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When we listen to our favorite song, what sounds like a continuous wave of music is actually transmitted as tiny packets of quantum particles called phonons.
The laws of quantum mechanics hold that quantum particles are fundamentally indivisible and therefore cannot be split, 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 Prof. Andrew Cleland used a device called an acoustic beamsplitter to ‘split’ phonons and thus demonstrate their quantum properties. By showing that the beamsplitter can be used both to induce a special quantum superposition state for a phonon and to create further interference between two phonons, the research team has taken the first critical steps towards creating a new type of quantum computer.
The results are published in the journal Science and built on years of groundbreaking work on phonons by the Pritzker Molecular Engineering team.
“Splitting” of a phonon in a superposition
In the experiments, the researchers used phonons that are about a million times higher in pitch than the human ear can hear. Earlier, Cleland and his team discovered how to create and detect single phonons and were the first to entangle two phonons.
To demonstrate the quantum capabilities of these phonons, the team, including Cleland’s graduate student Hong Qiao, created a beam splitter capable of splitting a beam of sound in half, transmitting half and reflecting the other half back to its source. (beam splitters already exist for light and have been used to demonstrate the quantum capabilities of photons). The entire system, including two qubits to generate and detect the phonons, operates at extremely low temperatures and uses single phonons of surface acoustic waves, which travel on the surface of a material, in this case lithium niobate.
However, quantum physics states that a single phonon is indivisible. So when the team sent a single phonon to the beamsplitter, instead of splitting, it entered a quantum superposition, a state in which the phonon is reflected and transmitted at the same time. Observing (measuring) the phonon collapses this quantum state into one of two outputs.
The team found a way to maintain that superposition state by capturing the phonon in two qubits. A qubit is the basic unit of information in quantum computing. Only one qubit actually captures the phonon, but researchers can’t tell which qubit until after the measurement. In other words, the quantum superposition is transferred from the phonon to the two qubits. The researchers measured this overlap of two qubits, providing “gold standard evidence that the beamsplitter is creating a quantum entangled state,” Cleland said.
Show that phonons behave like photons
In the second experiment, the team wanted to show another fundamental quantum effect that was first demonstrated with photons in the 1980s. Now known as the Hong-Ou-Mandel effect, when two identical photons are sent simultaneously from opposite directions into a beam splitter, the overlapping outputs interfere so that both photons are always found traveling together, in one direction or the other of exit.
Importantly, the same happened when the team conducted the phonon experiment: the overlapping output means that only one of the detector’s two qubits captures phonons, going in one direction but not the other. Although qubits only have the ability to capture a single phonon at a time, not two, the qubit positioned in the opposite direction never “feels” a phonon, demonstrating that both phonons are going in the same direction. This phenomenon is called two-phonon interference.
Getting phonons into these quantum entangled states is a much bigger leap than doing it with photons. The phonons used here, while indivisible, still require quadrillions of atoms working together quantum mechanically. And if quantum mechanics rules physics only in the smallest realm, it raises questions about where that realm ends and classical physics begins; this experiment further probes that transition.
“These atoms all have to behave consistently together to support what quantum mechanics says they should do,” Cleland said. “It’s quite amazing. The bizarre aspects of quantum mechanics aren’t limited by size.”
Creation of a new linear mechanical quantum computer
The power of quantum computers lies in the “strangeness” of the quantum realm. By harnessing the strange quantum powers of superposition and entanglement, researchers hope to solve previously unsolvable problems. One approach to doing this is to use photons, in what’s called a “linear optical quantum computer.”
A linear mechanical quantum computer that would use phonons instead of photons could have the ability to compute new types of calculations. “The successful two-phonon interference experiment is the final piece showing that phonons are equivalent to photons,” Cleland said. “The result confirms that we have the technology we need to build a linear mechanical quantum computer.”
Unlike photon-based linear optical quantum computing, the University of Chicago’s platform directly integrates phonons with qubits. This means that phonons could further 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 computer science, using phonons, which Cleland and his team are currently researching.
Other authors on the paper include . Dumur, G. Andersson, H. Yan, M.‑H. Chou, J. Grebel, CR Conner, YJ Joshi, JM Miller, RG Povey and X. Wu.
H. Qiao et al, Phonon Splitting: Building a Platform for Linear Quantum Mechanical Computing, Science (2023). DOI: 10.1126/science.adg8715. www.science.org/doi/10.1126/science.adg8715
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