Engineering Purely Nonlinear Coupling with the Quarton

TL;DR

This paper introduces the quarton, a device enabling purely nonlinear coupling between superconducting qubits, achieving significantly stronger cross-Kerr effects and linearizing qubits, which enhances quantum information processing capabilities.

Contribution

The quarton provides a novel approach for purely nonlinear coupling with giant cross-Kerr effects and mode linearization, surpassing existing dispersive coupling schemes.

Findings

Achieves gigahertz-level cross-Kerr coupling.

Cancels negative self-Kerr to linearize qubits.

Enables applications like microwave photon detection.

Abstract

Strong nonlinear coupling of superconducting qubits and/or photons is a critical building block for quantum information processing. Due to the perturbative nature of the Josephson nonlinearity, linear coupling is often used in the dispersive regime to approximate nonlinear coupling. However, this dispersive coupling is weak and the underlying linear coupling mixes the local modes which, for example, distributes unwanted self-Kerr to photon modes. Here, we use the quarton to yield purely nonlinear coupling between two linearly decoupled transmon qubits. The quarton's zero $\phi^2$ potential enables a giant gigahertz-level cross-Kerr which is an order of magnitude stronger compared to existing schemes, and the quarton's positive $\phi^4$ potential can cancel the negative self-Kerr of qubits to linearize them into resonators. This giant cross-Kerr between bare modes of qubit-qubit, qubit-photon, and even photon-photon is ideal for applications such as single microwave photon detection and implementation of bosonic codes.