Gate- and flux-tunable sin(2φ) Josephson element with planar-Ge junctions

Area of Science:

• Quantum Computing
• Condensed Matter Physics
• Materials Science

Background:

• Hybrid superconductor-semiconductor Josephson field-effect transistors (JoFETs) are Josephson junctions with gate-tunable critical current.
• These JoFETs can exhibit non-sinusoidal current-phase relations (CPRs) with multiple harmonics, a property not fully utilized.
• A π-periodic CPR is crucial for building protected superconducting qubits.

Purpose of the Study:

• To exploit the multi-harmonic CPR property of JoFETs.
• To engineer a Josephson circuit element with an almost perfectly π-periodic CPR.
• To demonstrate a new route towards parity-protected superconducting qubits.

Main Methods:

• Fabrication of a superconducting quantum interference device (SQUID) with low-inductance aluminum arms and two identical JoFETs.
• Utilizing a SiGe/Ge/SiGe quantum-well heterostructure with a high-mobility two-dimensional hole gas for JoFETs.
• Adjusting JoFET gate voltages and magnetic flux through the SQUID to achieve a specific operational regime.

Main Results:

• Achieved a Josephson circuit element with an almost perfectly π-periodic CPR.
• Demonstrated a dominant charge-4e supercurrent transport, accounting for over 95% of the total supercurrent.
• Successfully realized a key building block for protected superconducting qubits.

Conclusions:

• The multi-harmonic CPR of JoFETs can be effectively utilized to create π-periodic Josephson elements.
• This work presents a significant advancement in the development of parity-protected superconducting qubits.
• The engineered Josephson element offers a promising platform for future quantum computing applications.