alignment of atoms that takes place when certain materials are exposed to magnetic fields.

"This is pretty important that they've been able to demonstrate the principle," says John Chiaverini of Los Alamos National Laboratory in New Mexico.

"The experiment is an important initial step in the emerging field of quantum simulation," says David Wineland of the National Institute of Standards and Technology in Boulder, Colo., whose group in 2002 pioneered a more limited quantum simulation technique by trapping ions. The new experiment "demonstrates important tools that can potentially be implemented on much larger systems whose simulations are intractable by classical means," he adds.

The late physicist Richard Feynman pointed out in 1982 that ordinary computers can't possibly simulate true quantum behavior of a large number of particles because the phenomenon of superposition allows a particle to be in two states at once. For example, the spin of an atom--the quantum version of a bar magnet's orientation--can point simultaneously up and down. To simulate the spin states of an object made of two atoms, a computer has to keep track of four possible combinations of spins: up-up, up-down, down-up and down-down. For three atoms, eight possibilities exist, and the number keeps growing exponentially. For n atoms, the number is [2.sup.n], which gets very large very quickly. "This [2.sup.n]--that's what kills classical computers," Schatz says.

Chiaverini says even state-of-the-art supercomputers quickly get overwhelmed with all the calculations required to predict how all those spin states will evolve in time. "You run out of steam at about 40 spins," he says.

Simulating just 100 atoms would require keeping track of [2.sup.100] combinations, more than the number of protons in the visible universe. But a system of quantum objects, unlike a classical computer, is itself able to exist in a number of different states that grows exponentially. Several different teams of physicists are developing techniques for quantum simulation. The two leading approaches are to use ions in an electrostatic trap, as Schatz and colleagues have done, or to use atoms in an optical trap, which holds things in place using the pressure of light.

Last year, David Weiss of Pennsylvania State University in University Park and his colleagues demonstrated an optical trap able to hold hundreds of atoms in a cubic array, to image and manipulate the atoms individually, and to make them interact with one another. The researchers even took videos of glowing atoms staying in place or, occasionally, jumping from site to site along the array. "It took a couple of days until I could get my graduate student and my postdoc to stop taking pictures and actually start the experiment," Weiss said in March in New Orleans at a meeting of the American Physical Society.

The trapped-ion approach Schatz's team followed was first proposed by his Garching colleagues Diego Porras and Ignacio Cirac in 2004. In the experiment, the team suspended two magnesium ions in a vacuum, keeping them in place with electric fields. The positive ions were just a few micrometers apart--close enough to feel mutual electrostatic repulsion, but far enough not to feel each other's spins.

The researchers then used a laser to simulate the application of an external magnetic field, which could give the ions any initial state. "It's much better than a real magnetic field because, for example, you can individually address your atoms. It would be hard to have a real magnetic field 'on' on one atom, and 'off' on the other," Schatz says.

Using the laser, the researchers tuned the electrostatic interaction of the two particles. In the future, more-complex experiments could, for example, create a model of a superconductor and then selectively change the physical parameters to understand how the material conducts electricity with minimal loss of energy.

Such control would be impossible in a real solid material, such as a superconducting crystal. "If you tell a solid-state physicist, 'reduce your spin-spin interaction by a factor of two because I want to see the physics,' he can't do it," Schatz says.

Together with his colleague Warren Lybarger Jr. and others, Chiaverini is working on a similar setup, also based on Porras and Cirac's idea. And the team is developing an alternative approach that would use radio frequency fields, instead of lasers, to manipulate the ions' states.

Two ions, of course, don't make a real solid object, but researchers say that in the future they maybe able to scale up the Garching device to larger arrays. Currently, researchers who want to experiment with new materials, such as potential superconductors, have to first create actual crystals in the lab and then test their properties. Quantum simulations could make that task a lot easier. "Some day, hopefully, we can apply this to making designer materials from the ground up," Chiaverini says.

Feynman envisioned that eventually a general purpose, programmable quantum computer could carry out quantum simulations. But such machines are still decades away, while machines designed only for the simulations maybe available sooner.

Mimicking magnetic forces

Lasers in a quantum simulator push ions in different directions depending on the energy levels of the electrons.

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In the low-energy state, corresponding to parallel spins, both ions are pushed in the same direction.

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If the ions are in different states, they are pushed in different directions as if the ions are attracting or repelling each other.