Quantum weirdness applies to sound as well as to light and atomic particlesBy James R. Riordon
JUNE 16, 2023
Sound waves (illustrated) come in tiny quantum chunks called phonons. New experiments show how phonons exhibit the same quantum weirdness shared by photons and subatomic particles.
FLAVIO COELHO/MOMENT/GETTY IMAGES
You can’t divide the indivisible, unless you use quantum mechanics. Physicists have now turned to quantum effects to split phonons, the smallest bits of sound, researchers report in the June 9 Science.
It’s a breakthrough that mirrors the sort of quantum weirdness that’s typically demonstrated with light or tiny particles like electrons and atoms (SN: 7/27/22). The achievement may one day lead to sound-based versions of quantum computers or extremely sensitive measuring devices. For now, it shows that mind-bending quantum weirdness applies to sound as well as it does to light.
“There was no one that had really explored that,” says engineering physicist Andrew Cleland of the University of Chicago. Doing so allows researchers “to draw parallels between sound waves and light.”
Phonons have much in common with photons, the tiniest chunks of light. Turning down the volume of a sound is the same as dialing back the number of phonons, much like dimming a light reduces the number of photons. The very quietest sounds of all consist of individual — and indivisible — phonons.
Unlike photons, which can travel through empty space, phonons need a medium such as air or water — or in the case of the new study, the surface of an elastic material. “What’s really kind of, in my mind, amazing about that is that these sound waves [carry] a very, very small amount of energy, because it’s a single quantum,” Cleland says. “But it involves the motion of a quadrillion atoms that are all working together to [transmit] this sound wave.”
Phonons can’t be permanently broken into smaller bits. But, as the new experiment showed, they can be temporarily divided into parts using quantum mechanics.
Cleland and his team managed the feat with an acoustic beam splitter, a device that allows about half of an impinging torrent of phonons to pass through while the rest get reflected back. But when just one phonon at a time meets the beam splitter, that phonon enters a special quantum state where it goes both ways at once. The simultaneously reflected and transmitted phonon interacts with itself, in a process known as interference, to change where it ultimately ends up.
The lab demonstration of the effect relied on sound millions of times higher in pitch than humans can hear, in a device cooled to temperatures very near absolute zero. Instead of speakers and microphones to create and hear the sound, the team used qubits, which store quantum bits of information (SN: 2/9/21). The researchers launched a phonon from one qubit toward another qubit. Along the way, the phonon encountered a beam splitter.
Adjusting the parameters of the setup modified the way that the reflected and transmitted portions of the phonon interacted with each other. That allowed the researchers to quantum mechanically change the odds of the whole phonon turning up back at the qubit that launched the phonon or at the qubit on the other side of the beam splitter.
A second experiment confirmed the quantum mechanical behavior of the phonons by sending phonons from two qubits to a beam splitter between them. On their own, each phonon could end up back at the qubit it came from or at the one on the opposite side of the beam splitter.
If the phonons were timed to arrive at the beam splitter at the exact same time, though, they travel together to their ultimate destination. That is, they still unpredictably go to one qubit or the other, but they always end up at the same qubit when the two phonons hit the beam splitter simultaneously.
If phonons followed the classical, nonquantum rules for sound, then there would be no correlation in where the two phonons go after hitting the beam splitter. The effect could serve as the basis for fundamental building blocks in quantum computers known as gates.
“The next logical step in this experiment is to demonstrate that we can do a quantum gate with phonons,” Cleland says. “That would be one gate in the assembly of gates that you need to do an actual computation.”
Sound-based devices are not likely to outperform quantum computers that use photons (SN: 2/14/18). But phonons could lead to new quantum applications, says Andrew Armour, a physicist at the University of Nottingham in England who was not involved in the study.
“It’s probably not so clear what those [applications] are at the moment,” Armour says. “What you’re doing is extending the [quantum] toolbox…. People will build on it, and it will keep going, and there’s no sign of it stopping any time soon.”
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CITATIONSH. Qiao et al. Splitting phonons: Building a platform for linear mechanical quantum computing. Science. Vol. 380, June 9, 2023, p. 1,030. doi: 10.1126/science.adg8715.
The phonon*, which requires a medium or the surface of a medium for transmission, is the physical particle representing mechanical vibration and is responsible for the transmission of everyday sound and heat. Understanding and controlling the phononic properties of materials provides opportunities to thermally insulate buildings, reduce environmental noise, transform waste heat into electricity and develop earthquake protection. Advances in sonic and thermal diodes, optomechanical crystals, acoustic and thermal cloaking, hypersonic phononic crystals, thermoelectrics, and thermocrystals herald the next level of the technological revolution in phonon study and the associated phononics.
* phonon - in condensed-matter physics, a unit of vibrational energy that arises from oscillating atoms within a crystal. Any solid crystal, such as ordinary table salt(sodium chloride), consists of atoms bound into a specific repeating three-dimensional spatial pattern called a lattice. Because the atoms behave as if they are connected by tiny springs, their own thermal energy or outside forces make the lattice vibrate. This generates mechanical waves that carry heat and sound through the material. A packet of these waves can travel throughout the crystal with a definite energy and momentum, so in quantum mechanical terms the waves can be treated as a particle, called a phonon. A phonon is a definite discrete unit or quantum of vibrational mechanical energy, just as a photon is a quantum of electromagneticor light energy. Phonons and electrons are the two main types of elementary particles or excitations in solids. Whereas electrons are responsible for the electrical properties of materials, phonons determine such things as the speed of sound within a material and how much heat it takes to change its temperature.
In addition to their importance in the thermal and acoustic properties, phonons are essential in the phenomenon of superconductivity—a process in which certain metals such as lead and aluminum lose all their electrical resistance at temperatures near absolute zero (−273.15 °C; −459.67 °F). Ordinarily, electrons collide with impurities as they move through a metal, which results in a frictional loss of energy. In superconducting metals at sufficiently low temperatures, however, electrons—which ordinarily repel each other—slightly attract each other through the intermediate effect of phonons. The result is that the electrons move through the material as a coherent group and no longer lose energy through individual collisions. Once this superconducting state has been achieved, any initial flow of electrical current will persist indefinitely.
In 1986 a new class of materials, called high-temperature superconductors, was discovered; it is not known if the electron-phonon interaction is the basis for the superconducting behaviour of these materials.