In this illustration, the artificial atom on the right side of the image sends out sound waves that are picked up by the microphone on the left.

For the first time, physicists have figured out how to communicate with an artificial atom using sound instead of light.

Scientists already know a lot about how atoms and light interact. When atoms get charged up with energy, they often emit subatomic particles of light called photons. The photons belong to the wacky world of quantum mechanics where they behave as both particles and waves, and scientists have been studying their bizarre behavior for decades. But now researchers at the Chalmers University of Technology in Sweden have designed an artificial atom that can emit sound particles (called phonons) instead of photons after it's charged up.

"We have opened a new door into the quantum world by talking and listening to atoms," Per Delsing, a professor of microtechnology and nanoscience at Chalmers, said in a statement. [Wacky Physics: The Coolest Little Particles in Nature]

Making phonons

To create the stream of sound particles, the researchers used a superconducting circuit, which represented an "artificial atom." Artificial atoms can be charged up across multiple energy levels just like a real atom, and scientists can study the quantum behavior of the particles they emit.

For the experiment, the researchers cooled the artificial atom to near absolute zero so that heat would not disturb the delicate quantum system. The artificial atom the team used is only 0.0004 inches (0.01 millimeters) long. The setup also included a speaker and microphone to record the sound emitted.

Artificial atoms are usually coupled to light but for this experiment the researchers linked the artificial atom to sound. They put the superconducting circuit between two electrodes covered with piezoelectric fibers. The piezoelectric surfaces convert vibrations into an electric charge and then convert that electricity into a sound wave.

The researchers then fired the sound wave at the artificial atom. The artificial atom absorbed the sound wave and its energy level increased, reaching what scientists call an "excited state." As the atom relaxed back into a "ground state," it released phonons. The researchers measured and recorded the behavior of the phonons, and discovered the bond between an artificial atom and sound is much stronger than the bond created between an artificial atom and light. The stronger bond makes it easier to manipulate the phonons.

What does an atom sound like?

The stream of particles that came from the artificial atom is the weakest sound that can be detected, though the researchers didn't measure the actual decibels. It's much too high-pitched for the human ear to detect. The researchers measured the frequency at 4.8 gigahertz, not far from microwave frequencies used in wireless networks. On a musical scale, that's a D28 note, or about 20 octaves above the highest note on a grand piano.

Studying phonons instead of photons could provide new insights into the quantum world that scientists still don't fully understand.

"Due to the slow speed of sound, we will have time to control the quantum particles while they travel," lead study author Martin Gustafsson, a researcher at Columbia University, said in the statement. "This is difficult to achieve with light, which moves 100,000 times more quickly."

It's difficult to study the behavior of quantum particles, because their quantum state collapses as soon as researchers start poking around and measuring the particles. Artificial atoms already give scientists more control over quantum systems, but slow-moving sound waves will make it even easier to manipulate the particles. Learning more about quantum particles could help scientists get closer to developing technology like superfast quantum computers and quantum cryptography for secure communication.

Details of the experiment were published Sept. 11 in the journal Science Express.

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