Quantum Computing

How Does the New Quantum Computer Sound? Researchers Split Phonons to Find Out

Insider Summary

  • A team of scientists used a device called an acoustic beam splitter to “split” phonons to demonstrate their quantum properties.
  • Researchers say this is the first step towards a completely new quantum computer.
  • These linear mechanical quantum computers – which would use phonons instead of photons – could themselves have the ability to perform new kinds of calculations.
  • Image: Graduate student Hong Qiao (left) and graduate student Chris Conner working in Prof. Andrew Cleland. (Photo by Joel Wintermantle)

PRESS RELEASE — When we listen to our favorite songs, what sound like continuous musical waves are actually transmitted as tiny packets of quantum particles called phonons. The laws of quantum mechanics state that quantum particles are fundamentally indivisible and therefore indivisible, but researchers at the Pritzker School of Molecular Engineering (PME) at the University of Chicago are investigating what happens when you try to separate phonons.

In two trials – the first of its kind – a team led by Professor Andrew Cleland used a device called an acoustic beam splitter to “split” the phonons and thereby demonstrate their quantum nature. By demonstrating that a beam splitter can be used to induce a state of quantum superposition unique to a single phonon, and thereby create interference between the two phonons, the research team took the first critical step toward creating a new kind of quantum computer.

The result is recently published in a journal Science and builds on years of groundbreaking work on phonons by the team at Pritzker Molecular Engineering.

Authors of the new paper include (from left) graduate student Rhys Povey, graduate student Chris Conner, graduate student Jacob Miller, graduate student Yash Joshi, graduate student Hong Qiao (lead author of the paper), graduate student Haoxiong Yan, graduate student Xuntao Wu, and researcher postdoctoral Gustav Andersson. (Photo by Joel Wintermantle)

“Splitting” the phonons into superpositions

In the experiment, the researchers used phonons that have a pitch roughly one million times higher than that which can be heard by the human ear. Previously, Cleland and his team figured out how to generate and detect single phonons and first to involve two phonons.

To demonstrate this capability of the quantum phonon, the team – including graduate student Cleland Hong Qiao – created a beam splitter that can split a sound beam in half, transmit one half and reflect the other half back to the source (a beam splitter already exists for light and has been used to demonstrate this capability). photon quantum). The entire system, including two qubits for generating and detecting phonons, operates at very low temperatures and uses individual surface acoustic wave phonons, which travel on the surface of a material, in this case lithium niobate.

However, quantum physics says one phonon is indivisible. So when the team sent one phonon to a beam splitter, instead of splitting, it went into quantum superposition, a state in which the phonon is reflected and transmitted at the same time. Observing (measuring) phonons causes these quantum states to collapse into one of two outputs.

The team found a way to maintain this superposition state by capturing phonons in two qubits. Qubits are the basic unit of information in quantum computing. Only one qubit actually captures the phonon, but the researchers couldn’t determine which qubit until post-measurement: In other words, a quantum superposition transferred from the phonon to the two qubits. The researchers measured this superposition of two qubits, yielding “the gold standard evidence that beam splitting creates quantum entangled states,” said Cleland, who is also a scientist at the US Department of Energy’s Argonne National Laboratory.

“The results confirm that we have the technology we need to build a new kind of linear mechanical quantum computer.” Andrew Cleland, John A. MacLean Sr. Professor of Innovation and Molecular Engineering Company

Displays phonons behaving like photons

In the second experiment, the team wanted to demonstrate an additional 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 from opposite directions to the beam splitter at the same time, the superposed output interferes so that the two photons are always found traveling together, in one or the other output direction.

Importantly, the same thing happened when the team carried out the experiments with phonons – the superposition output meant that only one of the two detector qubits caught the phonons, going one way but not the other. Although qubits only have the ability to pick up on one phonon at a time, not two, qubits placed in opposite directions never “hear” the phonon, providing evidence that both phonons are headed in the same direction. This phenomenon is called two-phonon interference.

Getting phonons into this quantum entangled state is a much bigger leap than doing it with photons. The phonon used here, even though it is indivisible, still requires quadrillion atoms working together in a quantum mechanical fashion. And if quantum mechanics governs physics only on the smallest planes, it raises the question of where that realm ends and classical physics begins; this experiment further investigates that transition.

“All of those atoms have to behave together in a coherent way to support what quantum mechanics tells them to do,” said Cleland. “This is amazing. The strange aspects of quantum mechanics are not limited by size.”

Building a new linear mechanical quantum computer

The strength of a quantum computer lies in the “weirdness” of the quantum realm. By harnessing the strange powers of superposition and quantum entanglement, researchers hope to solve previously intractable problems. One approach to doing this is to use photons, which are called “linear optical quantum computers”.

Linear mechanical quantum computers – which would use phonons instead of photons – could themselves have the ability to perform new kinds of calculations. “The successful two-phonon interference experiment was the last bit to show that phonons are equivalent to photons,” said Cleland. “The results confirm that we have the technology we need to build a linear mechanical quantum computer.”

Unlike photon-based linear optical quantum computing, the UChicago platform directly integrates phonons with qubits. That means the next phonon could become part of a hybrid quantum computer that combines the best of a linear quantum computer with the power of a qubit-based quantum computer.

The next step is to create logic gates – a key part of computing – using phonons, which Cleland and his team are currently doing research on.

Other authors on the paper include É. Dumur, G. Andersson, H. Yan, M.-H. Chou, J. Grebel, CR Conner, YJ Joshi, JM Miller, RG Povey, and X.Wu.

Excerpt: “Splitting phonons: Building a platform for linear mechanical quantum computing.” Qiao et al. ScienceThursday, 8 June 2023, DOI: 10.1126/science. adg8715

Funding: Air Force Office of Scientific Research, Army Research Laboratory, Department of Energy Science Office, National Quantum Information Science Research Center, National Science Foundation

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