Sunday, July 27, 2008

Los Alamos National Labs and the Future of Simulation

One of the interesting things people are working on in New Mexico is the very highest form of simulation. This may not be immediately applicable to traditional media work, but... the products and techniques spun off may help drive the future of virtual reality.
BUILDING 'THE MATRIX'
By Davide Castelvecchi July 27th, 2008

Simulating the complexity of quantum physics would quickly overwhelm even the most advanced of today’s computers.

If The Matrix really existed, it would probably have to be a quantum simulator. The fictional computer in that story 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 real world, superconductors that work at room temperature, for example.

Tobias Schätz of the Max Planck Institute for Quantum Optics in Garching, Germany and his collaborators built a model of the smallest solid object imaginable — one made of two atoms — by suspending two ions in a vacuum. The researchers used laser light to vary the electrical repulsion of the ions in order to simulate the magnetic interaction of atoms. Essentially, the machine could use one force of nature to simulate the other.

In a paper published online by Nature Physics on July 27, 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 the Los Alamos National Laboratory in New Mexico.

“I feel 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 single ions. The new experiment “demonstrates important tools that can potentially be implemented on much larger systems whose simulations are intractable by classical means,” he says.

It was the late physicist Richard Feynman who pointed out in 1982 that ordinary computers can’t possibly simulate true quantum behavior of a large number of particles. That’s because of the phenomenon of superposition, which allows a particle to be in two states at the same time. For example, the spin of an atom — the quantum version of a bar magnet — can point simultaneously up and down.

Feynman reasoned that to simulate, say, 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 2n, which gets very large very quickly. “This 2n — that’s what kills classical computers,” says Schätz.

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.
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Eventually, Feynman envisioned, a general purpose, programmable quantum computer could itself carry out quantum simulations. But such machines are still decades away, most researchers say, while machines designed only for quantum simulations may become available sooner.
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