Non-abelian Majorana zero modes (MZMs) in hybrid superconductor-semiconductor system promise information processing robust to decoherence. The combination of a high spin-orbit semiconductor and an s-wave superconductor, submitted to an in-plane magnetic field, is predicted to host such MZMs and serve as a platform for topological quantum computation. Experimental evidence of MZMs has been reported using InAs and InSb nanowires [see here]. Comparable results on two-dimensional electron gas (2DEG) have only been recently observed with InAs quantum wells grown on InP substrates [see here], opening the possibility to design more and more complex devices via top-down nanofabrication techniques.

In Station Q Purdue, we are focusing on the growth of these InAs 2DEGs on InP substrates epitaxially coupled to Al superconducting layer. We frequently collaborate with the Center for Quantum Devices (Station Q Copenhagen) in Denmark and Qutech (Station Q Delft) in the Netherlands, for device fabrication and electronic transport measurements. As part of this collaborative work, MZMs have been recently evidenced in our InAs quantum wells grown on InP substrates [see here].

Our main efforts focus on the optimization of the InAs 2DEGS on InP substrates working especially on the heterostructure design and on engineering the coupling between the superconductor and the 2DEG. This experimental work is supported by a strong collaboration with theorists from Station Q. As part of this work, the in-situ deposition of the superconductor is also of prime importance. We have a Gen 620 chamber dedicated to these depositions and which is equipped with a cooling system allowing to reach very low temperature below -30°C. Such low temperature is required to obtain a crystalline Al film on As-terminated surfaces with low defect density.

Alternative studies with antimonides are also carried on trying to increase the effective g-factor of the combined superconductor-semiconductor systems by growing InSb quantum wells on GaAs substrates. We are also studying the growth of InAs quantum wells on GaSb substrates in order to reduce disorder induced by surface roughness and threading dislocations as in InAs 2DEGs on InP substrates.

Station Q Purdue team: Geoffrey Gardner, Sergei Gronin, Anthony Hatke, Ray Kallaher, Candice Thomas, Tian Wang