Fig. 1: Two-level quantum system (TLQS) designed with two types of point defects in oxide heterostructures.

a Schematic coordination diagram for a classical two-level systems (TLS). E_e represents the energy of electrons. The normal RTN involves the random transition between the state A and the state A’. Since shallow defects act as major charge traps with a small energy barrier (\({\phi }_{b}\)) the state transition is fast and unstable. b TLQS employs the shallow (Type-1) and the deep (Type-2) defects to control the state transition behavior. The fast and unstable transitions of the Type-1 defects are suppressed, as depicted by the gray-colored dashed lines, by controlling the internal potential profile. Instead, the complementary interaction between the different types of defects creates state A and B. The state transition depends on the temporal localization of electrons at Type-2 defects, resulting in the highly stable current fluctuation. c Schematic illustration of electron transport in TLQS. (Top) At a Schottky interface, Type-1 defects work as electron hopping sites, resulting in a specific electron transfer rate \({\varGamma }_{A}\). (Bottom) When an electron occupies the type 2 defects, the energy level of Type-1 defects upshifts and leads to the increased electron transfer rate \({\varGamma }_{B}\). Thus, depending on the charge state of the Type-2 defects, the tunneling current fluctuates between discrete two states. d TLQS device structure based on SrRuO₃/LaAlO₃/Nb-doped SrTiO₃ heterostructure.