A clean ballistic quantum point contact in strontium titanate

Abstract

Two dimensional electron gases based on SrTiO3 are an intriguing platform for exploring mesoscopic superconductivity combined with spin-orbit coupling, offering electrostatic tunability from insulator to metal to superconductor within a single material. So far, however, quantum effects in SrTiO3 nanostructures have been complicated by disorder. Here we introduce a facile approach to achieving high mobility and patterning gate-tunable structures in SrTiO3, and use it to demonstrate ballistic constrictions with clean normal state conductance quantization. Conductance plateaus show two-fold degeneracy that persists to magnetic fields of at least 5 T -far beyond what one would expect from the g−factor extracted at high fields -a potential signature of electron pairing extending outside the superconducting regime.Advances in the cleanliness of low-dimensional electron systems are typically produced by painstaking optimization of material quality. But occasionally, simplification of fabrication flows or material synthesis can play a key role.One prominent example is the invention of the mechanical exfoliation method to isolate monolayer graphene [1], a non-resource-intensive technique that democratized access to high quality 2D systems rich with new physics. In the same spirit, here we present a widely accessible fabrication method for a clean, ballistic quantum system in SrTiO 3 , a material known for its rich physics [2][3][4]. We forgo the expensive and complex epitaxial growth techniques typically used to achieve high mobility in dimensional electron gases (2DEGs), using only commercially available single crystals, standard ionic liquid gating, electron beam lithography, and widely available, low-temperature deposition techniques: sputtering and atomic layer deposition. Development of clean quantum systems is a central goal in condensed matter physics and materials science, driven in part by the promise of large-scale quantum computing. Architectures for solid-state quantum computing [5, 6] often involve superconductivity and nanoscale patterning, and can benefit from electrostatic tunability (as in gatemons [7, 8].) For topological qubits [9, 10], these three elements are required, along with spin-orbit coupling. A challenge for all routes towards large-scale quantum computation is in mitigating disorder, dissipation, and noise [5], which prevent high-fidelity quantum state control. Disorder-induced localized states are particularly problematic for demonstrating topological qubits, as they can mimic the most easily detectable signatures of Majorana states [10, 11]. The predominant approach for combining gate tunability and superconductivity is through proximitization of a high-mobility semiconductor (e.g. InAs, InSb) by a metallic superconductor (e.g. Al, Nb). Despite major progress in improving interfaces between such dissimilar materials, they remain major sources of the types of imperfections mentioned above [9]. An alternative approach is to construct a monolithic 51 quantum system from a single material that ...