The germanium quantum information route (2004.08133v1)

Abstract: In the worldwide endeavor for disruptive quantum technologies, germanium is emerging as a versatile material to realize devices capable of encoding, processing, or transmitting quantum information. These devices leverage special properties of the germanium valence-band states, commonly known as holes, such as their inherently strong spin-orbit coupling and the ability to host superconducting pairing correlations. In this Review, we initially introduce the physics of holes in low-dimensional germanium structures with key insights from a theoretical perspective. We then examine the material science progress underpinning germanium-based planar heterostructures and nanowires. We review the most significant experimental results demonstrating key building blocks for quantum technology, such as an electrically driven universal quantum gate set with spin qubits in quantum dots and superconductor-semiconductor devices for hybrid quantum systems. We conclude by identifying the most promising prospects toward scalable quantum information processing.

• The paper’s main contribution is demonstrating germanium’s robust spin-orbit interaction and superconducting integration for quantum device platforms.
• It employs a range of methodologies involving Ge/Si core/shell nanowires, hut wires, and planar heterostructures to showcase quantum confinement and qubit scalability.
• Results validate single-hole qubit operation and two-qubit logic, underscoring germanium's promise for scalable, fault-tolerant quantum architectures.

Germanium Quantum Information Pathways

The paper titled "The Germanium Quantum Information Route" provides a comprehensive investigation into the burgeoning potential of germanium (Ge) as a material foundation for advanced quantum technologies. Germanium, an element historically overshadowed by silicon in the realms of microelectronics, is re-emerging as a pivotal material in quantum computing due to its unique intrinsic properties and compatibility with existing CMOS technologies, making it a subject of interest for future scalable quantum information systems.

Key Features and Material Platforms

The paper emphasizes several intrinsic properties of germanium that make it advantageous for quantum technology:

• Spin-orbit coupling and superconducting capabilities: Germanium's high hole mobility, robust spin-orbit interaction, and the ability to integrate superconducting elements are core to its quantum suitability.
• Ge's inherent properties: The material’s ability to support quantum confined holes and exhibit superconducting pairing correlations is critical for developing spin qubits, topological quantum states, and hybrid systems.

Three primary germanium-based material platforms stand out in the pursuit of quantum devices:

• Ge/Si core/shell nanowires: These structures benefit from significant spin-orbit coupling, manageable heavy-hole and light-hole states, and strong confinement characteristics.
• Ge hut wires: These epitaxial nanostructures on silicon exhibit strong quantum confinement and uniformity, allowing for robust qubit integration.
• Ge/SiGe planar heterostructures: Recognized for yielding high-mobility hole channels, these structures support large-scale integration and quantum dot (QD) array formation necessary for spin-based qubit networks.

Advances in Quantum Devices

Germanium plays a pivotal role in cutting-edge quantum device design:

• Quantum dots and qubits: Ge devices have demonstrated single-hole qubit operation and two-qubit logic, with coherent qubit manipulation validated in Ge/SiGe heterostructures.
• Superconductor-semiconductor hybrids: The integration of superconducting elements in Ge-based devices such as Josephson field-effect transistors (JoFETs) and gatemons highlights the material's compatibility with superconducting quantum circuitry. High transparency interfaces between Ge and superconductors like aluminum are empirically confirmed, underscoring the feasibility of Majorana-based topological qubits.

Quantum Information Implications

The paper forecasts numerous implications and future directions:

• Logical qubits and error correction: The all-electronic control realized through Ge's spin-orbit coupling may facilitate scalable qubit arrays necessary for fault-tolerant quantum computation.
• Gate-based readout and long-range entanglement: Enhanced readout efficiencies via spin-cavity interactions can occur with Ge's spin-orbit capabilities, presenting pathways to entangling distant qubit systems.
• Topological quantum computation: Germanium’s material synergy with superconducting elements opens avenues for exploring topological states like Majorana zero modes, contributing to hybrid logical gate structures and long-range qubit connectivity.

In summary, the research on germanium as reviewed in this paper elucidates a clear trajectory towards it being a key player in scaling quantum technologies. Its intrinsic properties, paired with recent advancements in material fabrication and device integration, suggest that germanium will be integral to future quantum computers bridging superconductivity, spintronics, and topological phases, thereby steering towards a comprehensive quantum information technology infrastructure.