10.03.2021 change 10.03.2021

Researchers find ‘precise mixture’ of light and sound

Credit: Fotolia Credit: Fotolia

A Polish-German research team have mixed nanoscale sound waves with light quanta by using an artificial atom that converts vibrations of a sound wave into single quanta of light - photons with unprecedented precision.

Light and sound waves are the foundation of modern communication technologies. Light is used to transfer data over the global fibre optic network, while sound waves are used for wireless communication between routers, tablets and smartphones. These two key technologies now need to be adapted to the coming age of quantum communication.

The so-called hybrid quantum technologies are the key. They combine different quantum systems, taking advantage of each system's unique strengths while overcoming their limitations.

Professor Hubert Krenner, who heads research at the University of Augsburg said: “In this area, the vibrations of the crystal lattice are particularly promising,” adding that phonons stretch and compress literally every object embedded in a crystal, thus changing its physical properties.

In their research, scientists used surface acoustic waves on the nanometer scale, vibrating a single artificial atom called a quantum dot, and thus changing the colour of the emitted light.

Dr. Daniel Wigger, NAWA-ULAM scholarship holder, who studies the coupling between quantum dots and phonons at the University of Münster and the Wrocław University of Science and Technology said: “In our simulations, we were able to almost perfectly reproduce the spectra measured in Augsburg by incorporating nanoscale sound waves into our model, as if it were a phonon laser beam. 

“The presented research results are a milestone in the development of hybrid quantum technologies, because a quantum dot emits single light quanta, so-called photons, which are precisely clocked by the sound wave.”

Dr. Matthias Weiß, who defended his doctorate at the Institute of Physics in Augsburg, adds that it is fascinating that the spectral lines of quantum dots produced in Munich are so extremely sharp. According to the researcher, this allowed to observe how a small energy of a single phonon shifts the spectral line of a quantum dot.

The research team took another key step forward by using a second sound wave with a different frequency. New spectral lines appeared in the quantum dot spectrum, corresponding to the frequency sum or difference of two sound waves. Professor Hubert Krenner said that this phenomenon has been known in optics for decades as wave mixing.

Krenner explained that laser pointers use this process to generate green light. But in this case, lasers are sound waves that the researchers mix with light quanta.

Dr. Weiß added that when scientists changed the frequency of one of the two sound waves by one trillionth, they observed the spectrum oscillating over a period of about half a day, as predicted. The quantum dot itself represents the qubit, the basic unit in quantum computing.

In their model, the researchers considered a quantum dot as a qubit modulated by a sound wave. Apart from that, they didn't have to make any assumptions. The scientists believe that the exceptional consistence of calculations and experimental results proves that their very general model accurately describes all the key properties. Therefore, it should also apply to many other qubit implementations.

The research by the Wrocław University of Science and Technology, University of Augsburg, University of Münster and Technical University of Munich have been published in the prestigious journal Optica (https://doi.org/10.1364/OPTICA.412201).

The paper is the first step towards future quantum phononic technologies. Dr. Matthias Weiß has already conducted new experiments, the results of which are processed by Dr. Daniel Wigger at the Wrocław University of Science and Technology from the theoretical point of view.

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