Deep-level modeling of novel germanium-based superconductive quantum devices

A great scientific and technical challenges of our time is to engineer a scalable quantum computer, and proximity-induced superconductivity is one of the most important ingredients in many quantum devices.

Proximitized III-V semiconductors can host a hard superconducting gap and have been vastly studied in super-semi quantum devices.

Nevertheless, these material compounds are not suitable for spin-based qubits due to their large hyperfine interaction and are hence not ideal for use in hybrid devices.

One of the most promising, but so far unexplored researched materials to use in such hybrid quantum devices is germanium: it is a potentially ideal host for proximity-induced superconductivity, and exhibits a hard superconducting gap, but can also be used for spin-based qubits since it has suppressed hyperfine interaction and can be isotope purified to be nearly nuclear-spin free.

Additionally, it has an exceptionally large hole mobility, strong intrinsic spin-orbit coupling, as well as tunable g-factors, making it an ideal material to use in quantum devices. However, the mechanism for the superconducting proximity effect in germanium is still unknown.

In this project I will develop band models based on atomistic orbitals (kp theory), and combine them with T-matrix methods from Greens function theory, to show what mechanisms are responsible for proximity induced superconductivity in the hole bands of germanium.

I will apply the results to g-factor dependent Andreev spin qubits and phase- and gate-tunable long-range spin-qubit couplers.

I will predict effective spin-orbit coupling and g-factors in these superconducting quantum devices, using effective low-energy models (discretized on a lattice) to predict outcomes of future experiments on germanium-based quantum information devices. This will open up a new avenue of research, through the development of a new type of qubit.