Circuit quantum acoustodynamics with surface acoustic waves

Abstract: The experimental investigation of quantum devices incorporating mechanical resonators has opened up new frontiers in the study of quantum mechanics at a macroscopic level$^{1,2}$. Superconducting microwave circuits have proven to be a powerful platform for the realisation of such quantum devices, both in cavity optomechanics$^{3,4}$, and circuit quantum electro-dynamics (QED)$^{5,6}$. While most experiments to date have involved localised nanomechanical resonators, it has recently been shown that propagating surface acoustic waves (SAWs) can be piezoelectrically coupled to superconducting qubits$^{7,8}$, and confined in high-quality Fabry-Perot cavities up to microwave frequencies in the quantum regime$^{9}$, indicating the possibility of realising coherent exchange of quantum information between the two systems. Here we present measurements of a device in which a superconducting qubit is embedded in, and interacts with, the acoustic field of a Fabry-Perot SAW cavity on quartz, realising a surface acoustic version of cavity quantum electrodynamics. This quantum acoustodynamics (QAD) architecture may be used to develop new quantum acoustic devices in which quantum information is stored in trapped on-chip surface acoustic wavepackets, and manipulated in ways that are impossible with purely electromagnetic signals, due to the $10^{5}$ times slower speed of travel of the mechanical waves.

• The paper demonstrates that integrating SAWs with a high-Q cavity (~10^5) and a superconducting transmon qubit enables coherent exchange of quantum information.
• It employs a Fabry-Perot SAW cavity with superconducting Bragg mirrors to achieve an acoustic coupling strength of approximately 5.7 MHz.
• The research opens new avenues for hybrid quantum technologies such as quantum memories and delay lines by leveraging slow-propagating acoustic waves.

Analyzing Circuit Quantum Acoustodynamics with Surface Acoustic Waves

The paper "Circuit quantum acoustodynamics with surface acoustic waves" explores the integration of surface acoustic waves (SAWs) with superconducting qubits to form a system analogous to cavity quantum electrodynamics (QED), coined as circuit quantum acoustodynamics (QAD). The study pioneers the incorporation of SAWs into quantum computing architectures, with the promise of manipulating quantum information using the uniquely slow traveling speed of acoustic waves compared to electromagnetic signals.

Core Contributions

The research outlines the design and experimental validation of a SAW cavity, fabricated on a piezoelectric quartz substrate, integrated with a superconducting transmon qubit. The SAW cavity is formed using a Fabry-Perot configuration and superconducting Bragg mirrors, achieving high-quality factors (in the range of $10^5$). This enables the confinement and manipulation of surface acoustic wavepackets at microwave frequencies in the quantum regime. By embedding a transmon qubit within the SAW cavity, the study realizes a functional circuit QAD system. This setup allows for a coherent exchange between quantum information and mechanical oscillations, a novel paradigm shifting from traditional electromagnetic interactions.

Experimental Findings

Key experimental benchmarks include:

• Interaction Characterization: The observed acoustic coupling strength between the qubit and the cavity ($\lambda_{\text{m2}}$) is approximately 5.7 MHz. The coupling highlights the system's ability to induce frequency shifts on the SAW modes through quantum state manipulations, setting the stage for potential quantum information storage.
• Acoustic Stark Shifts: The research reports the qubit frequency's sensitivity to the phononic population within the cavity, supported by measuring acoustic Stark shifts. This indicates the qubit's strong interaction with the acoustic field.
• Time-Delayed Measurements: The paper describes time-delay induced frequency shifts in the qubit response as a direct consequence of SAW propagation, exploiting the mechanical wave's slow speed to control qubit behavior non-locally.

Implications and Speculation

The implications of such research are significant. On a theoretical front, integrating mechanical resonators with superconducting circuits opens pathways for extending cavity QED concepts to mechanical systems, enriching our understanding of quantum mechanics at macroscopic scales. Practically, high-quality SAW cavities provide a promising platform for developing new quantum technologies, such as quantum memories, delay lines, and signal processors that exploit the long coherence times and the large wavelength-to-chip-size ratio.

Future trajectories in this field may include increasing coupling efficiencies through the deployment of substrates with higher piezoelectric constants, such as lithium niobate, which would strengthen the coupling without compromising qubit coherence times. This escalation in interaction strength could also enable the study of hybrid systems involving other quantum platforms, like trapped ions or solid-state defects, opening multidiscipline collaborative ventures in quantum technology development.

Conclusion

The study presents a rigorous experimental approach to harnessing SAWs within quantum circuits, successfully demonstrating a new dimension of quantum control through the retarded acoustic response. This research not only enriches the collective understanding of hybrid quantum systems but also lays the groundwork for innovative devices in quantum information processing, beaconing the next era of quantum acoustical engineering.