Crystal resonators for hybrid acoustic quantum memories

The pursuit of robust quantum memories represents a critical challenge in developing practical quantum technologies, and researchers are increasingly turning to the unique properties of mechanical resonators. Yang Hu, Angad Gupta from University of Pittsburgh, and Jacob Repicky and Michael Hatridge from Yale University, alongside their colleagues, now demonstrate a significant advance in this field with the development of suspended quartz phononic crystal resonators operating at 100MHz. These resonators exhibit millisecond-long lifetimes at extremely low temperatures, offering a promising platform for storing quantum information, and crucially, the team achieves this performance using a novel contactless electrode design that minimises energy loss. This innovative approach enhances the interaction between the mechanical vibrations and superconducting circuits, paving the way for more efficient and scalable quantum acoustic devices and offering a potential pathway towards hybrid quantum systems that combine the strengths of different quantum platforms.

These systems offer potential for long-coherence quantum memories, essential for scaling quantum computation and enabling long-distance quantum communication. Achieving strong and controllable coupling between qubits and mechanical resonators remains challenging, often limited by weak interaction strengths and acoustic losses. Researchers are therefore exploring novel resonator designs and materials to enhance both coupling strength and coherence times.

Current mechanical resonators typically utilize silicon or silicon nitride, materials which, while possessing high quality factors, exhibit relatively weak piezoelectric coupling. This weak coupling hinders efficient transduction between microwave photons, used to control qubits, and the mechanical motion, limiting overall system performance. Furthermore, the size of these resonators often restricts their integration into complex quantum circuits, posing practical challenges for scalability. This work investigates quartz phononic crystal resonators as a potential solution for hybrid acoustic quantum memories.

Quartz possesses significantly stronger piezoelectric coupling than silicon or silicon nitride, promising improved transduction efficiency. Moreover, fabricating phononic crystals, periodic structures designed to manipulate acoustic waves, allows precise control over the resonator’s acoustic modes and the creation of band gaps that confine acoustic energy. By carefully designing the phononic crystal geometry, researchers aim to create resonators with high quality factors, strong piezoelectric coupling, and compact footprints, ultimately advancing the development of robust and scalable quantum acoustic devices. These long-lived mechanical modes serve as quantum memories.

The team demonstrates suspended quartz phononic crystal resonators operating at 100MHz with millisecond lifetimes at 8K. A contactless electrode geometry suppresses two-level system losses and other electrode-induced energy dissipation, allowing evaluation of the piezoelectric coupling rate between the mechanical modes and fluxonium qubits, and transmon qubits through a Josephson-junction-based three-wave mixer. Furthermore, the researchers discuss multi-period defect geometries for enhancing these coupling rates.

High Q-Factor Nanomechanical Resonators for Quantum Systems

This extensive research details the creation of high-quality, low-dissipation nanomechanical resonators, particularly for use as interfaces in quantum systems. The primary goal is to create resonators that efficiently couple to superconducting qubits, enabling new architectures for quantum information processing. These resonators act as transducers, converting between microwave and mechanical degrees of freedom. Achieving extremely high quality factors, representing low dissipation, is crucial for maintaining coherence in quantum systems. The research focuses on pushing the limits of quality factor in these resonators, intended to mediate interactions between distant qubits, potentially enabling modular quantum computing architectures.

Alpha quartz is identified as a promising material due to its piezoelectric properties and potential for high quality factors at cryogenic temperatures. Fabrication is complex, requiring precise control over etching to create well-defined resonator structures. Etching parameters are critical for minimizing surface roughness and defects. Advanced integration techniques like flip-chip bonding are explored to connect the nanomechanical resonators to superconducting circuits. A major source of dissipation is thermoelastic damping, where mechanical vibrations generate heat, leading to energy loss.

Surface roughness contributes significantly to thermoelastic damping and overall dissipation, necessitating optimized etching and surface treatment. Defects and impurities within the quartz can create two-level systems, which act as energy sinks and reduce quality factor. Understanding and mitigating these two-level systems is a significant challenge. The fundamental limit of noise due to thermal fluctuations is also considered. Losses related to the piezoelectric properties of the quartz are also discussed.

The geometry of the resonator is crucial for maximizing quality factor and coupling to qubits. Utilizing the piezoelectric properties of quartz allows for efficient electromechanical coupling. Controlling stress within the resonator can influence its properties and performance. Modeling and simulation are used to understand and optimize resonator designs. In essence, the research presents a comprehensive overview of the challenges and opportunities in creating high-performance nanomechanical resonators for quantum applications, emphasizing the importance of material selection, fabrication precision, loss mitigation, and theoretical understanding.

Contactless Resonators Enhance Quantum Coupling Potential

This research demonstrates the fabrication and characterization of suspended quartz phononic crystal resonators operating at 100MHz, achieving mechanical lifetimes of up to 1. 0 milliseconds at 8K. The team successfully developed a fabrication process for these devices, alongside a measurement system to assess their properties, and importantly, achieved this performance while minimizing energy dissipation through a novel contactless electrode design. This design eliminates common sources of decoherence found in low-temperature mechanical resonators, representing a significant step towards utilising these systems for quantum technologies.

The researchers further explored methods to enhance the coupling between the mechanical modes and superconducting qubits, proposing a scheme using a SNAIL to parametrically connect the resonators to transmon qubits for potential use as quantum memory units. Analysis of resonator designs with increased defect unit cells suggests a pathway to triple the effective coupling rate, although this introduces additional mechanical modes. The authors acknowledge that further improvements to the SNAIL capacitance and pumping strength could push the effective coupling rate beyond 100kHz. Future work will likely focus on optimising these parameters and experimentally verifying the proposed coupling scheme to assess the viability of these resonators as quantum memory elements.