Explained | Are phonons, sound particles, also quantum?

The motion of an insect on the surface of the water creates two sets of waves that interfere with each other, creating an interference pattern visible in the 11 to 12 o’clock region, New Mexico, May 29, 2019. | Photo Credit: Mike Lewinski/Unsplash

One of the two big news these days in the field of computing is quantum computers (the other is artificial intelligence). Recently, IBM published an article in which he claimed to have shown that a quantum computer could solve a useful problem that today’s conventional computers can’t, a result deserved by concerns that their calculations might become too unreliable when they also become complicated.

What are qubits?

Quantum computers use qubits as their basic units of information. A qubit can be a particle, like an electron; a collection of particles; or a quantum system designed to behave like a particle. Particles can do weird things that large objects like the semiconductors in classical computers cannot because they are guided by the rules of quantum physics. These rules allow each qubit to have the values ​​’on’ and ‘off’ at the same time, for example.

The premise of quantum computing is that information can be ‘encoded’ into some property of the particle, such as the spin of an electron, and then processed using these peculiar abilities. As a result, quantum computers are expected to perform complicated calculations that are beyond the reach of today’s best supercomputers.

Other forms of quantum computing use other units of information. For example, linear optical quantum computing (LOQC) uses photons, the particles of light, as qubits. Just as different pieces of information can be combined and processed by encoding them in electrons and then having the electrons interact in different ways, LOQC offers the use of optical equipment, such as mirrors, lenses, splitters, wave plates, etc., with photons. to process information. .

In fact, any particle that can be controlled and manipulated by quantum mechanical phenomena should, on paper, be usable as a unit of information in a quantum computer.

What are phonons?

That is why physicists wonder if they can also use phonons. Photons are packets of light energy; Similarly, phonons are packets of vibrational energy. So the question is: can we build a quantum computer whose unit of information is, colloquially speaking, sound?

according to a paper published in Science this month, it should be possible.

The problem is that researchers can manipulate electrons using electric currents, magnetic fields, etc., and they can manipulate photons with mirrors, lenses, etc., but what can they manipulate phonons with? To this end, in the new study, researchers at the University of Chicago reported developing an acoustic beam splitter.

What is a beam splitter?

Beam splitters are widely used in optical research. Imagine a torchlight shining along a straight line. This is basically a stream of photons. When a beam splitter is placed in the path of the light, it will split the beam in two: that is, it will reflect 50% of the photons to one side and let the other 50% pass directly.

While it seems simple, how a beam splitter actually works is based on quantum physics. If you shine a million photons at it, it will create two beams, each containing 500,000 photons. We can then reflect these two beams so that they cross each other, creating an interference pattern (remember Young’s double-slit experiment). But the researchers have discovered that an interference pattern appears even when they shine photons into the beam splitter one by one. What do photons interfere with? The answer is themselves.

This is because a) particles can also behave like waves, and b) until an observation is made, a quantum system exists in a superposition of all its possible states (such as a qubit that is partly “on” and partly “off”). ” at the same time). time). So when the single wave interacts with the beam splitter, it enters into a superposition of the two possible outcomes: reflected and transmitted. When these states are recombined, a interference pattern appears.

What did the new study do?

In the new study, the researchers developed an acoustic beam splitter, a tiny device resembling a comb with 16 metal bars protruding from it. It was placed in the middle of a 2 mm long lithium niobate channel. Each end of the channel had a superconducting qubit, a qubit whose circuit components were superconducting, that could emit and detect individual phonons. The entire setup was kept at an ultra-low temperature.

If these phonons were converted to sound, their frequency would be too high for humans to hear. Each phonon in the study represented, according to the article, the “collective” vibration of around one quadrillion atoms.

The team found that these phonons interacted with the comb just like photons interact with an optical beam splitter. When a phonon was emitted from the left side of the channel, it was reflected half the time and transmitted to the right side the other half. When phonons were emitted simultaneously from the left and right sides, they both ended up on one side (as expected).

A computer based on phonons…?

“The basic scientific question is whether phonons… really do behave the way quantum mechanics says they should,” said Andrew Cleland, a physicist at the Pritzker School of Molecular Engineering and a member of the study team. Physical magazine. The tests of his equipment showed that it was.

But there is still a long way to go from here to a working quantum computer that uses phonons as units of information. As University of Nottingham physicist Andrew Armor put it, in more general terms, science news: “What you are doing is expanding the (quantum) toolbox… People will build on it, and it will continue to work, and there are no signs of stopping anytime soon.”

• IBM published a paper claiming to have shown that a quantum computer could solve a problem. useful problem that today’s conventional computers can’t, a result deserved by concerns that their calculations might become too unreliable when they also become complicated.

• The team found that these phonons interacted with the comb just like photons interact with an optical beam splitter.

• Beam splitters are widely used in optical research. Imagine a torchlight shining along a straight line. This is basically a stream of photons. When a beam splitter is placed in the path of the light, it will split the beam in two: that is, it will reflect 50% of the photons to one side and let the other 50% pass directly.