Supersizing Quantum Behavior

New Ideas / by Veronique Greenwood /

A groundbreaking Caltech experiment may detect quantum physics where classical mechanics has ruled sovereign.

From the level of human existence down to the nanoscale, we’re governed by Newtonian laws. You drop something, it falls; you push something, it moves. On the level of atoms, those rules cease to apply: Atoms can exist in two states at once; they maintain connections with a sister atom across miles of empty space; they tunnel across surfaces. The atomic rules—quantum physics—are especially strange because we’re made of atoms, but don’t seem to have any of these properties. How can we find out whether a coffee mug or a sliver of aluminum, or any collection of atoms, has the strange properties of the atoms that make it up?

This question has dogged us ever since the discovery of quantum mechanics. The issue has always been finding a way to measure quantum effects beyond the scale of the atom, which would let us see at what size quantum mechanics give way to classical mechanics, or if quantum properties somehow manage to persist all the way up to the level of humans. Viennese physicist Anton Zeilinger, in groundbreaking experiments in 1999, found that molecules as large as buckyballs exhibit the wave-particle duality of atoms. He has since conducted, unsuccessfully, similar experiments with viruses, and researchers at Yale have explored whether electromagnetic radiation has quantum properties. Physicists keep pushing the envelope, trying to see how large of an object they can catch showing quantum effects.

Now, post-doc Matt LaHaye and his colleagues at the California Institute of Technology have built a miniature structure that can help them detect whether an object made of 10 billion atoms exhibits quantum properties. The specific property currently under study is a phenomenon called “quantized energy.” On the human scale, oscillating objects, like a pendulum, have a continuous energy curve, progressing from 100 percent of their possible energy to zero and back. But atoms oscillate between clear energy states—for example, an atom could have 100 percent energy or zero, but nowhere in between. They have quantized energy.

Keith Schwab, a co-author of the study, posited some years ago that if oscillating objects larger than atoms have quantized energy levels, too, one should be able to detect them by engineering the correct interaction with an atom-like system. By looking at how the energy levels of the atom change when it is coupled with an oscillating object, scientists could deduce the oscillating object’s energy levels, or quanta. At long last, Schwab, LaHaye, and colleagues, including Michael Roukes, co-director of Caltech’s Kavli Nanoscience Institute, have built a device that can do such an experiment.

On a silicon chip, they have created, side by side, a nanoscale aluminum bridge and a small loop of superconductor (a “Cooper pair box”) that acts like an artificial “atom.” This “atom” serves as a “qubit,” taking one of two states, like a quantum version of the binary bits in a computer. The bridge—made of 10 billion atoms—vibrates side to side when a current is applied, while the qubit jumps between its energy levels. Both generate electric fields, which are so close that they in turn interact with each other, allowing the movement of the bridge to telegraph the energy levels of the qubit.

The researchers’ first test, published recently in Nature, is a proof of principle: If they could deduce the atom’s quanta from the bridge’s vibrations, they would have proof that they could turn the experiment around and use the atom’s vibrations to measure the quanta of the bridge. And lo and behold, nested within the readouts from the bridge were the signatures of the qubit state, clear as day.

These results have something of a fairytale quality for the researchers. “These experiments sound like thought experiments you see in books, ones that when you build them won’t work,” Schwab says. “But what we’re realizing is that these work.”

With improvements to their technique, the researchers hope to probe the quanta of the bridge from the readout of the qubit. If they find quantum effects in their bridge, it will be the largest object yet to show them, a major breakthrough. And from there, the prospective experiments are exhilarating: By engineering even tighter coupling between the qubit and the bridge, the researchers hope to be able to probe whether the vibrating bridge exists in two places at once, a strange quantum property called superposition.

Within the physics community, the Caltech results have been viewed with interest. In a recent Nature editorial, Finnish physicists Pertti Hakonen and Mika A. Sillanpää admire the extreme delicacy of the measurements, remarking that this is “an important step,” and suggesting that quantum effects in macroscale moving objects could be achieved within a decade. Markus Aspelmeyer, of Zeilinger’s group in Austria, calls the work “very beautiful and demanding,” and says that the team has “an extremely promising system” for observing quantum effects. Schwab says that the number of physicists who take it for granted that the experiments will succeed has surprised him: “The primary criticism is, ‘You’ll succeed—then what?’”

If scientists do observe quantum effects in nanoscale objects, the question will then become: How does our understanding of physics change once Newtonian and quantum mechanics are proven to act on the same scale? “One of the ultimate questions we are going after is our understanding of how the classical world emerges from the quantum world,” LaHaye says. Aspelmeyer is more explicit: “If quantum theory can be shown to apply even to macroscopic objects, it would show us in the most direct way that we are still lacking a deeper, fundamental understanding of how the world is constituted.”

While the experiments with bridge and qubit continue, the group will be exploring ways to supercool nano-scale diving boards, while other research teams supercool tiny globes of glass. If they can cool these objects to the ground state, eliminating their natural vibrations, physicists will have another chance to peer into quantum effects of objects not too far removed from the human scale: Reaching the ground state would immediately make clear whether the Heisenberg uncertainty principle is acting on the level of the globes and boards. “In the field, there has been a huge amount of focus on cooling for the last few years,” Schwab says. “Everyone is racing to the ground state.”

Originally published July 29, 2009

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