Fadel’s group produced a state in which the crystal contained a superposition of a single and non-phonon. “In a sense, the crystal is in a state that is both stationary and vibrating,” says Fadel. To do this, they use microwave pulses to create a microscopic superconducting circuit that generates a force field that they can control with great precision. This force field pushes a small piece of material connected to the crystal to create single vibrating phonons. As the largest object exhibiting quantum weirdness to date, it advances physicists’ understanding of the interface between the quantum and classical worlds.
In particular, the experiment touches on a central mystery in quantum mechanics, known as the “measurement problem”. According to the most common interpretation of quantum mechanics, the act of measuring an object in a superposition using a macroscopic device (something relatively large, like a camera or a Geiger counter) would destroy superposition. For example, in the double slit experiment, if you use a device to detect an electron, you won’t see it at all of its potential wave positions, but fixed, seemingly random, at a specific point.
But other physicists have suggested alternative solutions to help explain quantum mechanics that don’t involve measurement, known as collapse models. These assumptions hold that quantum mechanics, as it is now accepted, is an approximate theory. As objects get larger, a number of unexplored phenomena prevent objects from remaining in a superposition—and it is this, not the act of measuring superposition, that prevents them. We see them in the world around us. Timothy Kovachy, a physics professor at Northwestern University who was not involved in the experiment, said that by pushing quantum superposition on larger objects, Fadel’s experiment limits what unknown phenomenon could be. What is that.
The benefits of controlling individual oscillations in crystals go beyond simply studying quantum theory—there are practical applications as well. Researchers are developing technologies that use phonons in objects like Fadel crystals as precision sensors. For example, objects containing individual phonons can measure the mass of extremely light objects, says Stanford University physicist Amir Safavi-Naeini. Extremely light forces can cause changes in these fragile quantum states. For example, if a protein lands on a crystal similar to Fadel’s, researchers can measure small changes in the crystal’s vibrational frequency to determine the protein’s mass.
In addition, the researchers are interested in using quantum vibration to store information for quantum computers, storing and manipulating superimposed encoded information. Safavi-Naeini said the vibrations tend to be relatively long-lived, making them a promising candidate for quantum memory. “Sound does not travel in a vacuum,” he said. “When a vibration on the surface of an object or inside it hits a boundary, it just stops there.” That property of sound tends to retain information longer than photons, which are commonly used in prototype quantum computers, although researchers still need to develop phonon-based technology. (Scientists are still exploring commercial applications of quantum computers in general, but many think their increased processing power could be useful in the design of new materials and drugs.) .)
In the future, Fadel wants to perform similar experiments on even larger objects. He also wanted to study how gravity can affect quantum states. Physicists’ theory of gravity describes the behavior of large objects precisely, while quantum mechanics accurately describes extremely small objects. “If you think about quantum computers or quantum sensors, they will certainly be large systems. So it’s important to understand whether quantum mechanics breaks down for larger sized systems,” Fadel said.
As researchers dig deeper into quantum mechanics, its weirdness has evolved from a thought experiment into a real question. Understanding where the boundary is between the quantum and classical worlds will influence the development of future scientific devices and computers—if this knowledge can be found. “These are basic, almost philosophical experiments,” says Fadel. “But they’re also important for future technologies.”
