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Qubits surf sound waves between quantum nodes

Qubits surf sound waves between quantum nodes

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Inspired by the functioning of pulsed lasers, scientists from France and Japan have developed an acoustic counterpart that enables the precise and controlled transmission of single electrons between quantum nodes.

Riding the waves

The spin of an electron can serve as a basis for creating qubits—the basic unit of information of quantum computing. In order to process or store that information, the information in qubits may have to be transported between quantum nodes in a network.

One option is transporting the electrons themselves, something that can now be done by having them ride sound waves. “More than 10 years ago, we demonstrated it for the first time,” said lead researcher Christopher Bauerle of the Grenoble-based Institute Néel.

However, this technique had a significant drawback. Like any wave, a sound wave takes a sinusoidal form, consisting of many maxima and minima, which makes it difficult to predict the location of the electron.

Bauerle and his team have now gotten around this problem by engineering a wave that has a single minimum or a single maximum. “By using a technique called Fourier synthesis, we superposed many waves with different frequencies in such a way that there was only one minimum or one maximum depending on whether you apply positive or negative voltage,” he said.

Bauerle compares these concentrated sound waves to laser pulses. “If you want to perform time-resolved measurements, you excite a system with a short laser pulse. We use a similar technique in our system using sound. Since we have a focused acoustic pulse, we know exactly at what time the electron will arrive at a node,” he said.

Paulo Santos, a Berlin-based nanoelectronics expert from the Paul Drude Institute for Solid State Electronics, likens the technique to a surfer riding a wave. “Just like a surfer gets transported by a wave in an ocean, the electron qubit rides the surface acoustic wave to move in the quantum network,” Santos, who was not part of the study, remarked.

Making waves

To generate these sound waves, a chip containing quantum nodes was embedded in a gallium-arsenide crystal linked to two gold-plated electrodes deposited onto a piezoelectric substrate. An electric field is generated by applying an alternating voltage to these electrodes. The varying electric field deforms the piezoelectric material and generates surface acoustic waves. They are accompanied by a moving electric field (generated by the inverse piezoelectric effect) that helps to transport the electrons.

Bauerle listed several advantages of this system, which works at temperatures between 20 milliKelvin to one Kelvin. “The electrons get transported between the nodes at the speed of sound (3,000 m/s). This, along with the precise and controlled manner of electron transmission, enables us to manipulate the quantum information in real time. If you compare it to the photonic quantum system, the manipulations have to be done beforehand because information is transmitted at the speed of light, which is too fast for real-time manipulation,” he said.

Moreover, this technique can potentially be scaled up because of the large size of the waveform. “A single acoustic wave can carry electrons from different quantum nodes at the same time,” Bauerle said, adding that they achieved 99.4 percent transmission efficiency during their experiments.

According to Santos, this technique’s unique ability to precisely transport qubits as well as manipulate them on the fly on a chip could have a number of distinct applications in the future. “The next big step is demonstrating the entanglement of these flying qubits. The other big effort is going to be to transfer this technology from gallium arsenide to other materials like silicon.”

He added, however, that it could take years before seeing practical applications based on this research.

Santos highlighted that electron spins are just one of many approaches for processing quantum information; other options include photons, superconducting qubits, and cold atoms. He pointed out that photon qubits will remain a mainstream approach in quantum systems.

“There are more people working on photon-based quantum processing because there already exists a huge infrastructure. For example, silicon-based chips also have integrated optics. The ‘electron surfing’ technique is compatible with on-chip integration and can profit from these developments,” he said, suggesting that advances in one can help with the other.

Physical Review X, 2022. DOI: 10.1103/PhysRevX.12.031035

Dhananjay Khadilkar is a journalist based in Paris.

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