This Is the Quietest Sound in the Universe

Fadel’s group created a state where the crystal contained a superposition of a single phonon and zero phonons. “The crystal is sort of in a state where it’s still and vibrating at the same time,” says Fadel. To do this, they use microwave pulses to create a tiny superconducting circuit that creates a force field that they can control with great precision. This force field pushes a small piece of material attached to the crystal to vibrate individual phonons. As the largest object yet to exhibit quantum weirdness, it is advancing physicists’ understanding of the interface between the quantum world and the classical world.

Specifically, the experiment touches on a central puzzle in quantum mechanics known as the “measurement problem”. According to the most common interpretation of quantum mechanics, measuring an object in superposition with a macroscopic device (something relatively large, like a camera or a Geiger counter) destroys the superposition. For example, if you detect an electron with a device in the double-slit experiment, you do not see it in all possible wave positions, but in a certain place apparently at random.

However, other physicists have proposed alternatives for explaining quantum mechanics that do not involve measurement, so-called collapse models. These assume that quantum mechanics, as currently accepted, is an approximate theory. As objects get larger, an as yet undiscovered phenomenon prevents the objects from existing in superposition states – and that it is this, and not the measurement of superpositions, that prevents us from encountering them in the world around us. By extending quantum superposition to larger objects, Fadel’s experiment narrows what this unknown phenomenon can be, says Timothy Kovachy, a professor of physics at Northwestern University who was not involved in the experiment.

The benefits of controlling single vibrations in crystals go beyond just studying quantum theory—there are also practical applications. Researchers are developing technologies that use phonons in objects like Fadel’s crystal as precise sensors. For example, objects that host single phonons can measure the mass of extremely light objects, says Stanford University physicist Amir Safavi-Naeini. 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 the small changes in the crystal’s vibrational frequency to determine the protein’s mass.

In addition, researchers are interested in using quantum vibrations to store information for quantum computers that store and manipulate information encoded in superposition. Oscillations tend to last relatively long, making them a promising candidate for quantum memory, says Safavi-Naeini. “Sound doesn’t propagate in a vacuum,” he says. “When a vibration hits a boundary on the surface or inside an object, it just stops there.” This property of sound tends to retain information longer than photons, which are commonly used in prototype quantum computers, although researchers still have phonon based technology need to develop. (Scientists are still exploring the commercial applications of quantum computing in general, but many believe their increased computing power could be useful in the development of new materials and drugs.)

In future work, Fadel hopes to conduct similar experiments on even larger objects. He also wants to study how gravity might affect quantum states. Physicists’ theory of gravitation precisely describes the behavior of large objects, while quantum mechanics precisely describes microscopic objects. “When you think of quantum computers or quantum sensors, they are inevitably large systems. Therefore, it is important to understand whether quantum mechanics fails in larger-scale systems,” says Fadel.

As researchers delve deeper into quantum mechanics, its oddity has evolved from a thought experiment to a practical question. Understanding where the boundaries lie between the quantum world and the classical world 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.”