Quantum Computing with Germanium-Based Josephson Junctions

Researchers have demonstrated direct microwave spectroscopy of Andreev-bound states in planar Josephson junctions defined in Ge high-mobility two-dimensional hole gases contacted by superconducting platinum germano-silicide. The devices were fabricated using a strained Ge quantum well and characterized by DC and microwave spectroscopy techniques. The results show hybridization between the Andreev bound states and the resonator mode, which is consistent with a Jaynes-Cummings model. The induced superconducting gap was estimated to be around 1.48 μeV, and the parity lifetime in Ge-based hybrid systems was observed to be seconds. This work provides a viable platform for isolating discrete Andreev-bound states in Ge quantum wells and probing them via microwave spectroscopy.

Andreev-bound states (ABSs) are discrete energy excitations at the interface between superconductors and semiconductors. These states play a crucial role in quantum computing applications, as they can be used to harness the charge and spin properties of particles. In recent years, researchers have employed isolated ABSs in new qubit architectures, which has led to significant advancements in the field.

However, current platforms for studying ABSs, such as group II-V materials, have limitations that hinder their potential for quantum computing applications. These limitations include short spin dephasing times and the need for advanced qubit control techniques. Therefore, it is essential to develop alternative platforms to investigate the physics of ABSs further and mitigate existing limitations.

The 2D hole gases in Ge might constitute a promising platform for studying ABSs due to their high mobility, strong spin-orbit interaction, and the possibility of providing a nuclear-spin-free environment by isotopic purification. Furthermore, a planar geometry allows for the realization of complex devices beyond the options of semiconducting nanowires.

The experimental setup consists of high-mobility 2D hole gases proximitized by PtSiGe contacts. Elect electrostatic gating defines tunable planar Josephson junctions (JJs), and the devices are characterized by dc and microwave spectroscopy techniques. The JJs are incorporated in radiofrequency superconducting quantum interference devices, which enables tunability of the phase drop across the junction.

The results show that the devices can be operated in a regime where few ABSs at low energy dominate the microwave response. Single-tone spectroscopy reveals hybridization between the ABSs and the resonator mode, which is consistent with a Jaynes-Cummings model. Two-tone microwave spectroscopy enables the estimation of the induced superconducting gap Δ1 and the mode transmission τ. Monitoring of the readout resonator operated in the dispersive regime enables the observation of JJ parity fluctuations on a time scale of seconds, providing an estimate of the parity lifetime in Ge-based hybrid systems.

In conclusion, this work demonstrates a viable platform for isolating discrete ABSs in Ge quantum wells, probing them via microwave spectroscopy, and performing real-time parity measurements. The results show that 2D hole gases in Ge can be used to study ABSs, which is an essential step towards developing new qubit architectures and mitigating existing limitations in quantum computing applications.

Future directions include further optimization of the device design and materials to improve the coherence times of the ABSs. Additionally, exploring the potential of 2D hole gases in Ge for other quantum computing applications, such as topological quantum computing, is an exciting area of research.