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See Inside June 2010

"Quantum Microphone" Puts Visible Object in Two Places at Once

What is the sound of one molecule clapping? Researchers have demonstrated a device that can pick up single quanta of mechanical vibration similar to those that shake molecules during chemical reactions and have shown that the device itself, which is the width of a hair, acts as if it exists in two places at once. This type of “quantum weirdness” feat so far had only been observed at the scale of molecules.

“This is a milestone,” says Wojciech Zurek, a theoretical physicist at Los Alamos National Laboratory. “It confirms what many of us believe, but some continue to resist—that our universe is quantum to the core.”

Aaron O’Connell, a graduate student at the University of California, Santa Barbara, used computer-chip manufacturing techniques to create a mechanical resonator—akin to a small tuning fork. It was one micron thick and 40 microns long, just big enough to be visible with the naked eye. He and his collaborators then attached the resonator to a superconducting circuit and cooled everything to within 0.025 of a degree above absolute zero. At those temperatures, the resonator would either be completely still or possess a quantum of vibrational energy, called a phonon. Vibrations could be detected using the superconducting circuit—in which case the device acted as a “quantum microphone.” Alternatively, running currents in the circuit would force the resonator to vibrate in sync. Thus, when the team put the circuit into a superposition of two states, one with a current and the other without, the resonator was in a superposition of vibrating and not vibrating.

In a vibrating state each atom in the resonator moved only by an extremely small distance—less than the size of the atom itself. Thus, in the superposition of states the resonator was never really in two totally distinct places. But still, the experiment showed that a large object (made of about 10 trillion atoms) can display just as much quantum weirdness as single atoms do. O’Connell presented the results in March at a meeting of the American Physical Society, and the findings appeared in the April 1 Nature. (Scientific American is part of Nature Publishing Group.)

This article was originally published with the title "Quantum Microphone Puts Visible Object in Two Places at Once."

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