Delocalized Oxygen as the Origin of Two-Level Defects in Josephson Junctions

Abstract

One of the key problems facing superconducting qubits and other Josephson junction devices is the decohering effects of bistable material defects. Although a variety of phenomenological models exist, the true microscopic origin of these defects remains elusive. For the first time we show that these defects may arise from delocalization of the atomic position of the oxygen in the oxide forming the Josephson junction barrier. Using a microscopic model, we compute experimentally observable parameters for phase qubits. Such defects are charge neutral but have nonzero response to both applied electric field and strain. This may explain the observed long coherence time of two-level defects in the presence of charge noise, while still coupling to the junction electric field and substrate phonons.

Atomic delocalization as a microscopic origin of two-level defects in Josephson junctions

“…≫ E 100 01 GHz). | | Z values larger than 2.75 Å show similar phase behaviour to that of figure 7, which in completely unbound in z. Large| | Z separation distances also decrease the validity of the 2+1D model, in addition: the radial distribution analysis in [18] suggests large separation distances for nearest neighbour atoms have a low probability of occurrence.…”

“…To begin we plot a phase space diagram akin to those introduced in [18], where ξ is plotted as a function of the distance to the confining aluminium atoms (| | | | X Y , ). Each phase diagram is split into at least four domains, where the properties of these domains can be explained through the interplay of potential configuration and dipole alignment (discussed in section 6).…”

“…The dipole magnitudes in figures 13 and 14 are calculated against the electron charge for simplicity, although as we are discussing an oxygen atom, the dipole elements may in fact be larger. Using our JJ DFT models [18] we partition the charge density associated with atoms across the lattice into Bader volumes [42]. The charge enclosed within each Bader volume is a good approximation to the total electronic charge of an atom.…”

“…As described in previous work [18], we consider the origin of some defects to be within the amorphous oxide layer itself, specifically an oxygen atom in a spatially delocalized state. This has important ramifications for materials science based efforts to reduce the effects of TLSs.…”

“…As an illustrative example, consider an interstitial oxygen defect in crystalline silicon: the harmonic approximation for atomic positions cannot be applied due to the rotational symmetry of the defect as oxygen delocalizes around the Si-Si bond axis [26]. This forms an anharmonic system with a quasi-degenerate [18] ground state, even in a 'perfect' crystal. This ansatz allows the existence of many different spatial configurations throughout the layer, causing unique TLS properties based solely on atomic positions and rotation in relation to the external electric field.…”