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Researchers at the University of Washington have found they can detect atomic “breathing,” or the mechanical vibration between two layers of atoms, by looking at the kind of light those atoms emit when stimulated by a laser. The sound of this atomic “breath” could help researchers encode and transmit quantum information.
Researchers also developed a device that could serve as a new type of building block for quantum technologies, which are expected to have many future applications in fields such as computing, communications and sensor development.
The researchers published these findings on June 1 Nature Nanotechnology.
“This is a new atomic-scale platform, using what the scientific community calls ‘optomechanics’, where the motions of light and mechanics are intrinsically coupled together,” said senior author Mo Li, professor of physics and electrical and computer engineering. . “It provides a new type of involved quantum effect that can be used to control single photons passing through integrated optical circuits for many applications.”
Previously, the team had studied a quantum-level quasiparticle called an ‘exciton’. Information can be encoded in an exciton and then released as a tiny particle of photon energy considered the quantum unit of light. The quantum properties of each emitted photon, such as the polarization, wavelength, and/or timing of the photon’s emission, can function as a quantum bit of information, or “qubit,” for quantum computing and communication. And since this qubit is carried by a photon, it travels at the speed of light.
“The bird’s-eye view of this research is that to have a viable quantum network, we need to have ways to reliably create, operate, store, and transmit qubits,” said lead author Adina Ripin, a PhD student in UW Physics. “Photons are a natural fit for transmitting this quantum information because optical fibers allow us to transport photons long distances at high speeds, with low energy or information losses.”
The researchers were working with excitons to create a single photon emitter, or ‘quantum emitter’, which is a key component for light- and optical-based quantum technologies. To do this, the team placed two thin layers of tungsten and selenium atoms, known as tungsten diselenide, on top of each other.
When the researchers applied a precise pulse of laser light, they knocked the electron of a tungsten diselenide atom away from the nucleus, which generated an exciton quasiparticle. Each exciton consisted of a negatively charged electron on one layer of the tungsten diselenide and a positively charged hole where the electron was located on the other layer. And because opposite charges attract, the electron and hole in each exciton were tightly bound to each other. After a brief moment, as the electron fell back into the hole it previously occupied, the exciton emitted a single photon encoded with quantum information, producing the quantum emitter the team set out to create.
But the team found that tungsten diselenide atoms emitted another type of quasiparticle, known as a phonon. Phonons are a product of atomic vibration, which is similar to respiration. Here, the two atomic layers of tungsten diselenide acted like tiny drumheads that vibrated relative to each other, generating phonons. This is the first time that phonons have been observed in a single photon emitter in this type of two-dimensional atomic system.
When the researchers measured the spectrum of the emitted light, they noticed several evenly spaced peaks. Each single photon emitted by an exciton was paired with one or more phonons. This is somewhat similar to climbing a quantum energy ladder one step at a time, and in the spectrum, these energy peaks were visually represented by the evenly spaced peaks.
“A phonon is the natural quantum vibration of tungsten diselenide material, and it has the effect of vertically stretching the exciton electron-hole pair found in the two layers,” said Li, who is also a member of the steering committee for UW’s QuantumX, and is a faculty member of the Institute for Nano-Engineered Systems. “This has an extraordinarily strong effect on the optical properties of the exciton-emitted photon that has never been reported before.”
The researchers were curious that phonons could be harnessed for quantum technology. They applied electrical voltage and saw that they could vary the interaction energy of the associated phonons and emitted photons. These variations were measurable and controllable in ways relevant to encoding quantum information in a single photon emission and all of this was accomplished in an integrated system, a device involving only a small number of atoms.
Next the team plans to build a fiber waveguide on a chip that captures single photon emissions and directs them where they need to go, then magnifies the system. Instead of controlling just one quantum emitter at a time, the team wants to be able to control multiple emitters and their associated phonon states. This will allow quantum emitters to “talk” to each other, a step towards building a solid foundation for quantum circuits.
“Our overall goal is to create an integrated system with quantum emitters that can use single photons passing through newly discovered optical and phonon circuits to perform quantum computations and quantum sensing,” said Li. “This advance will certainly contribute to this effort and help further develop quantum computing which, in the future, will have many applications.”
Other co-authors are Ruoming Peng, Xiaowei Zhang, Srivatsa Chakravarthi, Minhao He, Xiaodong Xu, Kai-Mei Fu and Ting Cao.
Tunable phonon coupling in exciton quantum emitters, Nature Nanotechnology (2023). DOI: 10.1038/s41565-023-01410-6 , www.nature.com/articles/s41565-023-01410-6
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