Quantum computers could be built based on sound
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A crucial building block for sound-based quantum computers appears to work for the first time.
A popular way to build quantum computers is to encode information into quantum states of light particles and then send them through a maze of mirrors and lenses to manipulate that information. Andrew Cleland at the University of Chicago and his colleagues wanted to do the same with sound particles.
Sound occurs when an object or a substance, such as air, vibrates. We hear it as a continuous sound, but it’s actually a collection of tiny bits of vibration, or particles of sound, called phonons.
“Making a phonon requires quadrillions of atoms to move collectively, but in our experiment each is a single quantum object. Physicists sometimes make phonons sound like they’re just a neat trick for thinking about sound, but here they’re very real,” says Cleland.
His team built a chip-sized device with components made of perfectly conductive 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 refrigerator at a temperature of one hundredth of a Kelvin so that the photons exhibit quantum effects. Each phonon was about a million times higher than audible sound.
The researchers previously built similar chips, but now they have one component called a beamsplitter. It consists of 16 small, parallel aluminum strips that are designed so that any sound that hits them is 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 in which the entire particle was capable of being reflected and emitted simultaneously.
Cleland says this is exactly what they hoped, because this process is a necessary step for performing calculations on quantum computers that rely on light particles. To make their chip even more like a sound-based quantum computer, the researchers also successfully mimicked the way two light particles usually “talk to each other” and how their behavior is controlled during light-based calculations.
Here they simultaneously sent two phonons into the beam splitter from opposite directions and saw their respective superposition states influence each other. In the future, they will use this procedure to perform simple operations that make up computer programs.
Dirk Bouwmeester at the University of California, Santa Barbara says that for light particles, procedures such as quantum teleportation or the creation of entanglement depend on the use of beam splitters, and now they could be done with sound particles as well. “It’s really spectacular that the team was able to 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 computer chip, says Yiwen Chu at the Swiss Federal Institute of Technology in Zurich.
For Cleland, building a sound-based quantum computer is more exciting than having one more way to build a device that could eventually solve problems currently unsolvable by conventional computers. “I hope to learn how much quantum physics we can do with mechanical objects. Phonons are somehow more tangible, ‘meatier’ than light, but they exhibit the same quantum behavior. This is great for me,” he says.
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