Phonons may be chiral: Study claims to settle debate

Phonons may be chiral: Study claims to settle debate

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To demonstrate the existence of chiral phonons, the researchers used X-ray resonant inelastic scattering (RIXS). Circularly polarized light shines on the quartz. The angular momentum of the photons is transferred to a crystal, in this case causing anions (orange spheres with p-orbitals) to revolution with respect to neighboring cations (green spheres). Credits: Paul Scherrer Institute / Hiroki Ueda and Mahir Dzambegovic

Results published in Nature settle the dispute: phonons can be chiral. This fundamental concept, discovered using circular X-ray light, sees phonons twisting like a corkscrew through quartz.

Throughout nature, at all scales, you can find examples of chirality or handedness. Imagine trying to eat a sandwich with two hands that weren’t enantiomers, non-superimposable mirror images of each other. Consider the drug disasters caused by administering the wrong drug enantiomer or, on a subatomic scale, the importance of the concept of parity in particle physics. Now, thanks to a new study led by researchers at the Paul Scherrer Institute PSI, we know that phonons can also possess this property.

A phonon is a quasiparticle that describes the collective vibrational excitations of atoms in a crystal lattice; imagine it as the Irish Riverdance of the atoms. Physicists have predicted that if phonons can demonstrate chirality they could have important implications for the fundamental physical properties of materials. With the rapid increase in recent years of research into topological materials exhibiting curious electronic and magnetic surface properties, interest in chiral phonons has grown. However, experimental proof of their existence has remained elusive.

What makes phonons chiral is their dance steps. In the new study, atomic vibrations dance in a twist that moves forward like a corkscrew. This corkscrew movement is one of the reasons there has been such a push to discover the phenomenon. If phonons can spin like this, like the coil of wire that forms a solenoid, perhaps they could create a magnetic field in a material.

A new perspective on the problem

It is this possibility that motivated Urs Staub’s group at PSI, which led the study. “It’s because we’re at the junction of ultrafast X-ray science and materials research that we might be approaching the problem from a different perspective,” he says. Researchers are interested in manipulating the chiral modes of materials using circularly polarized chiral light.

He was using such light that the researchers could make their own test. Using quartz, one of the best-known minerals whose atoms silicon and oxygen form a chiral structure, they showed how circularly polarized light couples to chiral phonons. To do this, they used a technique known as resonant inelastic X-ray scattering (RIXS) at the Diamond Light Source in the UK. This was complemented with supporting theoretical descriptions of how the process would create and enable the detection of chiral phonons by groups from ETH Zurich (Carl Romao and Nicola Spaldin) and MPI Dresden (Jeroen van den Brink).

“It doesn’t usually work like that in science”

In their experiment, circularly polarized light shines on the quartz. Photons of light possess angular momentum, which they transfer to the atomic lattice, launching the vibrations in their corkscrew motion. The direction in which the phonons spin depends on the intrinsic chirality of the quartz crystal. As phonons spin, they release energy in the form of scattered light, which can be detected.

Imagine standing on a roundabout and throwing a Frisbee. If you throw the Frisbee in the same direction of movement as the roundabout, you would expect it to zip. Throw it the other way and it will spin less, as the angular momentum of the roundabout and the Frisbee cancel each other out. Similarly, when circularly polarized light twists in the same way as the phonon it excites, the signal is enhanced and chiral phonons can be detected.

A well-planned experiment, accurate theoretical calculations and then something strange happened: almost everything went according to plan. As soon as they analyzed the results, the difference in the response to the light chirality flip was undeniable.

“The results were convincing almost immediately, especially when we compared the difference with the other enantiomers of quartz,” recalls Hiroki Ueda, PSI scientist and first author of the publication. Sitting at his computer analyzing the data, Ueda was the first to see the results: “I kept checking my analysis codes to make sure it was true.” Staub points out, “That’s not normal! It doesn’t usually work like that in science!”

While searching for chiral phonons, there were several false alarms. Will this settle the debate? “Yes, I think so, that’s the beauty of this work,” believes Staub, whose opinion was shared by reviewers of Nature. “Because it’s simple and beautiful and straightforward. It’s obvious. It’s so simple, it’s obvious that this is chiral motion.”

More information:
Hiroki Ueda et al, Chiral phonons in quartz probed by X-rays, Nature (2023). DOI: 10.1038/s41586-023-06016-5

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Researchers have ‘split’ phonons in the move towards a new kind of quantum computer

Researchers have 'split' phonons in the move towards a new kind of quantum computer

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Artist’s impression of a platform for linear quantum mechanical computing (LMQC). The central transparent element is a phonon beam splitter. The blue and red marbles represent individual phonons, which are the collective mechanical motions of quadrillions of atoms. These mechanical motions can be visualized as surface acoustic waves entering the beam splitter from opposite directions. Two-phonon interference at the beamsplitter is critical to LMQC. The output phonons emerging from the image are in a two-phonon state, with a “blue” and a “red” phonon clustered together. Credit: Peter Allen

When we listen to our favorite song, what sounds like a continuous wave of music is actually transmitted as tiny packets of quantum particles called phonons.

