How splitting sound could lead to a new kind of quantum computer

When you turn on a lamp to light up a room, you’re experiencing light energy transmitted as photons, which are small, discrete quantum packets of energy. These photons have to obey the sometimes strange laws of quantum mechanics, which, for example, dictate that photons are indivisible, but at the same time allow a photon to be in two places at once.

Similar to the photons that make up light rays, indivisible quantum particles called phonons form a beam of sound. These particles emerge from the collective motion of quadrillions of atoms, just as the ripple of a stadium in a sports arena is due to the motion of thousands of individual fans. When you hear a song, you hear a flow of these very small quantum particles.

Originally conceived to explain the thermal capabilities of solids, phonons are predicted to obey the same quantum mechanical rules as photons. The technology for generating and detecting single phonons, however, has lagged behind that for photons.

That technology is only now being developed, in part by my research group at the Pritzker School of Molecular Engineering at the University of Chicago. We’re exploring the fundamental quantum properties of sound by splitting phonons in half and twisting them together.

My group’s fundamental research on phonons may one day allow researchers to build a new type of quantum computer, called a mechanical quantum computer.

Sound splitting with poor mirrors

To explore the quantum properties of phonons, our team uses acoustic mirrors, which can direct sound beams. Our latest experiments, published in a recent issue of Science, however, involve bad mirrors, called beam splitters, that reflect about half of the sound sent back to them and let the other half through. Our team set out to explore what happens when we direct a phonon towards a beam splitter.

Since a phonon is indivisible; it cannot be divided. Instead, after interacting with the beam splitter, the phonon ends up in what is called a superposition state. In this state the phonon is, paradoxically, both reflected and transmitted, and you are equally likely to detect the phonon in both states. If you step in and detect the phonon, you’ll measure half the time it was reflected and half the time it was transmitted; in a sense, the state is chosen at random by the detector. In the absence of the detection process, the phonon will remain in the superposition state of being transmitted and reflected.

A short Ted-Ed explanation on superposition, which occurs when particles can exist in multiple places at the same time.

This superposition effect was observed many years ago with photons. Our results indicate that phonons have the same property.

Entangled phonons

After demonstrating that phonons can enter quantum superpositions just like photons, my team asked a more complex question. We wanted to know what would happen if we sent two identical phonons into the beam splitter, one in each direction.

It turns out that each phonon will go into a similar superposition state of transmitted medium and reflected medium. But because of the physics of the beam splitter, if we time the phonons precisely, they will interfere quantum mechanically with each other. What emerges is actually a superposition state of two phonons going in one direction and two phonons going in the other, the two phonons are therefore quantum entangled.

In quantum entanglement, each phonon is in a superposition of reflected and transmitted, but the two phonons are locked together. This means that detecting one phonon as transmitted or reflected forces the other phonon to be in the same state.

So if you detect, you will always detect two phonons, going one way or the other, never one phonon going both ways. This same effect for light, the combination of overlapping and interference of two photons, is called the Hong-Ou-Mandel effect, after the three physicists who first predicted and observed it in 1987. Now, my group has demonstrated this effect with sound.

The future of quantum computing

These results suggest that it may now be possible to build a mechanical quantum computer using phonons. There are ongoing efforts to build optical quantum computers that require only single photons to be emitted, sensed, and interfered with. These parallel efforts to build electric quantum computers, which through the use of large numbers of entangled particles promise exponential acceleration for some problems, such as factoring large numbers or simulating quantum systems.

A quantum computer using phonons could be very compact and self-contained, built entirely on a chip similar to the processor in a laptop computer. Its small size could make it easier to implement and use, if researchers can further expand and improve phonon-based technologies.

My group’s experiments with phonons use qubits, the same technology that powers electronic quantum computers, which means that as technology for phonons catches up, there is the potential to integrate phonon-based computers with electronic quantum computers. This could produce potentially unique new computational capabilities.The conversation

Andrew N. Cleland, Professor of Molecular Engineering Innovation and Enterprise, University of Chicago Pritzker School of Molecular Engineering

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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