The phonon is simultaneously both reflected and transmitted
At the University of Chicago, researchers have demonstrated that sound, like light, obeys the deepest laws of quantum mechanics — that even the smallest packet of vibration can exist in two places at once, and that two such particles can become so entangled that the fate of one instantly determines the fate of the other. This is not merely a curiosity of physics but a quiet announcement that the building blocks of a new kind of computing may be found not in light or electricity alone, but in the collective hum of matter itself. Humanity has long listened to sound; it is only now beginning to think with it.
- For over a century, phonons were mathematical ghosts — theorized but never isolated and manipulated at the quantum level, leaving a stubborn gap between prediction and proof.
- The Chicago team forced that gap closed by splitting individual, indivisible sound particles across a beam splitter, catching them in the disorienting quantum state of being simultaneously reflected and transmitted.
- They then pushed further, sending two phonons toward each other and watching them lock into entanglement — the Hong-Ou-Mandel effect, first seen with light in 1987, now replicated with sound for the first time.
- The discovery carries urgent practical weight: phonon-based quantum computers could live on a single chip, compact and integrable with existing electronic quantum systems in ways optical approaches cannot easily match.
- The laboratory door has opened, but the corridor to a working phonon computer remains long — the science is proven, the engineering has only just begun.
Light has long been known to break common sense. A single photon can exist in two places at once — a superposition — collapsing into one definite location only when observed. Sound, it turns out, plays by the same strange rules.
Sound travels as phonons, quantum particles born from the collective motion of vast numbers of atoms, the way a stadium wave rises from thousands of fans moving in sequence. Phonons were theorized over a century ago to explain how solids store heat, and the mathematics always suggested they should behave like photons. But generating and measuring individual phonons proved far harder than doing the same with light — until now.
Researchers at the University of Chicago's Pritzker School of Molecular Engineering have demonstrated that phonons can be placed into quantum superposition. Using beam splitters that reflect roughly half the sound directed at them, the team showed that a single phonon — indivisible by nature — enters a state of being simultaneously reflected and transmitted. Measurement forces it one way or the other, but before that moment, it genuinely occupies both paths.
The team then asked what happens when two identical phonons approach the splitter from opposite sides at the same instant. Rather than each going its own way, the phonons interfere quantum-mechanically and become entangled — bound so that measuring one instantly reveals the state of the other. They always exit together, never apart. This phenomenon, the Hong-Ou-Mandel effect, was first observed with photons in 1987. It has now been replicated with sound.
The implications extend well beyond fundamental physics. Because phonons arise from atomic motion within materials, a phonon-based quantum computer could theoretically fit on a single chip — compact and self-contained in a way optical systems struggle to achieve. The Chicago experiments already incorporate the same qubits used in electronic quantum computers, suggesting that phonon technology could one day be woven into existing systems, producing hybrid machines of unprecedented capability. The road from demonstration to device is long, but the first steps have been taken.
Light behaves in ways that seem to violate common sense. A single photon—the smallest unit of light energy—can exist in two places at once, a phenomenon called superposition. It can be both here and there until the moment you look, at which point it becomes definitively one or the other. For decades, physicists have known this is how light works. Sound, it turns out, follows the same rules.
Sound travels as phonons, quantum particles born from the collective motion of trillions upon trillions of atoms, much the way a stadium wave emerges from thousands of individual fans standing and sitting in sequence. When you hear music, you're listening to a stream of these infinitesimal packets. Phonons were theorized over a century ago to explain why solids hold heat the way they do, and the mathematics suggested they should obey the same quantum laws as photons. But building the tools to actually generate and measure individual phonons proved far harder than doing the same for light. That gap is only now closing.
Researchers at the University of Chicago's Pritzker School of Molecular Engineering have just demonstrated that phonons can be coaxed into the same quantum superposition states as photons. The work, published in Science, used what the team calls "bad" mirrors—beam splitters that reflect roughly half the sound directed at them and let the other half pass through. When a single phonon encounters such a splitter, something strange happens. The phonon cannot actually be divided; it is indivisible by definition. Instead, it enters a superposition state where it is simultaneously both reflected and transmitted. If you measure it, you will find it went one way or the other, each outcome equally likely. But before measurement, it exists in both states at once.
This behavior had been observed with photons for many years. Demonstrating it with sound was the first major milestone. The team then posed a harder question: what happens when two identical phonons approach the beam splitter from opposite directions at precisely the right moment? Each phonon enters its own superposition of reflected and transmitted. But because of how the beam splitter's physics works, the two phonons interfere with each other quantum-mechanically. The result is a superposition state where either both phonons exit one way or both exit the other way. The two phonons become quantum-mechanically entangled—locked together such that measuring one instantly determines the state of the other. You will never find one phonon going each direction; they move as a pair. This effect, called the Hong-Ou-Mandel effect, was first predicted and observed with photons in 1987. The Chicago team has now replicated it with sound.
The implications reach beyond fundamental physics. A quantum computer built from phonons could be radically different from the optical and electrical quantum computers now under development. Because phonons emerge from the collective motion of atoms in a material, a phonon-based quantum computer could theoretically be built entirely on a single chip, compact and self-contained like a laptop processor. The Chicago group's experiments already use qubits—the same quantum bits that power electronic quantum computers—which means that as phonon technology matures, it could potentially be integrated with existing electronic systems. The result might be a hybrid machine with computational abilities no single approach could achieve alone. The path from laboratory demonstration to practical machine remains long, but the door has opened.
Notable Quotes
A quantum computer using phonons could be very compact and self-contained, built entirely on a chip similar to that of a laptop computer's processor.— A.N. Cleland, University of Chicago
The Hearth Conversation Another angle on the story
Why does it matter that sound behaves like light at the quantum level? Isn't that just confirming a theory?
It matters because the theory was never tested at this scale. We had the math, but building the actual instruments to measure individual phonons took decades longer than it did for photons. Now that we can do it, we know the theory holds in practice.
And the beam splitter experiment—why is that significant?
It shows that phonons can exist in two states simultaneously, just like photons. That's the foundation for quantum computing. You need particles that can be in superposition to build a quantum computer.
What about the entanglement part? Two phonons locked together—how is that useful?
Entanglement is what gives quantum computers their power. When particles are entangled, measuring one instantly tells you about the other. That correlation is what lets quantum computers solve certain problems exponentially faster than classical computers.
So why build a quantum computer from sound instead of light or electricity?
Size and integration. A phonon-based computer could fit on a single chip. Light-based systems need more space. And because phonons use the same qubits as electronic computers, you could potentially merge the two technologies on one device.
How close are we to an actual phonon quantum computer?
We're still in the fundamental research phase. We've proven the physics works. Now comes the engineering—scaling it up, making it reliable, integrating it with other systems. That's years of work ahead.
What happens if this doesn't pan out?
Even if phonon computers never become practical, we've learned something true about how nature works. And the tools we're building to study phonons will have applications we probably haven't imagined yet.