September 23, 2023
Technology applications of quantum nanoscience

Scientists Harness Quantum Mechanics to Split Sound Waves into Indivisible Units, Opening Path to Sound-Based Quantum Technologies

In a groundbreaking development, physicists have successfully utilized quantum effects to split phonons, the smallest units of sound, according to a report published in the journal Science on June 9. This achievement mirrors the quantum weirdness often observed with light or subatomic particles like electrons and atoms, demonstrating that the mind-bending principles of quantum mechanics are applicable to sound as well.

The breakthrough holds tremendous potential for the development of sound-based versions of quantum computers and highly sensitive measuring devices. By delving into the quantum properties of sound, researchers may unlock new avenues for technological advancements. The study represents a significant step toward understanding the parallels between sound waves and light.

Phonons, the fundamental particles of sound, share similarities with photons, the smallest units of light. Just as reducing the volume of sound corresponds to decreasing the number of phonons, dimming a light source reduces the number of photons emitted. The quietest sounds comprise individual and indivisible phonons.

However, unlike photons, which can traverse empty space, phonons require a medium like air, water, or, in the case of the study, the surface of an elastic material. What makes this discovery fascinating is that sound waves carry an exceptionally small amount of energy, as they consist of a single quantum, yet they involve the synchronized motion of quadrillions of atoms working together to transmit the sound wave.

Although phonons cannot be permanently divided into smaller entities, the researchers demonstrated that they can be temporarily separated using quantum mechanics. They accomplished this feat through an acoustic beam splitter, a device that allows approximately half of a stream of phonons to pass through while reflecting the remainder. However, when a single phonon encounters the beam splitter, it enters a special quantum state in which it simultaneously travels in both directions. The phonon interacts with itself through a phenomenon called interference, ultimately influencing its final destination.

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The laboratory experiment involved sound waves with frequencies millions of times higher than the range perceivable by humans, conducted in an environment cooled to temperatures approaching absolute zero. Instead of traditional speakers and microphones, the researchers employed qubits, which store quantum information. They launched a phonon from one qubit toward another, with the phonon encountering the beam splitter along the way.

By adjusting the experimental parameters, the scientists could modify the interaction between the reflected and transmitted portions of the phonon, exploiting quantum mechanics to alter the probability of the phonon ending up either back at the originating qubit or at the qubit on the opposite side of the beam splitter.

A subsequent experiment confirmed the quantum behavior of the phonons by sending phonons from two qubits to a beam splitter placed between them. When the phonons were timed to arrive at the beam splitter simultaneously, they traveled together to the same destination. While the ultimate outcome was still unpredictable, with the phonons randomly going to one qubit or the other, they consistently ended up at the same qubit when they hit the beam splitter simultaneously.

In classical sound theory, where quantum effects do not apply, there would be no correlation between the paths taken by the two phonons after encountering the beam splitter. This finding has the potential to serve as the foundation for essential components of quantum computers, known as gates.

The successful manipulation of sound waves at the quantum level represents a significant achievement and could pave the way for advancements in sound-based quantum technologies. It offers new avenues for exploring the intersection of quantum mechanics and acoustics, bringing researchers closer to harnessing the full potential of quantum phenomena in the realm of sound.