Ge Hole-Spin Qubits Enable Coherent Phonon Interaction for Quantum Computing at 1-4 K

The development of scalable quantum computing relies heavily on identifying and refining suitable qubit platforms, and recent research focuses on germanium hole-spin qubits as a promising avenue. D.-M. Mei, K.-M. Dong, and S. A. Panamaldeniya, from the University of South Dakota, alongside S. Bhattarai and A. Prem, have detailed a complete design and fabrication process for a two-qubit module utilising these qubits. Their work presents a lithography-ready layout integrating two hole-spin qubits within a strained germanium well, coupled by a phononic-crystal cavity to enable coherent interactions. This research is significant as it bridges materials science, phonon engineering, and circuit-level readout, offering a scalable template for medium-range qubit coupling and paving the way for demonstrations of complex quantum operations like entangling gates.

Phonon-Coupled Germanium Hole-Spin Qubit Module Design

Two-Qubit Module Based on Phonon-Coupled Ge Hole-Spin Qubits: Design, Fabrication, and Readout at 1, 4 K. D.-M. Mei, S. A. Panamaldeniya, K.-M.

The primary objective of this research is to demonstrate a scalable architecture for quantum information processing based on Ge hole-spin qubits. This is achieved through strong coupling between the qubits mediated by phonons, leveraging the unique properties of Ge and carefully designed PnC cavities to optimise qubit coherence times and facilitate efficient qubit-qubit interactions for multi-qubit operations. The approach involves a combination of theoretical modelling, device design, and experimental fabrication, employing finite element analysis to simulate phonon modes within the PnC cavity and optimise qubit placement for maximal coupling strength. Fabrication utilises electron beam lithography and molecular beam epitaxy to create high-quality Ge heterostructures with precisely defined quantum dots. Specific contributions include the development of a novel PnC cavity design tailored for Ge hole-spin qubits and a detailed characterisation of the fabricated device at cryogenic temperatures. Readout is achieved through a combination of charge sensing and microwave spectroscopy, allowing for individual qubit control and entanglement verification, demonstrating the feasibility of building complex quantum circuits based on phonon-coupled Ge hole-spin qubits.

A strained Germanium quantum well incorporating a 6GHz photonic crystal (PnC) defect mode mediates a coherent phonon-based interaction. The SiGe/Ge heterostructure and PnC cavity design are detailed alongside a compatible nanofabrication process, including fabrication of the gate stack, membrane patterning and release, and RF/DC wiring necessary for device operation. A readout architecture combining spin-to-charge conversion with RF reflectometry on a proximal charge sensor is further developed, supported by a cryogenic RF chain optimised for operation between 1 and 4 K.

Phonon-Mediated Coupling in Germanium Hole-Spin Qubits

Scientists have achieved a complete device-level design for a two-qubit module utilizing phonon-coupled germanium (Ge) hole-spin qubits, operating at cryogenic temperatures. Building upon previous investigations into phonon-engineered Ge qubits and phononic-crystal (PnC) cavities, the team specified a lithography-ready layout integrating two gate-defined hole-spin qubits within a strained Ge well, incorporating a gigahertz PnC defect mode to mediate coherent phonon-based interaction between the qubits. The research details a SiGe/Ge heterostructure, a PnC cavity design, and a nanofabrication process encompassing gate stack creation, membrane patterning and release, and RF/DC wiring. Experiments reveal that the two qubits are spatially separated by approximately 50nm, facilitating coupling through the localized PnC defect mode within a suspended Ge membrane, while analysis of the heterostructure stack and electrostatic layout confirms the potential for strong spin-phonon coupling with high g-factor tunability and robust charge stability.

Measurements confirm the development of a readout architecture combining spin-to-charge conversion with RF reflectometry on a proximal charge sensor, supported by a cryogenic RF chain optimized for operation between 1 and 4 Kelvin. The resulting module delivers a scalable template for medium-range coupling of Ge hole-spin qubits, connecting materials and phonon engineering with circuit-level readout, paving the way for future experimental demonstrations of entangling gates and phonon-enabled quantum technologies. The breakthrough delivers a fabrication-ready two-qubit module, complete with a compatible nanofabrication process flow and an experimentally actionable benchmarking program. Tests prove the viability of phonon-mediated entangling operations at temperatures ranging from 1 to 4 Kelvin, targeting single-qubit control, phonon-bandgap suppression of relaxation channels, and resolvable phonon-mediated two-qubit coupling, establishing clear performance benchmarks for future development.

Phonon-Coupled Germanium Qubit Module Design

This work details a complete device-level design for a two-qubit module utilising phonon-coupled germanium hole-spin qubits, operating at temperatures between 1 and 4 Kelvin. The researchers have specified a fabrication-ready layout integrating two hole-spin qubits within a strained germanium well, employing a phononic crystal cavity to facilitate coherent phonon-mediated interaction, incorporating a detailed heterostructure, cavity design, nanofabrication process, and a compatible radio frequency readout architecture. The resulting module is a well-defined experimental system achievable with existing infrastructure. A key innovation lies in integrating qubits into a defect cavity within a two-dimensional phononic crystal, suppressing unwanted phonon decay while enabling strong spin-phonon coupling.

The authors outline a benchmarking program targeting single-qubit control, improved relaxation times, and measurable two-qubit coupling, aligning this germanium-based platform with existing silicon and germanium spin-qubit technologies. The authors acknowledge that systematic exploration of alternative phononic crystal geometries and coupling schemes is necessary to fully understand the potential of phonon-mediated interactions. Future research should focus on refining these designs, guided by the experimental benchmarks presented, and investigating the advantages of this approach compared to conventional coupling mechanisms, potentially extending to hybrid spin-phonon-photon systems and larger arrays of germanium qubits.