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While many plans for quantum computers transmit data using the particles of light known as photons, researchers from the University of Chicago Pritzker School of Molecular Engineering (UChicago PME) are turning to sound.

In out today in Nature Â鶹ÒùÔºics, a team uniting UChicago PME's experimentalist Cleland Lab and theoretical Jiang Group demonstrated deterministic phase control of phonons, tiny mechanical vibrations that, on a much larger scale, would be considered sound.

By removing the randomness inherent in photon-based systems, this phase control could give sound an edge over light in building tomorrow's quantum computers.

"The deterministic nature of our phonon platform implies this may prove a better platform for quantum computing than photons, although there are still many open questions," UChicago PME Prof. Andrew Cleland said.

By scattering a phonon off a superconducting qubit—the quantum equivalent of the data "bit" that makes up a regular computer—and mediating the electrical interaction, the UChicago PME researchers were able to deterministically control the phonon's phase. This means they can send phonon-based data through a quantum computer free from the randomness that, by nature, will always hinder platforms based on photons.

Theoretically, this could lead to systems as fast and powerful as the best quantum computers, but as predictable as the laptop on your desk.

"Having this deterministic quantum operations gives this hybrid platform an advantage over pure linear optical approaches," said first author Hong Qiao, a postdoctoral researcher in Cleland's lab.

Deterministic vs. probabilistic

Deterministic systems are simple cause-and-effect with no randomness involved. This means that, unless something is broken, if scientists do A, they'll always get B.

But many aspects of quantum mechanics are probabilistic, also known as non-deterministic. This means that even in a perfectly working system, there's a certain chance researchers won't get the result they want. If scientists do A, they could get B. Or they could get C, maybe D or beyond depending on the type of quantum system they're using.

Engineering better systems can improve the likelihood of success, but there's always some randomness—and researchers can't find out if a process worked until after the fact.

"Based on your measurement result of optical photons, you can determine whether this operation was successful or not," Qiao said. "And it's not like if you do this operation, you get what you want. It's more like you do the operation, you measure and, based on your measurement result, you have certain probability of success or failure."

The ability to control phonons' phases could be applicable to a wide variety of computer architectures, including a novel quantum random access memory system the same team developed earlier this year.

"Quantum phononics is advancing as a field, with new theoretical architectures allowing for increasingly compact devices and enabling integration at larger scales," said UChicago PME Prof. Liang Jiang.

Brief lives

The next hurdle for the team is increasing the phonons' lifespans, making the vibrations last long enough to be useful for quantum computing.

"Currently, these phonons have lifetimes in the microsecond range," Qiao said. "We would like to give this phonon a lifetime maybe 100 times longer to be useful for computational tasks."

That goal, while ambitious, is well within the theoretical possibilities, said co-author Zhaoyou Wang, a postdoctoral researcher in Jiang's theoretical group. The reason the lifespan is so short is because the phonons are coupled to a qubit. This connection, while allowing the team to control the phonons, vastly shortens the vibrations' lifespan, like grabbing a ringing bell to muffle it.

On their own, phonons can have coherence times up to seconds, far beyond what photons can muster. That's because light glows, emitting itself into the subatomic void. Vibrations just vibrate by themselves.

"Photons are an electromagnetic wave, so there are many leakage channels to the outside," Wang said. "Phonons will decay if you contact them, but they don't leak into the vacuum. In principle, if you have very well-isolated phononic resonators, and if you don't contact them, it doesn't give you decay."

The current short lifespan limits scaling to around 10 phonons, Cleland said. But he said the team hopes to address the loss limitation in a future work.

"We're excited by the advances in this paper, which point to the future possibility of using phonons for quantum computing in a completely solid-state, chip-based format," Cleland said.