On-Chip Andreev Devices: Hard Superconducting Gap and Quantum Transport in Ballistic Nb–In0.75Ga0.25As-Quantum-Well–Nb Josephson Junctions

A superconducting hard gap in hybrid superconductor–semiconductor devices has been found to be necessary to access topological superconductivity that hosts Majorana modes (non-Abelian excitation). This requires the formation of homogeneous and barrier-free interfaces between the superconductor and semiconductor. Here, a new platform is reported for topological superconductivity based on hybrid Nb–In_0.75Ga_0.25As-quantum-well–Nb that results in hard superconducting gap detection in symmetric, planar, and ballistic Josephson junctions. It is shown that with careful etching, sputtered Nb films can make high-quality and transparent contacts to the In_0.75Ga_0.25As quantum well, and the differential resistance and critical current measurements of these devices are discussed as a function of temperature and magnetic field. It is demonstrated that proximity-induced superconductivity in the In_0.75Ga_0.25As-quantum-well 2D electron gas results in the detection of a hard gap in four out of seven junctions on a chip with critical current values of up to 0.2 µA and transmission probabilities of >0.96. The results, together with the large g-factor and Rashba spin–orbit coupling in In_0.75Ga_0.25As quantum wells, which indeed can be tuned by the indium composition, suggest that the Nb–In_0.75Ga_0.25As–Nb system can be an excellent candidate to achieve topological phase and to realize hybrid topological superconducting devices.

The proximity-induced superconductivity in a high-mobility 2D electron gas in indium gallium arsenide (In_0.75Ga_0.25As) quantum well is reported. The result opens a new road toward hybrid topological quantum systems and helps pave the way for the development of on-chip Andreev devices for realizing the next generation of quantum processors.

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