Most of us don’t think that atoms have their own unique vibrations, but they do. In fact, it’s such a fundamental feature of the building blocks of nature that a team of researchers at the University of Washington recently observed and used this phenomenon in their research study. By studying the light atoms emit when stimulated by a laser, they were able to detect vibrations sometimes called atomic respiration.
The result is a breakthrough that may one day allow us to build better tools for many types of quantum technologies.
Led by Mo Li, a professor of photonics and nanodevices in both the UW Department of Electrical and Computer Engineering and the UW Department of Physics, the researchers set out to build a better quantum emitter, or QE, that could be incorporated into circuits opticians .
QEs are an essential part of the quantum technology toolkit as they provide a way to generate individual quantum particles that can be used as qubits. Similar to bits of information in everyday computing, qubits are used in quantum computing to perform calculations far beyond what can be achieved with classical computers. Typically, a qubit is built from an electron or photon due to the unique quantum properties of these particles.
This is a new atomic-scale platform, using what the scientific community calls optomechanics, where the motions of light and mechanics are inherently coupled together, Li said. It provides a new type of quantum effect involved that can be used to control single photons passing through integrated optical circuits for many applications.
To build their QE, the team started with tungsten diselenide, a molecule composed of tungsten and selenium. This was formed in the thinnest of sheets, each only a single atom thick. Two of these sheets were then stacked on top of each other and placed on a series of nanopillars, just 200 nanometers wide.
This placement on the nanopillars caused the sheets to warp at the point of contact, resulting in an array of regularly spaced quantum dots. Quantum dots are semiconductor particles a few nanometers in size, with unique optical and electronic properties, and are a common method of constructing QE for quantum applications. Due to the deformation caused by the nanopillars, these are more specifically referred to as strain-induced quantum dots.
By applying a precise pulse of laser light to one of the quantum dots, an electron is knocked away from the nucleus of tungsten diselenide atoms. This briefly creates a quasiparticle known as an excite them. This exciton is composed of the negatively charged electron and the corresponding positively charged hole in the opposing sheet. Because they are strongly bonded, the electron quickly returns to the atom. When it does, it releases a single photon encoded with very specific quantum information.
To have a viable quantum network, we need ways to reliably create, operate, store and transmit qubits, said Adina Ripin, lead author of the paper, a member of the Mo Li group and a doctoral student in the physics department. . Photons are a natural fit for transmitting this quantum information because optical fibers allow us to transport photons over long distances at high speeds, with low energy or information losses.
This approach has led to the production of very coherent, high-quality photons that could potentially be used as qubits. By itself, this would make the project a success. However, some details soon appeared in the data, deserving a closer look.
Researchers have discovered that a quasiparticle called a phonon it was also produced in the process of creating each photon. Phonons are an optomechanical phenomenon based on the vibration between atoms and occur in all matter. Phonons can be thought of as acoustic analogs to photons, with their own quantum waveforms. While we can’t see or hear it directly, Li says the vibrations can be visualized as the breath between atoms.
In this study, the phonons were generated by the vibration between two wafer-thin layers of tungsten diselenide, which acted like tiny drumheads vibrating relative to each other. The UW team found that these phonons were closely related to the photon that was being generated.
You can think of phonons in terms of a little spring attached to the layers, Li said. This spring is vibrating, so it directly changes how the electron and the hole can recombine. For this reason, the emitted photon also changes.
Previously, phonons had never been observed in this type of single photon emitter system. Furthermore, by analyzing the spectrum of the emitted light, the team found equally spaced peaks representing the different quantum energy levels of the phonons. Expert analysis by Ting Cao, a quantum theorist and assistant professor of materials science and engineering, revealed that every single photon emitted by an exciton was paired with one, two, three or more phonons.
A phonon is the natural quantum vibration of the tungsten diselenide material and has the effect of vertically stretching the exciton electron-hole pair found in the two layers, Li continued. This has a remarkably strong effect on the optical properties of the exciton-emitted photon that has never been reported before.
The team was also able to regulate the phonon-exciton-photon interaction by applying electrical voltage across the materials. By varying the voltage, they found they could alter the interaction energy of the associated phonons and the emitted photons. This was controllable in ways relevant to encoding specific quantum information in a single photon.
I find it fascinating that we have been able to observe a new type of hybrid quantum platform, said Ruoming Peng, also a lead author on the paper, who graduated with his doctorate from UW ECE in 2022. Studying how where phonons interact with quanta emitters, we have discovered a whole new realm of possibilities for controlling and manipulating quantum states. This could lead to even more exciting discoveries in the future.
Li and his team want to extend their system further, monitoring more emitters and their associated phonon states. This would allow quantum emitters to talk to each other, building the foundation for new types of quantum circuits. Future applications of these approaches include quantum computing, quantum communications and quantum sensing.
The UW team includes Adina Ripin, Ruoming Peng, Xiaowei Zhang, Srivatsa Chakravarthi, Minhao He, Xiaodong Xu, Kai-Mei Fu, Ting Cao and Mo Li. The research is supported by the National Science Foundation. Their research article, Tunable phononic coupling in excitonic quantum emitters, was recently published in the journal Nature Nanotechnology.
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