Splitting phonons has taken a step toward a new type of quantum computer

When we listen to our favorite song, what sounds like a continuous wave of music is actually traveling as tiny packets of quantum particles called phonons.

The laws of quantum mechanics state that quantum particles are fundamentally indivisible and thus cannot be divided, but researchers at the University of Chicago’s Pritzker School of Molecular Engineering (PME) are exploring what happens when you try to split a phonon.

In two experiments – the first of their kind – a team led by Professor Andrew Cleland used a device called an acoustic beam splitter to ‘split’ the phonons and then show them their quantum properties. By showing that a beam transmitter can be used to induce a special quantum superposition state for a single phonon, as well as create interference between two phonons, the research team has taken the first crucial steps toward creating a new type of quantum computer.

The results are published in the journal Sciences And it built on years of groundbreaking work on phonons by the team at Pritzker Molecular Engineering.

“split” a phonon into a superposition

In the experiments, the researchers used phonons with a pitch roughly a million times louder than what can be heard by the human ear. Previously, Cleland and his team figured out how to create and detect individual phonons and were the first to link two phonons.

To prove the quantum capabilities of these phonons, the team—including Cleland graduate student Hong Qiao—created a device to split a sound beam in half, transmit the other half and reflect the other half back to its source (ray splitters already exist for light and have to demonstrate the quantum capabilities of photons). The entire system, including two qubits for generating and detecting phonons, operates at extremely low temperatures and uses individual surface acoustic wave phonons, which travel on the surface of a material, in this case lithium niobates.

However, quantum physics says that a single phonon is indivisible. So when the team sent a single phonon to the ray transmitter, instead of splitting, it entered a quantum superposition state, a state in which a phonon is reflected and transmitted at the same time. Observing (measuring) the phonon causes this quantum state to collapse into one of the two exits.

The team found a way to maintain this superposition state by capturing the phonon in two qubits. A qubit is the basic unit of information in quantum computing. Only one qubit captures the phonon, but the researchers can’t tell which qubit is even after measurement. In other words, the quantum superposition is transferred from the phonon to the two qubits. The researchers measured this superposition of two qubits, yielding “gold standard evidence that the beam splitter creates a quantum entangled state,” Cleland said.

Show phonons behave like photons

In the second experiment, the team wanted to demonstrate an additional fundamental quantum effect that was first demonstrated using photons in the 1980s. Now known as the Hong-Ou-Mandel effect, when two identical photons from opposite directions are sent to a beam splitter at the same time, the superimposed outputs are so that both photons are always found traveling together, in one or the other of the output directions.

More importantly, the same thing happened when the team ran the experiment with phonons — the superimposed output means that only one of the two detector qubits picks up the phonons, going in one direction but not the other. Although qubits only have the ability to pick up one phonon at a time, not two, qubits placed in the opposite direction never “hear” a phonon, which indicates that both phonons travel in the same direction. This phenomenon is called two-phonon interference.

Getting phonons into quantum entanglement is a much bigger leap than doing it with photons. The phonons used here, despite being indivisible, still require quadrillionths of atoms working together in the manner of quantum mechanics. And if quantum mechanics governs physics only in the smallest world, it raises questions about where this world ends and classical physics begins; This experiment makes this transition further.

“These atoms must behave together coherently to support what quantum mechanics says they should do,” Cleland said. “It’s kind of amazing. The weird aspects of quantum mechanics aren’t just about size.”

Creation of a new linear quantum mechanical computer

The strength of quantum computers lies in the “strangeness” of the quantum realm. By harnessing the strange quantum forces of superposition and entanglement, researchers hope to solve previously intractable problems. One way to do this is to use photons, in what’s called a “linear optical quantum computer”.

A linear mechanical quantum computer — which uses phonons instead of photons — could have the potential to compute new types of computations. “The success of the phonon interference experiment is the latest piece to show that phonons are equivalent to photons,” Cleland said. “The result confirms that we have the technology we need to build a linear quantum mechanical computer.”

Unlike photon-based linear optical quantum computing, the University of Chicago platform directly integrates phonons with qubits. This means that phonons could also be part of a hybrid quantum computer that combines the best of linear quantum computers with the power of qubit-based quantum computers.

The next step is to create a logic gate — an essential part of computing — using phonons, which Cleland and his team are currently researching.

Other authors on the paper are É. Dumore, J. Anderson, H. Yan, M. -H. Zhou, J. Greibel, CR Conner, YJ Joshi, JM Miller, RJ Buffay, and X Wu.