High-quality Ge/SiGe Cavities Enable Coherent Control of Hole Spin Qubits

The development of robust superconductor-semiconductor interfaces represents a significant hurdle in advancing technologies such as quantum computing and novel electronic devices, and researchers are actively seeking materials that combine the best properties of both worlds. Franco De Palma, Elena Acinapura, and Wonjin Jang, alongside colleagues at École Polytechnique Fédérale de Lausanne and the University of Basel, now demonstrate a method for creating high-quality microwave cavities on germanium/silicon-germanium heterostructures, materials promising for hosting advanced qubit devices. The team addresses the challenge of defects in these layered materials by employing a tapered etching approach, effectively creating a smooth transition for superconducting circuits to connect with the underlying semiconductor. This technique yields resonators with remarkably high performance, comparable to those fabricated on conventional silicon wafers, and establishes a practical pathway towards integrating superconducting and semiconductor technologies on a single platform for emerging planar germanium devices.

However, realising the full potential of these materials in quantum technologies requires compatibility with high-quality superconducting circuits, a challenge often limited by defects within the material stack. Researchers have now developed a technique to enhance the coherence of cavity modes on reverse-graded Ge/SiGe heterostructures, addressing a key limitation and paving the way for improved qubit performance and scalability by optimising material growth and fabrication techniques to minimise decoherence mechanisms.

Germanium Resonators and Circuit Quality Enhancement

Significant progress has been made in designing and fabricating high-performance superconducting circuits for quantum computing applications. This work focuses on improving superconducting resonators, crucial components for strong coupling in circuit QED and building parametric amplifiers. Researchers are exploring high impedance and high kinetic inductance resonators, often utilising spiral geometries to enhance quality factors. Key materials include germanium, for creating hole spin qubits, and aluminium, for fabricating Josephson junctions. Advanced fabrication techniques, such as lift-off photoprocessing, dry etching, and offset masks, are employed to create these intricate circuits.

A major theme in this research is understanding and mitigating loss and noise that degrade qubit performance. Two-level systems, ubiquitous defects in materials, contribute significantly to loss and noise, and reducing their density is a central goal. Dielectric loss, material defects, and surface effects also contribute to performance limitations. Researchers are actively investigating techniques to reduce these effects, including material purification, surface treatment, design optimisation, shielding, and annealing. Understanding and controlling critical current fluctuations and low-frequency noise are also vital for improving qubit coherence.

Addressing magnetic field noise through careful shielding is also a key focus. This research supports the development of various qubit technologies, including hole spin qubits in germanium and widely used transmon qubits. Circuit Quantum Electrodynamics, the field combining superconducting circuits with quantum optics, is central to this work. Achieving strong coupling between qubits and resonators is essential for quantum information processing, and maintaining quantum coherence is crucial for performing computations. The ultimate goal is to build and control quantum systems for performing complex calculations.

Characterisation techniques, such as measuring scattering parameters and quality factors, are used to assess resonator performance. Noise spectroscopy and microwave techniques are employed to identify and mitigate noise sources. Emerging areas of research include exploring the superconductor-insulator transition, investigating dissipative quantum chaos, and designing circuits resilient to magnetic field fluctuations. Researchers are also developing high impedance resonators for hybrid architectures, connecting different quantum systems. This concerted effort to build better superconducting quantum circuits focuses on materials, fabrication, design, and characterisation techniques to achieve high-quality qubits and resonators for quantum computing applications.

Germanium Etching Enables Superconducting Circuit Fabrication

Researchers have achieved a breakthrough in fabricating high-quality superconducting circuits on germanium-silicon heterostructures, paving the way for advanced quantum devices. A key challenge is ensuring compatibility between these semiconductor platforms and high-performance superconducting resonators, often limited by defects in the material stack. The team developed a novel etching process to remove much of the germanium-silicon material, leaving a smooth transition down to an underlying silicon substrate. This allows for direct fabrication of superconducting circuits on the silicon, while still enabling access to the germanium layer for potential device integration.

The etching process creates a tapered mesa structure, carefully engineered to facilitate the fabrication of superconducting circuits directly onto the germanium layer from the silicon substrate. Electrical tests on numerous structures confirmed that this fabrication process introduces no disconnections, maintaining circuit integrity. To quantify the impact of this approach, the team fabricated and characterised high-impedance Josephson junction array resonators on three different substrates: bare germanium-silicon, etched germanium-silicon, and a silicon reference wafer. Measurements performed at extremely low temperatures, 10 millikelvin, revealed exceptional performance.

Resonators fabricated on the etched heterostructure exhibited internal quality factors limited only by the fabrication of the Josephson junctions themselves. Notably, these quality factors were comparable to those achieved on the pristine silicon reference wafer, demonstrating that the etching process does not degrade resonator performance. Further analysis revealed a self-Kerr coefficient consistent with theoretical predictions, indicating strong nonlinearity, a desirable trait for certain quantum applications. These results demonstrate a practical path toward integrating superconducting circuits with germanium-based semiconductors, opening new possibilities for planar Ge-based quantum technologies and advanced devices.

Germanium Resonators Achieve High Coherence

Scientists have developed a technique to significantly enhance the coherence of superconducting circuits built on germanium-silicon germanium heterostructures, materials promising for advanced quantum devices. Recognising that defects within these layered materials limit performance, the team devised a method to etch the heterostructure down to a high-resistivity silicon substrate. This process allows for the direct fabrication of superconducting resonators on the silicon, effectively isolating them from the defect-prone layers above. The researchers demonstrated substantial improvements in the internal quality factor of these resonators, reaching values comparable to those achieved on pristine silicon wafers.

Specifically, high-impedance resonators exhibited quality factors of tens of thousands, and 50-ohm resonators achieved values exceeding 100,000. Importantly, the etching process itself does not introduce additional loss, and the small overlap between the resonators and the remaining heterostructure does not degrade performance. Furthermore, the team successfully demonstrated tunable resonance frequencies in the high-impedance resonators using magnetic fields compatible with spin qubit operation. This tapered etching technique is broadly applicable to any heterostructure containing defects, offering a versatile path toward highly coherent light-matter interfaces in semiconductor materials and paving the way for more robust and scalable quantum technologies.