(Nanowerk News) University of Washington researchers have found that they can detect the “breathing” of atoms, or mechanical vibrations between two layers of atoms, by observing the type of light those atoms emit when stimulated by a laser. These atomic “breathing” sounds can help researchers encode and transmit quantum information.
The researchers are also developing devices that can serve as a new type of building block for quantum technologies, which are widely anticipated to have many future applications in fields such as computing, communications, and sensor development.
The researchers published these findings in Natural Nanotechnology (“Tuneous phononic coupling in an excitonic quantum transmitter”).
“This is a novel atomic-scale platform, using what the scientific community calls ‘optomechanics,’ in which light and mechanical motion are intrinsically coupled together,” said senior author Mo Li, a UW professor of electrical and computer engineering and physics. . “This provides a new type of involved quantum effects that can be used to control single photons traveling through integrated optical circuits for many applications.”
Previously, the team had studied quantum-level quasiparticles called “excitons”. Information can be encoded into excitons and then released in the form of a photon — a tiny particle of energy that is considered the quantum unit of light. The quantum properties of each emitted photon — such as photon polarization, wavelength, and/or emission time — can serve as quantum information bits, or “qubits”, for quantum computing and communications. And because this qubit is carried by a photon, it travels at the speed of light.
“The broad view from this research is that in order to have a viable quantum network, we need to have a reliable way to create, operate, store, and transmit qubits,” said lead author Adina Ripin, a UW physics doctoral student. “Photons were the natural choice for transmitting this quantum information because optical fiber allows us to transport photons long distances at high speeds, with low energy or information loss.”
The researchers worked with stimuli to create a single photon emitter, or “quantum emitter”, which is a critical component for quantum technologies based on light and optics. To do this, the team placed two thin layers of tungsten and selenium atoms, known as tungsten selenide, on top of each other.
When the researchers applied pulses of precise laser light, they knocked electrons out of the tungsten selenide atom’s nucleus, which generated exciton quasiparticles. Each exciton consists of negatively charged electrons in one tungsten selenide layer and positively charged holes where the electrons used to reside in the other layer. And because opposite charges attract each other, the electrons and holes in each excitation are tightly bound to each other. After a brief moment, as the electron falls back into the hole it was previously occupied, the excitation emits a single photon encoded with quantum information — resulting in the quantum emitter the team set out to create.
But the team found that tungsten selenide atoms emit another type of quasi-particle, known as phonons. Phonons are products of atomic vibrations, which are similar to respiration. Here, two layers of tungsten selenide atoms act like tiny drumheads vibrating relative to each other, which generates 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 saw several evenly spaced peaks. Each photon emitted by the exciton is coupled with one or more phonons. This is somewhat akin to climbing the quantum energy ladder one rung at a time, and on the spectrum these energy spikes are represented visually by equally spaced peaks.
“Fonons are the natural quantum vibrations of the tungsten diselenide material, and have the effect of vertical stretching of the exciton electron-hole pairs sitting 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 a very strong effect on the optical properties of photons emitted by excitons that have not been reported before.”
The researchers wanted to know if they could harness phonons for quantum technology. They applied an electrical voltage and saw that they could vary the interaction energy of the associated phonon and the emitted photon. This variation can be measured and controlled in ways relevant to encoding quantum information into single photon emissions. And this is all accomplished in one unified system — a device involving only a small number of atoms.
Next, the team plans to create waveguides—fibers on a chip that capture single-photon emissions and direct them where they need to go—and then upgrade the system. Instead of just controlling one quantum emitter at a time, the team wanted to be able to control multiple emitters and their associated phonon states. This will allow quantum transmitters to “talk” to each other, a step toward building a solid foundation for quantum circuits.
“Our overarching goal is to create an integrated system with a quantum transmitter that can use single photons traveling through newly discovered optical circuits and phonons to perform quantum computing and quantum sensing,” said Li. “This advance will definitely contribute to that effort, and it helps develop further quantum computing which, in the future, will have many applications.”