Qubits ride sound waves between quantum nodes
Inspired by how pulsed lasers work, French and Japanese scientists have developed an acoustic counterpart that enables the precise and controlled transmission of single electrons between quantum nodes.
riding the waves
An electron’s spin can serve as the basis for creating qubits, the basic unit of information in quantum computing. In order to process or store this information, the information contained in the qubits may need to be transported between quantum nodes in a network.
One option is to transport the electrons themselves, which can now be done by having them ride sound waves. “More than 10 years ago, we demonstrated this for the first time,” said lead researcher Christopher Bauerle of the Institut Néel in Grenoble.
However, this technique had a significant drawback. Like any wave, a sound wave takes a sinusoidal shape, composed of many maxima and minima, which makes it difficult to predict the location of the electron.
Bauerle and his team have now circumvented this problem by designing a wave that has a single minimum or a single maximum. “Using a technique called Fourier synthesis, we superimposed many waves with different frequencies such that there was only a minimum or a maximum depending on whether you applied positive or negative voltage,” a- he declared.
Bauerle compares these concentrated sound waves to laser pulses. “If you want to make 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 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 is carried by a wave in an ocean, the electron qubit rides the surface acoustic wave to move through the quantum network,” Santos, who was not part of the study, remarked.
To generate these sound waves, a chip containing quantum nodes was embedded in a gallium arsenide crystal connected to two gold-plated electrodes deposited on 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) which makes it possible to transport the electrons.
Bauerle listed several advantages of this system, which operates at temperatures between 20 milliKelvin and one Kelvin. “Electrons are transported between nodes at the speed of sound (3,000 m/s). This, together with the precise and controlled way of electron transmission, allows us to manipulate quantum information in real time. If you compare it to the photonic quantum system, the manipulations have to be done beforehand because the information is transmitted at the speed of light, which is too fast for real-time manipulation,” he said.
Additionally, this technique can potentially be scaled due to the large size of the waveform. “A single acoustic wave can transport electrons from different quantum nodes at the same time,” Bauerle said, adding that they achieved 99.4% transmission efficiency during their experiments.
According to Santos, the 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 to demonstrate the entanglement of these flying qubits. The other big effort will be to transfer this technology from gallium arsenide to other materials like silicon.”
He added, however, that it could be years before we see practical applications based on this research.
Santos pointed out that electron spins are just one of many approaches to quantum information processing; 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 is already a huge infrastructure. For example, silicon-based chips also have optics built in. The ‘electron surfing‘ technique is compatible with on-chip integration and can benefit from these developments,” he said, suggesting that advances in one can help the other.
Physical Examination X, 2022. DOI: 10.1103/PhysRevX.12.031035
Dhananjay Khadilkar is a journalist based in Paris.