# Materials for Quantum Computing

Quantum computing is emerging as the next digital revolution. In solid-state implementations computing errors ultimately originate from the materials comprising the quantum bits (qubits). Fault tolerant qubits may be encoded in exotic quantum states that exist in topological low dimensional systems. One of the most promising material systems for this purpose is hybrid superconductor/ semiconductor/ ferromagnet interfaces. The desired quantum computing functionality, or even the very existence of exotic topological quantum states, depend critically on the nature of the interface at the atomistic level. Epitaxial, atomically sharp, defect free interfaces are particularly desirable.

Our goal is to explore the materials space of hybrid superconductor/ semiconductor/ ferromagnet interfaces in the context of quantum computing. We aim to discover new material combinations for epitaxial interfaces, in which topological phases may exist, as well as metastable interface phases with desirable properties that may be stabilized by epitaxial templating. To this end, we employ a computational approach combining density functional theory (DFT) with lattice matching and genetic algorithm optimization. Lattice matching is used for screening a large number of candidate interfaces and genetic algorithms are used for detailed studies of the most promising candidates. This may guide the experiments of our collaborators in promising directions and advance the realization of new quantum computing schemes.

#### Topological properties of the SnSe/EuS(111) interface

SnSe in a topological crystalline insulator (TCI), a material that is insulating in the bulk but has a conducting state at the surface, which is topologically protected by time reversal symmetry and the crystal symmetry. We conducted DFT simulations of an epitaxial interface between SnSe and the ferromagnetic insulator (FMI), EuS. The magnetic proximity effect breaks the time reversal symmetry and opens a gap of 21 meV at Γ and 9 meV at M in the topological interface state of SnSe, shown in red. Because the magnetic proximity effect is short ranged and confined to the interface, the topological state at the top surface of SnSe, shown in green, in unperturbed. Charge transfer at the interface leads to band bending and shifts the gapped interface states below the Fermi level by 88 meV at Γ and above the Fermi level by 47 meV at M. The topological interface state of SnSe/EuS (111) has a larger gap and a smaller binding energy than e.g., Bi2Se3/EuS. Hence, it could be a potential candidate for implementing quantum/ topological devices.