Superconducting qubits need nonlinearity—without it, the energy levels are equally spaced and you cannot address a single transition. Josephson junctions provide this through their sinusoidal current-phase relationship. But thin superconducting nanowires at low supercurrents have an increasingly linear current-phase relationship. Individual nanowires are useless for qubits.
Sun and Bezryadin show that two linear nanowires in parallel—a Dayem loop—can produce the nonlinearity that neither possesses alone. Threading magnetic flux through the loop creates quantum interference between the two superconducting condensates via the Little-Parks effect. This interference restores cubic nonlinearity in the combined current-phase relationship.
The restoration is sharp: the oscillatory dependence on flux means there are specific flux values where the nonlinearity is maximized and others where it vanishes again. At the optimal flux, the anharmonicity is sufficient for a functional transmon qubit.
Each wire alone is too linear. Together, under a magnetic field that creates a phase difference between them, the interference pattern reintroduces exactly what was missing. The nonlinearity is not a property of either component—it is a property of their relationship. Linearity in the parts produces nonlinearity in the whole, mediated by quantum coherence.
The authors examine both generic cubic current-phase relations and realistic microscopic models with higher-order terms, confirming the mechanism is robust.
Properties absent from every component individually can emerge from interference between components. The relationship creates what the elements lack.