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**UNC Physics Colloquium**

*Evgeny Mikheev, Stanford University*

**Quantum point contacts in an oxide superconductor**

**Abstract**

Superconductors and semiconductors are typically thought of as distinct material categories. Each has fascinating and technologically useful electronic properties. What if one could combine them in one material? This is rare but possible: for example, the oxide SrTiO3 superconducts at carrier densities so low that its superconductivity can be tuned by applying electric fields. I will explain how this can be done locally by applying voltages to nanopatterned gate electrodes. This presents a unique opportunity to create superconductor/normal state junctions without the complexity of interfacing two dissimilar materials.

I will present my recent work [1, 2] on defining narrow constrictions between larger superconducting regions in SrTiO3. I will demonstrate how the combination of quantum confinement and local gate tuning results in quantized staircase shapes in normal state conductance. This is a hallmark of clean ballistic behavior that typically requires working with pristine high-mobility semiconductors. An even more distinctive quantization signature is observed in the superconducting state: each ballistic mode can only carry a finite quantum of supercurrent, and a staircase shape is seen in the critical current. This work inches us closer than ever to experimentally realizing superconducting junctions coupled by one or few perfectly transmitting ballistic modes. This a difficult but important technological goal: such junctions are key enablers for several approaches to protect quantum information from dephasing and bit-flip errors. My future target is to integrate oxide nanostructures into gate-tunable transmon qubits (gatemons), Andreev qubits, and nanowires with topological superconductivity.

[1] E. Mikheev, I. T. Rosen, D. Goldhaber-Gordon, Science Advance 7, eabi6520 (2021)[2] E. Mikheev, I. T. Rosen, M. A. Kastner, D. Goldhaber-Gordon, arXiv:2110.11535