Macro-Weirdness: "Quantum Microphone" Puts Naked-Eye Object in 2 Places at Once

A new device tests the limits of Schrödinger's cat

O'Connell et al./Nature

PORTLAND, Ore.—What's 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—a "quantum weirdness" feat that so far had only been observed at the scale of molecules.

"This is a milestone," says Wojciech Zurek, a theorist at the Los Alamos National Laboratory in New Mexico. "It confirms what many of us believe, but some continue to resist—that our universe is 'quantum to the core'."

Physicists have long known that, following the laws of quantum mechanics, objects at the scale of atoms or smaller can exist in multiple simultaneous states. For example, a single electron can move along multiple different paths or an atom can be placed in two different places, simultaneously. This so-called superposition of states should in principle apply to larger objects, as well, as in the proverbial thought experiment in which a cat is simultaneously dead and alive. And in recent years various teams have shown that the weird phenomenon does occur among objects as big as molecules, and also in truly macroscopic systems such as electrical currents in superconductors.

In the new experiment Aaron O'Connell, a graduate student at the University of California, Santa Barbara, and his co-workers have shown for the first time that larger objects can also be in two places at once. "It tells us that quantum mechanics works for macroscopic objects in space," says O'Connell, who presented the results here at a meeting of the American Physical Society. The results were also published online Wednesday in Nature. (Scientific American is part of Nature Publishing Group.)

The team used computer-chip manufacturing techniques to create a mechanical resonator—akin to a small tuning fork. The device is a piece of piezoelectric material (a material that expands or contracts in the presence of an electric field as well as generates an electrical field when put under stress) sandwiched between two layers of aluminum, which act as electrodes. It is one micron thick and 40 microns long, just enough to be visible "with your naked eye," O'Connell says.

The resonator's electrodes are attached to an electronic readout based on superconducting circuits, and the whole contraption is kept in a vacuum and cooled to within 20 thousandths of a degree above absolute zero. But the electronic circuitry can also be used to apply a voltage to the electrodes, so that the team can get the resonator to expand and contract at will. This motion takes place at a characteristic, or resonant, frequency of six gigahertz, or six billion cycles per second. (Tuning forks also have a resonant frequency—in the order of kilohertz—but the mode of resonant vibration in that case is to oscillate sideways rather than to expand and contract.)

The team's first result was to show that at such chilly temperatures the width, or amplitude, of the resonator's vibration becomes quantized—in other words, there is a small amount of vibrational energy, called a phonon, below which the resonator is essentially still. The existence of discrete packets of energy is a hallmark of quantum behavior, and phonons are the mechanical equivalent of light's photons—they are the ultimate, indivisible quanta of vibration, whether thermal or acoustic.

Next, the team put the superconducting circuit into a superposition of two states, one with a current and the other one without. Correspondingly, the resonator was in a superposition of vibrating and not vibrating. These quantum states continued for about six nanoseconds—about as long as the team expected—before fading away.

In a vibrating state each atom in the resonator only moves by an extremely small distance—less than the size of the atom itself. Thus, in the superposition of states the resonator is never really in two totally distinct places. But still, the experiment showed that a large object (the resonator is made of about 10 trillion atoms) can display just as much quantum weirdness as single atoms do. "Yup, quantum mechanics still works," says U.C.S.B.'s Andrew Cleland, O'Connell's co-author and adviser. As to how the day-to-day reality of objects that we observe, such as furniture and fruit, emerges from such a different and exotic quantum world, that remains a mystery.

In addition to its theoretical implications, the device could also find applications in the study of phonons that occur in nature, because a phonon that perturbs the resonator can be detected through the electronic circuit—it is essentially a quantum microphone. "This is a fantastically sensitive detector of acoustic vibration," Cleland says. In principle, one could even place molecules on the resonator and "hear them" interact, chemically or otherwise.

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