New Detector May Test Heisenberg's Uncertainty Principle

ost motion sensors are designed with only a certain amount of sensitivity in mind. An infrared sensor built to detect the movements of a burglar, say, will overlook the nocturnal meanderings of the family cat (thankfully for the homeowner, who won't be awakened by false alarms).

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But scientists at the University of California at Santa Barbara have taken sensitivity to new extremes. They have devised an extremely precise detector, one that is capable of sensing movements far smaller than those of a human or a pet. Coupled with a tiny crystal beam, the device can detect displacement on an atomic scale — a fraction of a nanometer.

Using such a device, the researchers hope to determine whether Heisenberg's uncertainty principle, a fundamental notion of quantum mechanics that is normally thought to apply on the atomic level, still holds when billions of atoms are assembled as an object.

"We're trying to see whether or not quantum mechanics breaks down when you get too many atoms in a system," said Dr. Andrew N. Cleland, a physicist at the university and co-author of a paper with Dr. Robert G. Knobel on the device published in the current issue of Nature.

Dr. Miles Blencowe, a theoretical physicist at Dartmouth and the author of a perspective article that accompanied the Cleland-Knobel paper, said, "There is nothing really in the laws of quantum mechanics that says they shouldn't apply to larger objects."

"But you have to think long and hard about how one would do an experiment to test this," he said.

The experiment Drs. Cleland and Knobel thought long and hard about, along the lines of what Dr. Blencowe had proposed in an earlier paper, is almost, but not quite, capable of measuring the quantum forces affecting the crystal beam to the exclusion of everything else. It is a transistor, a variant on the simple electronic component that, by the millions, fills the typical computer chip.

Simply put, a transistor uses a voltage to control a current, or the flow of electrons. In the transistor that Dr. Cleland and Dr. Knobel built using semiconductor fabrication techniques, the current flows one electron at a time, and the controlling voltage is affected by the deflection of the tiny crystal beam. So a change in current acts as a measure of how much the beam moved.

The beam, a single gallium-arsenide crystal, is about three microns long and 250 by 200 nanometers in cross section. That's very small, yet still contains billions of atoms. So it is an adequate testing ground for applying the uncertainty principle beyond the atomic level.

Put forth by the German physicist Werner Heisenberg in the 1920's, the principle holds that the position and momentum of an object cannot both be precisely determined. Measuring the position affects the momentum, and vice versa. This uncertainty leaves physicists, when speaking about an atomic particle like an electron, able to talk only about the probabilities of where it might be at a given time.

An electron is one thing; a tangible object, though, is another. Take a bridge, for example. When it flexes due to wind or traffic, it may be difficult to determine its position and momentum. But remove those forces and eventually a precise determination of those two qualities will be possible. Quantum forces act on the bridge, but it is so big that they mostly can be ignored.

The crystal beam is, in effect, an extremely small bridge, so small that when all the "outside" forces acting on it (not traffic or wind, obviously, but thermal forces) are removed, the quantum effects should be detectable. Drs. Cleland and Knobel cooled their device to a fraction of a degree above absolute zero. At that temperature, any change in the position of the beam would be related to quantum uncertainty, so-called zero-point fluctuations.

"Usually we don't measure things accurately enough to see these effects," Dr. Blencowe said.

The Cleland-Knobel device is extremely accurate, able to detect a flexing of the beam of about one one-thousandth of a nanometer. Yet it still only comes close to being able to detect the quantum effects. Improvements of several orders of magnitude are needed.

Dr. Keith Schwab, a physicist with the National Security Agency who is conducting similar research, said: "What they've done is really hard; it's a really hard piece of fabrication. But they still have a factor of a hundred to go."

One improvement would be to make a smaller crystal beam. "To make one that is roughly a factor of 10 smaller is not a problem," Dr. Cleland said. "The more challenging problem is to reduce the noise in the transmitter."

Aside from eventually demonstrating that the uncertainty principle applies beyond the atomic level, the research also may lead to the development of a new type of laboratory instrument, one that can measure the extremely small forces of individual atoms.

Atomic force microscopes already exist that can show the general topography — the lineup of atoms — of a molecule. But a microscope based on their sensitive device, Dr. Cleland said, should be able to identify the atoms themselves.

Such an instrument might, for example, be able to sequence a DNA molecule just by looking at it. "It would be a very powerful tool," Dr. Cleland said.