Quantum manipulation of low-energy bound states in superconductors

While superconducting-circuit-based qubits have been widely used in the recent progress of quantum computing despite their limitations, an alternative platform with inherent protection from quantum decoherence as a future generation of qubits is of great interest. Among many, low-energy bound states in superconductors are promising candidates as they are protected by the superconducting gap and further constitute the building block of possible novel topological qubits. Here, by using spin-polarized scanning tunneling microscopy/spectroscopy, we study Yu-Shiba-Rusinov (YSR) states realized as magnetic impurities on a superconductor surface. We use interface-engineered superconductors that are robust to external magnetic fields, allowing us to determine the ground state and manipulate the quasiparticle spins without breaking the superconducting order. Our results provide a promising platform to realize novel topological phases by using a combination of superconductivity, magnetism, and spin-orbit coupling under magnetic fields at various directions.