Fadell’s group created a superposition of single and zero phonons in crystals. “In a sense, the crystal is both at rest and in motion,” says Fadell. To do this, they use microwave pulses to force tiny superconducting circuits to generate force fields that can be controlled with great precision. This force field pushes small pieces of material attached to the crystal and introduces single-phonon vibrations. As the largest object ever to demonstrate quantum strangeness, it has pushed physicists’ understanding of the boundary between the quantum and classical worlds.
Specifically, the experiment touches on a central mystery of quantum mechanics known as the ‘measurement problem’. According to the most popular interpretation of quantum mechanics, the act of superimposing and measuring objects using a macroscopic device (something relatively large, such as a camera or a Geiger counter) destroys superposition. For example, in the double-slit experiment, when the device is used to detect an electron, the electron is seemingly randomly fixed at one particular point rather than being observed at all positions of its potential waves.
But other physicists have proposed an illustrative alternative to quantum mechanics without measurements, known as the decay model. These postulate that the currently accepted quantum mechanics is an approximation theory. As objects get larger, as yet undiscovered phenomena prevent them from existing in superposition. And it is this, not the act of measuring superposition, that prevents us from encountering them in the world around us. Northwestern University physics professor Timothy Covacy, who wasn’t involved in the experiment, said Fadell’s experiment is about applying quantum superposition to larger objects to see what the unknown phenomenon might look like. It states that it constrains
The advantage of controlling individual vibrations in crystals is not limited to studying quantum theory, but has practical applications. Researchers are developing techniques to use the phonons of objects like Fadell’s crystals as precision sensors. For example, objects that retain individual phonons can measure the mass of very light objects, says physicist Amir Safavi Naini of Stanford University. Very light forces can induce changes in these delicate quantum states. For example, if a protein lands on a crystal similar to Fadell’s crystal, researchers can measure small changes in the crystal’s vibrational frequency to determine the mass of the protein.
Additionally, researchers are interested in using quantum oscillations to store information in quantum computers. Quantum computers store and manipulate superimposed and encoded information. Oscillations tend to persist for relatively long periods, making them good candidates for quantum memory, Safavi-Naini said. “Sound doesn’t travel in a vacuum,” he says. “Once vibrations on or inside an object reach a boundary, they stop there.” That property of sound tends to store information longer than photons, which are commonly used in prototypes of quantum computers, but research researchers need to develop phonon-based technology. (Scientists in general are still investigating commercial applications of quantum computers, but many believe that increased quantum computing power could help design new materials and medicines.)
Fadell hopes to do similar experiments with larger objects in future work. He also wants to study how gravity affects quantum states. Physicists’ gravitational theories accurately describe the behavior of large bodies, while quantum mechanics accurately describes tiny bodies. “When you think about quantum computers and quantum sensors, you inevitably end up with large-scale systems, so it’s very important to understand whether quantum mechanics breaks down for larger-sized systems,” said Fadell. say.
As researchers delved deeper into quantum mechanics, its weirdness evolved from a thought experiment to a practical problem. If we can understand where the boundary between the quantum and classical worlds lies, it will influence the development of future scientific instruments and computers. “These are basic, almost philosophical experiments,” Fadell says. “But they are also important for future technologies.”
