This is the quietest sound in the universe

Fadel’s group created a state in which the crystal contained a superposition of a single phonon and zero phonons. “In a sense, the crystal is in a state where it is still and vibrating at the same time,” says Fadel. To do this, they use microwave pulses to cause a small superconducting circuit to produce a force field that they can control with high precision. This force field pushes on a small piece of material attached to the crystal to introduce single phonons of vibration. As the largest object to exhibit quantum oddities to date, it furthers physicists’ understanding of the interface between the quantum and classical worlds.

Specifically, the experiment touches on a central mystery in quantum mechanics, known as the “measurement problem.” According to the most popular interpretation of quantum mechanics, the act of measuring an overlapping object using a macroscopic device (something relatively large, like a camera or Geiger counter) destroys the overlap. For example, in the double-slit experiment, if you use a device to detect an electron, you don’t see it in all its possible wave positions, but fixed, seemingly randomly, at a particular point.

But other physicists have proposed alternatives to help explain quantum mechanics that do not involve measurements, known as collapse models. These assume that quantum mechanics, as currently accepted, is an approximate theory. As objects get larger, some as yet undiscovered phenomenon prevents objects from existing in superposition states, and it is this, and not the act of measuring superpositions, that prevents us from finding them in the world around us. By bringing quantum superposition to larger objects, Fadel’s experiment constrains what that unknown phenomenon might be, says Timothy Kovachy, a professor of physics at Northwestern University who was not involved in the experiment.

The benefits of controlling individual vibrations in crystals extend beyond just investigating quantum theory; there are also practical applications. Researchers are developing technologies that use phonons in objects like the Fadel crystal as precise sensors. For example, objects that harbor individual phonons can measure the mass of extremely light objects, says physicist Amir Safavi-Naeini of Stanford University. Extremely light forces can cause changes in these delicate quantum states. For example, if a protein landed on a crystal similar to Fadel’s, researchers could measure small changes in the crystal’s vibrational frequency to determine the protein’s mass.

Furthermore, the researchers are interested in using quantum vibrations to store information for quantum computers, which store and manipulate encoded information in superposition. The vibrations tend to last for a relatively long time, making them a promising candidate for quantum memory, says Safavi-Naeini. “Sound doesn’t travel in a vacuum,” she says. “When a vibration on the surface of an object or in its interior hits a limit, it simply stops there.” That property of sound tends to preserve information longer than photons, commonly used in prototype quantum computers, though researchers still need to develop phonon-based technology. (Scientists are still exploring the commercial applications of quantum computers in general, but many think their increased processing power could be useful for designing new materials and drugs.)

In future work, Fadel wants to perform similar experiments on even larger objects. He also wants to study how gravity could affect quantum states. Physicists’ theory of gravity accurately describes the behavior of large objects, while quantum mechanics accurately describes microscopic objects. “If you think about quantum computers or quantum sensors, they are inevitably going to be big systems. Therefore, it is crucial to understand if quantum mechanics breaks down into larger systems,” says Fadel.

As researchers delve deeper into quantum mechanics, its weirdness has evolved from thought experiment to practical question. Understanding where the boundaries lie between the quantum and classical worlds will influence the development of future scientific devices and computers, if this knowledge can be found. “These are fundamental, almost philosophical experiments,” says Fadel. “But they are also important for future technologies.”

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