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New Experiment Probes Weird Zone Between Quantum and Classical

Mark Anderson Email 04.11.07 | 2:00 AM

Scientists at the Max Planck Institute for Quantum Optics in Germany have created a tiny silicon cantilever arm on a chip that, after being cooled down to 0.0001 degrees above absolute zero, will sway back and forth in multiple modes at once, becoming the world's first macroscopic system in a purely quantum mechanical state. Image: Max Planck Institute, Munich/Jorg Kotthaus, Universtiy of Munich

The strange boundary between the macroscopic world and the weird realm of quantum physics is about to be probed in a unique experiment.

Scientists have created a minute cantilever arm on the surface of a silicon chip that they hope will leave the world of classical physics and enter the quantum realm when cooled to near absolute zero.

The experiment will be the first time scientists have ever scaled an object in the observable world down into the slippery world of quantum mechanics.

"I think it's really possible to observe quantum effects (in the cantilever arm) with this experiment," said Peter Rabl of the University of Innsbruck in Austria, who isn't part of the experiment.

"Either you have a real, macroscopic object in a quantum state -- or you find out that quantum mechanics doesn't work for the macroscopic world," he said. "In either case, it would be quite fascinating."

Albert Einstein was famously skeptical of quantum physics, finding some of its more outlandish suppositions too strange and counterintuitive to be true.

Although many of quantum physics' predictions have long been experimentally confirmed, scientists have never been able to observe its effects directly.

In a paper published on the arXiv physics pre-print server, Philipp Treutlein and colleagues at the Max Planck Institute for Quantum Optics in Munich describe an experiment that may change that.

Treutlein proposes a chip with a cantilever arm -- a tiny diving board 0.0007 centimeters (7 microns) long, made from a billion silicon atoms.

With a minute magnet attached at one end, the cantilever will make ultra-sensitive measurements of a cloud of several thousand laser-cooled rubidium atoms hovering mere microns above it.

With carefully tuned laser pulses, the cloud of rubidium will be kept in a pristine state of matter -- mere nanodegrees above absolute zero -- called a Bose-Einstein condensate, or BEC.

In a process first predicted in 1925 by Einstein and the Indian physicist Satyendra Nath Bose, these supercooled atoms settle into a single, coherent quantum state.

BECs were first created in the lab in 1995 and have been used to study superfluids and to slow light rays to a crawl.

A group led by Jörg Kotthaus at the University of Munich has fabricated a first version of Treutlein's chip, with the cantilever and magnet mechanism (see photo). Treutlein's group will soon be ready to prepare a rubidium cloud that will hover over the chip and interact electromagnetically with the cantilever arm.

"One of the motivations for our experiment is to investigate a system which is on the border between quantum and classical," Treutlein said.

The real challenge, said Keith Schwab of Cornell University, is the next step: cooling the cantilever arm to one-ten-thousandth of a degree above absolute zero. At any temperature above this threshold, the cantilever arm would remain as classical and uninteresting as a regular diving board with a magnet attached to it, said Schwab.

But below the threshold, the cantilever arm would itself enter into a quantum mechanical state -- and ultimately, via the attached magnet, communicate back and forth with the quantum states in the rubidium cloud.

At least, everyone expects this to happen: No one has ever taken something as big as Treutlein's micro-cantilever system into the quantum realm before.

This means, Schwab said, that the bleeding edge of atomic physics necessary to create and maintain a rubidium cloud in a Bose-Einstein condensate would need to be combined with the state of the art in cryogenic physics.

"You'd need amazing technology from both sides," Schwab said.

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