ETH Zurich Details Loss Mechanisms in High-Coherence Resonators

A collaborative effort spanning three Swiss institutions, ETH Zurich, EPFL, and EMPA, has identified key loss mechanisms limiting the coherence of advanced mechanical resonators.

Researchers Raquel Garcia-Belles and Alexander Anferov, who contributed equally to the work at ETH Zurich’s Department of Physics and Quantum Center, focused on multimode mechanical resonators coupled to superconducting circuits, a combination promising for building blocks of quantum systems. The study reveals that defects within the piezoelectric material and its interface with the resonator bulk are critical factors impacting performance; measurements show phonon lifetimes up to seconds and coherence times approaching one millisecond in the quantum regime. The paper reports that these results “represent a new milestone for the performance of cQAD devices and offer concrete paths forward for further improvements.” Current investigations into circuit quantum acoustodynamics (cQAD) increasingly focus on high-overtone bulk acoustic-wave (HBAR) resonators as prime candidates for realizing robust quantum systems. These devices, leveraging the mechanical coherence of acoustic waves, now demonstrate performance metrics competitive with other cQAD platforms like those based on phononic crystals or surface acoustic waves. A recent study, involving researchers from ETH Zurich, EPFL, and EMPA, meticulously examines loss mechanisms within these HBARs, identifying the piezoelectric material’s defect density and its interface with the bulk crystal as critical limiting factors. This level of coherence, when coupled with a superconducting qubit, yields a hybrid system boasting a large quantum coherence cooperativity. Researchers at ETH Zurich are meticulously examining loss mechanisms within mechanical resonators, and this work benefits from collaboration spanning ETH Zurich, EPFL in Lausanne, and EMPA in Dubendorf, integrating expertise in physics, materials science, and nanoelectromechanical systems. This performance is crucial for circuit quantum acoustodynamics (cQAD) devices, where mechanical coherence directly impacts functionality. A detailed understanding of loss mechanisms allows for optimized material growth and device fabrication, ultimately enhancing the performance of future quantum acoustic devices. The pursuit of stable quantum states within hybrid systems has yielded a significant leap forward in coherence cooperativity, a crucial metric for viable quantum technologies. This study, a collaborative effort spanning ETH Zurich, EPFL (Lausanne), and EMPA (Dubendorf), focused on AlN-sapphire HBARs, high-overtone bulk acoustic-wave resonators, and their acoustic dissipation. The combination of high mechanical coherence and strong electromechanical coupling, while preserving qubit coherence, is critical for realizing strong coupling regimes essential for full quantum control and measurement. This advancement positions cQAD systems as increasingly promising candidates for applications ranging from high-precision sensing to quantum memories and transducers. Researchers are increasingly focused on high-coherence multimode mechanical resonators, specifically, high-overtone bulk acoustic-wave resonators (HBARs), coupled to superconducting circuits, a configuration known as circuit quantum acoustodynamics (cQAD). This approach leverages the ability of HBARs to maintain high quality factors, even when incorporating piezoelectric films essential for electrical coupling. These improvements are particularly significant given the challenges of maintaining coherence at low temperatures and phonon populations, where two-level systems become dominant sources of decoherence. The resulting hybrid systems exhibit a large quantum coherence cooperativity, as the paper reports. Researchers have identified a direct link between defects within piezoelectric materials and the coherence of mechanical resonators, a crucial factor for advancing circuit quantum acoustodynamics (cQAD) technologies. The study, focused on aluminum nitride-sapphire heterostructures, demonstrates that the density of imperfections in the piezoelectric layer and its interface with the bulk material significantly constrains performance.

The team meticulously probed acoustic dissipation, revealing that while high-quality sapphire substrates offer excellent baseline characteristics, the introduced AlN film presents a vulnerability. The researchers state, highlighting the importance of material purity. This detailed analysis, employing both superconducting qubits and microwave spectroscopy, revealed a shift in dominant dissipation mechanisms at extremely low temperatures, around 10 millikelvin. Efforts to build practical circuit quantum acoustodynamics (cQAD) devices increasingly focus on minimizing energy loss within mechanical resonators, a challenge addressed through careful material selection and interface engineering. Researchers are not simply seeking higher quality factors; they are targeting coherence times long enough to enable complex quantum operations. The study of AlN-sapphire heterostructures reveals that the piezoelectric AlN layer and its interface with the sapphire substrate represent critical areas for loss mitigation, impacting the overall performance of these resonators. This collaborative work, spanning ETH Zurich, EPFL, and EMPA, demonstrates a focus on multimode resonators, devices capable of supporting multiple acoustic modes simultaneously, coupled to superconducting circuits. This approach allows for detailed probing of dissipation mechanisms at cryogenic temperatures, where thermal noise is significantly reduced. These improvements are particularly notable given the inherent challenges of introducing a piezoelectric film, necessary for electrical coupling, which can introduce defects and interfaces. This leads to a large quantum coherence cooperativity. Researchers have identified that the quality of the piezoelectric aluminum nitride layer, and crucially its interface with the sapphire substrate, significantly limits performance in circuit quantum acoustodynamics (cQAD) devices. This detailed analysis, spanning ETH Zurich, EPFL, and EMPA, moves beyond simply achieving high quality factors to pinpointing specific dissipation sources. The study highlights that defects within the AlN film and imperfections at the AlN-sapphire interface contribute to energy loss, particularly at cryogenic temperatures where thermal effects are minimized. Source: http://link.aps.org/doi/10.1103/vszq-z1sh Stay currentSee today’s quantum computing news on Quantum Zeitgeist for the latest breakthroughs in qubits, hardware, algorithms, and industry deals. Tags: