Sound-based quantum computers could be built using chip-sized device

A crucial building block for quantum computers based on sound has been shown to work for the first time.

One popular way of building quantum computers is to encode information into quantum states of particles of light, then send them through a maze of mirrors and lenses to manipulate that information. Andrew Cleland at the University of Chicago and his colleagues set out to do the same with particles of sound.

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Sound is created when an object or a substance, like air, vibrates. We hear it as a continuous noise, but it is actually a collection of tiny chunks of vibration, or particles of sound, called phonons.

“Making a phonon requires that quadrillions of atoms move collectively, but in our experiment, each is a single quantum object. Physicists sometimes make phonons sound like they are just a convenient trick for thinking about sound, but here they are very real,” says Cleland.

His team built a chip-sized device that has components made of a perfectly conducting material and can create phonons one at a time before sending them to other parts of the device. The chip is kept in a powerful fridge at a temperature a hundredth of a kelvin so that the photons exhibit quantum effects. Each phonon was about a million times higher pitched than audible sound.

The researchers have previously built similar chips, but now they have added a component called a beam splitter. It consists of 16 tiny, parallel aluminium strips designed so that any sound that hits them gets reflected and transmitted in equal parts. But when the researchers sent a phonon into it, instead of splitting in two, it assumed a quantum superposition state where the whole particle was simultaneously in the state of being reflected and transmitted.

Cleland says that this is exactly what they hoped would happen because this process is a necessary step for running calculations on quantum computers that rely on particles of light. To make their chip even more like a sound-based quantum computer, the researchers also successfully recreated the way two particles of light are commonly made to “talk to each other” and how their behaviour is controlled during light-based computations.

Here, they simultaneously sent two phonons from opposite directions into the beam splitter and saw their respective superposition states influence each other. In the future, they will use this procedure to implement simple operations that make up computer programs.

Dirk Bouwmeester at the University of California, Santa Barbara, says that for particles of light, procedures like quantum teleportation or creating entanglement hinge on using beam splitters, and now they could be done with particles of sound as well. “It is truly spectacular that the team could replace photons with phonons,” he says.

Because many different quantum objects interact with sound, future experiments could also use phonons to transfer quantum information between different, hard-to-connect components of a quantum computing chip, says Yiwen Chu at the Swiss Federal Institute of Technology in Zurich.

For Cleland, building a sound-based quantum computer is exciting beyond having one more way to construct a device that could eventually solve problems that are currently unsolvable for conventional computers. “I hope to learn how much quantum physics we can do with mechanical objects. Phonons are somehow more tangible, more ‘meaty’ than light, but they have been showing the same quantum behaviours. This is amazing to me,” he says.