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Sound Waves Enable Single-Photon Control at High Speed

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Light and sound are foundational to modern communication technology. Glass fibers — with laser light form the bedrock of the internet. Nanoscale sound waves on chips serve to process signals at gigahertz frequencies for wireless transmission between devices.

One of the most pressing questions for the future is how these technologies can be adapted to quantum systems. Future applications of quantum technology require flexible architectures to overcome limitations of current devices such as limited tunability or a lack of built-in quantum light sources. On-chip generation of single photons is necessary to avoid the inevitable coupling losses incurred when using an off-chip source.

A team of researchers has demonstrated control of individual light quanta, at an extremely high degree of precision, using sound waves to switch individual photons on a chip back and forth between two outputs at gigahertz frequencies. The method can be used in acoustic quantum technologies or complex integrated photonic networks.

A focused laserbeam (left, blue) generates single photos by a single quantum dot inside the photonic waveguides (red), which are fabricated on top of crystalline gallium arsenide- (GaAs-) aluminum gallium arsenide (Al0.2Ga0.8As). Two interdigitating electrodes (interdigital transducers, IDT) generate nanoscale soundwaves (surface acoustic waves, SAWs), which dynamically strain the waveguides. The nanoscale soundwave generated by the left IDT switches the color of the emitted single photons. The two waveguides are coupled by two so-called multimode interference beamsplitters (MMIs). The soundwave generated by the right IDT sorts the single photons according to their color (red and green) between the two outputs on the right. Courtesy of Dominik Bühler/University of Valencia.
A focused laser beam (left, blue) generates single photons by a single quantum dot inside the photonic waveguides (red), which are fabricated on top of crystalline gallium arsenide- (GaAs-) aluminum gallium arsenide (Al0.2Ga0.8As). Two interdigitating electrodes (interdigital transducers, IDTs) generate nanoscale sound waves, which dynamically strain the waveguides. The nanoscale sound wave generated by the left IDT switches the color of the emitted single photons. The two waveguides are coupled by two so-called multimode interference beamsplitters. The sound wave generated by the right IDT sorts the single photons according to their color (red and green) between the two outputs on the right. Courtesy of Dominik Bühler/University of Valencia.

The researchers fabricated a dynamically reconfigurable integrated photonic circuit comprising a Mach-Zender interferometer and surface acoustic wave (SAW) transducers, directly on a monolithic semiconductor platform. In the system, the SAWs dynamically strained the waveguides, which were approximately 30× thinner than a human hair.


In addition, the circuit comprised an integrated quantum light source, in the form of quantum dots.

“These quantum dots, just a few nanometers in size, are islands inside the waveguides which emit light as individual photons,” said Matthias Weiß, a researcher at the University of Münster. “The quantum dots are included in our chip and so we don’t have to use complicated methods to generate individual photons by means of another source.”

Using nanoscale sound waves, the researchers directly switched the photons on the chip back and forth between two outputs at unprecedented speed as they propagated the waveguides, said Dominik Bühler, who designed the quantum chips as part of his Ph.D. at the University of Valencia.

The team believes its results to be a milestone on the way toward hybrid quantum technologies, as they combine three quantum systems: a quantum light source in the form of quantum dots; the light quanta created; and phonons, the quantum particles in the sound wave.  

The team has taken a further step toward acoustic quantum technologies. Mauricio de Lima, a researcher at the University of Valencia, said, “We are already working flat out to enhance our chip so that we can program the quantum state of the photons as we wish, or even control several photons with different colors between four or more outputs.” 

The research was published in Nature Communications (www.doi.org/10.1038/s41467-022-34372-9).


Published: December 2022
Glossary
quantum
The term quantum refers to the fundamental unit or discrete amount of a physical quantity involved in interactions at the atomic and subatomic scales. It originates from quantum theory, a branch of physics that emerged in the early 20th century to explain phenomena observed on very small scales, where classical physics fails to provide accurate explanations. In the context of quantum theory, several key concepts are associated with the term quantum: Quantum mechanics: This is the branch of...
quantum dots
A quantum dot is a nanoscale semiconductor structure, typically composed of materials like cadmium selenide or indium arsenide, that exhibits unique quantum mechanical properties. These properties arise from the confinement of electrons within the dot, leading to discrete energy levels, or "quantization" of energy, similar to the behavior of individual atoms or molecules. Quantum dots have a size on the order of a few nanometers and can emit or absorb photons (light) with precise wavelengths,...
waveguide
A waveguide is a physical structure or device that is designed to confine and guide electromagnetic waves, such as radio waves, microwaves, or light waves. It is commonly used in communication systems, radar systems, and other applications where the controlled transmission of electromagnetic waves is crucial. The basic function of a waveguide is to provide a path for the propagation of electromagnetic waves while minimizing the loss of energy. Waveguides come in various shapes and sizes, and...
Research & TechnologyLasersOpticsquantumLight Sourcesquantum dotsUniversity of MünsterUniversity of MunichUniversity of Valenciasingle photonWaveguidephotonic integrated circuitsEuropeTechnology News

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