The laws of quantum mechanics hold that quantum particles are fundamentally indivisible and therefore cannot be split, but researchers at the University of Chicago’s Pritzker School of Molecular Engineering (PME) are exploring what happens when you try to split a phonon.

In two experiments, the first of their kind, a team led by Prof. Andrew Cleland used a device called an acoustic beamsplitter to ‘split’ phonons and thus demonstrate their quantum properties. By showing that the beamsplitter can be used both to induce a special quantum superposition state for a phonon and to create further interference between two phonons, the research team has taken the first critical steps towards creating a new type of quantum computer.

The results are published in the journal Science and built on years of groundbreaking work on phonons by the Pritzker Molecular Engineering team.

“Splitting” of a phonon in a superposition

In the experiments, the researchers used phonons that are about a million times higher in pitch than the human ear can hear. Earlier, Cleland and his team discovered how to create and detect single phonons and were the first to entangle two phonons.

To demonstrate the quantum capabilities of these phonons, the team, including Cleland’s graduate student Hong Qiao, created a beam splitter capable of splitting a beam of sound in half, transmitting half and reflecting the other half back to its source. (beam splitters already exist for light and have been used to demonstrate the quantum capabilities of photons). The entire system, including two qubits to generate and detect the phonons, operates at extremely low temperatures and uses single phonons of surface acoustic waves, which travel on the surface of a material, in this case lithium niobate.

However, quantum physics states that a single phonon is indivisible. So when the team sent a single phonon to the beamsplitter, instead of splitting, it entered a quantum superposition, a state in which the phonon is reflected and transmitted at the same time. Observing (measuring) the phonon collapses this quantum state into one of two outputs.

The team found a way to maintain that superposition state by capturing the phonon in two qubits. A qubit is the basic unit of information in quantum computing. Only one qubit actually captures the phonon, but researchers can’t tell which qubit until after the measurement. In other words, the quantum superposition is transferred from the phonon to the two qubits. The researchers measured this overlap of two qubits, providing “gold standard evidence that the beamsplitter is creating a quantum entangled state,” Cleland said.

Show that phonons behave like photons

In the second experiment, the team wanted to show another fundamental quantum effect that was first demonstrated with photons in the 1980s. Now known as the Hong-Ou-Mandel effect, when two identical photons are sent simultaneously from opposite directions into a beam splitter, the overlapping outputs interfere so that both photons are always found traveling together, in one direction or the other of exit.

Importantly, the same happened when the team conducted the phonon experiment: the overlapping output means that only one of the detector’s two qubits captures phonons, going in one direction but not the other. Although qubits only have the ability to capture a single phonon at a time, not two, the qubit positioned in the opposite direction never “feels” a phonon, demonstrating that both phonons are going in the same direction. This phenomenon is called two-phonon interference.

Getting phonons into these quantum entangled states is a much bigger leap than doing it with photons. The phonons used here, while indivisible, still require quadrillions of atoms working together quantum mechanically. And if quantum mechanics rules physics only in the smallest realm, it raises questions about where that realm ends and classical physics begins; this experiment further probes that transition.

“These atoms all have to behave consistently together to support what quantum mechanics says they should do,” Cleland said. “It’s quite amazing. The bizarre aspects of quantum mechanics aren’t limited by size.”

Creation of a new linear mechanical quantum computer

The power of quantum computers lies in the “strangeness” of the quantum realm. By harnessing the strange quantum powers of superposition and entanglement, researchers hope to solve previously unsolvable problems. One approach to doing this is to use photons, in what’s called a “linear optical quantum computer.”

A linear mechanical quantum computer that would use phonons instead of photons could have the ability to compute new types of calculations. “The successful two-phonon interference experiment is the final piece showing that phonons are equivalent to photons,” Cleland said. “The result confirms that we have the technology we need to build a linear mechanical quantum computer.”

Unlike photon-based linear optical quantum computing, the University of Chicago’s platform directly integrates phonons with qubits. This means that phonons could further be part of a hybrid quantum computer that combines the best of linear quantum computers with the power of qubit-based quantum computers.

The next step is to create a logic gate, an essential part of computer science, using phonons, which Cleland and his team are currently researching.

Other authors on the paper include . Dumur, G. Andersson, H. Yan, M.‑H. Chou, J. Grebel, CR Conner, YJ Joshi, JM Miller, RG Povey and X. Wu.

More information:
H. Qiao et al, Phonon Splitting: Building a Platform for Linear Quantum Mechanical Computing, Science (2023). DOI: 10.1126/science.adg8715. www.science.org/doi/10.1126/science.adg8715

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