The principles of acoustics and the interaction between sound waves and the physical properties of materials are used to test devices and control qubits using sound. Researchers have created a system that measures the stability and quality of acoustic resonators using atomic vacancies in silicon carbide. Furthermore, these vacancies could be used for acoustically controlled quantum information processing, opening up a new avenue for manipulating quantum states embedded in this widely used material.
Acoustic resonators can be found everywhere. In fact, there’s a good chance you’re holding one right now. To filter out noise that could degrade a signal, most smartphones today use bulk acoustic resonators as radio frequency filters. Most Wi-Fi and GPS systems use these filters as well.
Although acoustic resonators are more stable than electrical resonators, they can degrade over time. There is currently no simple way to actively monitor and analyze the material quality degradation of these widely used devices.
Wafer-scale manufacturable silicon carbide resonators in particular are known to have the best-in-class performance for quality factors. But crystal growth defects such as dislocations and grain boundaries as well as resonator manufacturing defects such as roughness, tether stress, and micro-scale craters can cause stress-concentration regions inside the MEMS resonator.
Sunil Bhave
Researchers at Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a system that uses atomic vacancies in silicon carbide to measure the stability and quality of acoustic resonators in collaboration with researchers at Purdue University’s OxideMEMS Lab. Furthermore, these vacancies could be used for acoustically controlled quantum information processing, opening up a new avenue for manipulating quantum states embedded in this widely used material.
“Silicon carbide, which is the host for both the quantum reporters and the acoustic resonator probe, is a readily available commercial semiconductor that can be used at room temperature,” said Evelyn Hu, the Tarr-Coyne Professor of Applied Physics and of Electrical Engineering and the Robin Li and Melissa Ma Professor of Arts and Sciences, and senior author of the paper. “As an acoustic resonator probe, this technique in silicon carbide could be used in monitoring the performance of accelerometers, gyroscopes, and clocks over their lifetime and, in a quantum scheme, has potential for hybrid quantum memories and quantum networking.”
The research was published in Nature Electronics.
A look inside acoustic resonators
Silicon carbide is a widely used material in microelectromechanical systems (MEMS), including bulk acoustic resonators.
“Wafer-scale manufacturable silicon carbide resonators in particular are known to have the best-in-class performance for quality factor,” said Sunil Bhave, co-author of the paper and professor at Purdue’s Elmore Family School of Electrical and Computer Engineering. “But crystal growth defects such as dislocations and grain boundaries, as well as resonator manufacturing defects such as roughness, tether stress, and micro-scale craters, can cause stress-concentrations regions inside the MEMS resonator.”
Today, the only way to see what’s happening inside an acoustic resonator without destroying it is with super powerful and very expensive X-rays, such as the broad-spectral X-ray beam at the Argonne National Lab.
“These types of expensive and difficult-to-access machines are not deployable for doing measurements or characterization in a foundry or somewhere where you’d actually be making or deploying these devices,” said Jonathan Dietz, a graduate student at SEAS and co-first author of the paper. “Our motivation was to try to develop an approach that would allow us to monitor the acoustic energy inside of a bulk acoustic resonator so you can then take those results and feed them back into the design and fabrication process.”
Naturally occurring defects in silicon carbide occur when an atom is removed from the crystal lattice, resulting in a spatially local electronic state whose spin can interact with sound waves via material strain, such as that generated by an acoustic resonator.
Acoustic waves move through the material, putting mechanical strain on the lattice, which can cause the defect’s spin to flip. Changes in the spin state can be observed by shining a laser through the material and counting the number of defects that are “on” or “off” after being perturbed.
“How dim or bright the light indicates how strong the acoustic energy is in the local environment where the defect is,” said Aaron Day, co-author of the paper and a graduate student at SEAS. “Because these defects are the size of single atoms, the information they give you is very local and, as a result, you can actually map out the acoustic waves inside the device in this non-destructive way.”
This map can indicate where and how the system is degrading or not performing optimally.
Acoustic control
Those same silicon carbide defects can function as qubits in a quantum system. Many quantum technologies today are based on spin coherence: how long spins will remain in a particular state. A magnetic field is frequently used to control coherence. However, Hu and her colleagues demonstrated that they could control spin by mechanically deforming the material with acoustic waves, achieving control quality comparable to other approaches that use alternating magnetic fields.
“To use the natural mechanical properties of a material – its strain – expands the range of material control that we have,” said Hu. “When we deform the material, we find that we can also control the coherence of spin and we can get that information just by launching an acoustic wave through the material. It provides an important new handle on an intrinsic property of a material that we can use to control the quantum state embedded within that material.”
