Physicists have demonstrated behavior governed by rules of the quantum world, which operate at the level of atoms, in mechanical objects large enough to see.
The accomplishment fulfills a long-held dream to bridge the quantum and everyday worlds. One day, researchers say, mechanical devices in a laboratory might be manipulated according to the rules of single atoms — paving the way to quantum information processing or probing other unusual behaviors of the subatomic world.
“This is groundbreaking work,” says Markus Aspelmeyer, a physicist at the University of Vienna in Austria who was not affiliated with the work. “Now the door is open. Now the fun begins.”
Multiple teams have competed for years to link the quantum and everyday realms by building a tiny vibrating device and draining out as much of its energy as physically possible, reducing it to the “quantum ground state.” Most groups have tried to do this by building ever more powerful refrigerators to chill the material down to nearly absolute zero, or zero on the Kelvin temperature scale.
But physicist Andrew Cleland of the University of California, Santa Barbara, decided instead to take a shortcut. “If I took a tuning fork and wanted to get it to the quantum ground state, I would have to cool it below 50 billionths of a kelvin,” he explains. “There is no technology that will allow you to do that, not now. But if you push the frequency of that tuning fork up” by orders of magnitude, “then you only have to cool it to 50 millionths of a degree above absolute zero.”
Thus, by choosing a material that vibrated at extremely high frequencies — in this case, 6 billion times a second — Cleland and colleagues were able to use a commercially available refrigerator to reach the quantum ground state, because they didn’t have to cool the system as much as they would with a material at lower frequency.
The researchers also figured out a way to measure activity using a quantum bit — a unit of quantum information—rather than light, which can impart energy back into the cooled-down system. “The real key for us getting this experiment to work was using this particular flavor of a quantum bit,” says Cleland.
In the end, the system that showed quantum behavior is a simple-looking film of aluminum nitride layered between two aluminum electrodes. Cleland and colleagues were able to show not only that the device had reached its quantum ground state, but that they could control it. The scientists created a phonon, the smallest measure of vibrational energy, and watched as it moved back and forth between the resonating device and the quantum bit, they report in a paper published online March 17 in Nature.
“There is huge potential for using these mechanical systems in the quantum regime,” says Aspelmeyer. “Now we have to exploit all the possibilities that we have.”
Potential applications, he says, include using arrays of these resonators to control multiple quantum systems in information processing or to test predictions about “Schrödinger cat” states — named for a hypothetical feline simultaneously alive and dead — in which a system exists in a mix of states known as a superposition. Cleland’s team showed, somewhat indirectly, that a form of superposition existed inside their resonator. If the researchers could make a resonator with longer-lasting vibrations, scientists might be able to test superposition on the macroscopic scale.