Scientists have just shown how to make a quantum computer using sound waves

Scientists have just shown how to make a quantum computer using sound waves

A strange and wonderful array of technologies are vying to become the standard bearer of quantum computing. The last contender wants to encode quantum information into sound waves.

One thing that all quantum computers have in common is that they manipulate information encoded in quantum states. But that’s where the similarities end, because those quantum states can be induced in everything from superconducting circuits to trapped ions to supercooled atoms, photons and even silicon chips.

While some of these approaches attracted more investment than others, they were still a long way from establishing the industry on a common platform. And in the world of academic research, experimentation still abounds.

Now, a team from the University of Chicago hasSt took the first crucial steps toward building a quantum computer that can encode information into phonons, the fundamental quantum units that make up sound waves in much the same way way that photons form beams of light.

The basic principles of how to create a phonon quantum computer are quite similar to those used in photonic quantum computers. Both involve the generation and detection of single particles, or quasiparticles, and their manipulation using beamsplitters and phase shifters. Phonons are quasiparticles, because although they behave like particles as far as quantum mechanics is concerned, they are actually made up of the collective behavior of large numbers of atoms.

The Chicago group had already proven that they could generate single phonons using surface acoustic waves, which travel along the surface of a material at frequencies about a million times higher than a human can hear, and use them to transfer quantum information between two superconducting qubits.

But in a new card inside Science, researchers demonstrate the first phonon beamsplitter, which, as the name suggests, is designed to split acoustic waves. This component is a key ingredient for a phonon quantum computer as it allows exploiting quantum phenomena such as superposition, entanglement and interference.

Their configuration involves two superconducting qubits fabricated on flat pieces of sapphire, joined together by a lithium niobate channel. Each qubit is connected via a tunable coupler to a device called a transducer, which converts electrical signals into mechanical signals.

This is used to generate vibrations that create individual phonons in the channel connecting the qubits, which has a beam splitter made up of 16 parallel metal fingers at its center. The entire setup is cooled to just above absolute zero.

To demonstrate the capabilities of their system, the researchers first excited one of the qubits to make it generate a single phonon. This traveled down the channel to the beamsplitter, but since quantum particles like phonons are basically indivisible, instead of splitting it went into a quantum superposition.

This refers to skill From a quantum system must be in several states at once, until they are measured and collapse into one of the possibilities. In this case the phonon was both reflected back to the original qubit and transmitted to the second qubit, which was able to capture the phonon and store the quantum superposition.

In a second experiment, the researchers managed to replicate a quantum phenomenon fundamental to how logic gates in photonic quantum computers are created called the Hong-Ou-Mandel effect. In optical setups, this results in two identical photons being input simultaneously into a beam splitter from opposite directions. Both then enter an overlap, but these outputs interfere with each such that both photons end up traveling together to only one of the detectors.

Researchers have shown that they can replicate this effect using phonons and, more importantly, that they can use qubits to alter the characteristics of the phonons so that they can control which direction the output travels. computer, says Andrew Cleland, who led the study.

The successful two-phonon interference experiment is the final piece showing that phonons are equivalent to photons, Cleland said in a Press release. The result confirms that we have the technology we need to build a linear mechanical quantum computer.

The researchers admit that the approach is unlikely to compete directly with optical approaches to quantum computing, because the components are much larger and slower. However, their ability to interface seamlessly with superconducting qubits could make them promising for hybrid computing schemes that combine the best of both worlds.

It is likely to be a long time before the underlying components reach the sophistication and industrial readiness of other quantum approaches. But it seems the race for quantum advantage has just begunten a little more crowded.

Image credit: BroneArtUlm / Pixabay

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