A symphony in quantum

While entanglement has been demonstrated in very small particles, new research from the lab of University of Chicago Pritzker School of Molecular Engineering (UChicago PME) Prof. Andrew Cleland is thinking big, demonstrating high-fidelity entanglement between two acoustic wave resonators.

The paper was published Friday in Nature Communications.

"A lot of research groups have demonstrated that they can entangle very, very small things down to the single electron. But here we can demonstrate entanglement between two massive objects," said co-first author Ming-Han Chou, a former UChicago PME and Physics doctoral researcher now at the Amazon Web Services Center of Quantum Computing. "The second thing we demonstrate in this research is that our platform is scalable. If you can imagine building a big quantum processor, our platform would be like a unit cell within that."

The entanglement isn't between the molecules, atoms or any other particles that make up the resonators, but between the "phonons" that occupy the resonators. These are the nanoscale mechanical vibrations that, were there ears small enough to hear them, would be considered sound.

"Phonons are quantum particles of sound," said co-first author Hong Qiao, a UChicago PME postdoctoral researcher in Cleland's lab. "A phonon is not an elementary particle. It's the collective motion of maybe quadrillions of particles behaving together. This is macroscopic compared to other quantum systems where you are entangling single electrons, single atoms, single photons."

The Quantum Concerto

Entangling this collective, sound-like motion has long been a research focus for Cleland. His lab was the first to figure out how to create and detect single phonons and the first to entangle two phonons. While PhD candidates at UChicago PME, Qiao was on the research team for the latter breakthrough and Chou was involved with both.

More recently, the Department of Defense named Cleland a 2024 Vannevar Bush Faculty Fellow to pursue phonon-based quantum computing.

"The conventional wisdom has been that quantum mechanics rules physics at the smallest scale while classical physics rules the human scale," Cleland said. "But our ability to entangle massive objects by entangling their collective motion pushes that boundary. The domain where Erwin Schrödinger's cat exists gets bigger with each advance."

The device the team built is centered on two surface acoustic wave resonators, each on its own chip with its own mechanical support structure and each connected to its own superconducting qubit. The qubits are used to generate and detect the entangled phonon states. With this device, the researchers from Cleland's group showed that the large resonators could be quantum-entangled both while physically separate and with high fidelity.

"Previously, people have demonstrated there is entanglement, but with limited fidelity," Qiao said. "What we have shown here is we can go one step further to prepare more complicated entangled states, maybe even potentially add logical encodings."

The next hurdle is time, lengthening the resonator's lifetime to increase the quantum coherence. A longer-lasting entanglement would allow more powerful communication or distributed quantum computing, two major goals in building quantum networks.

"Our mechanical resonator has a relatively short lifetime, and that has quite limited the performance in this approach," Chou said. "The next step is very clear: We will try to improve the mechanical resonator lifetime."

The group hopes to extend the resonator lifetime from its current level of about 300 nanoseconds to more than 100 microseconds. It might sound daunting, but there are several existing strategies to hit this more than 300-fold increase, Chou said.

"There are different device geometries or different approaches in quantum acoustics that can already achieve such a long lifetime, but just to simplify our experiment we didn't use them in this initial research," Chou said.