Circuit design implementing longitudinal coupling: a scalable scheme for superconducting qubits

Abstract

We present a circuit construction for a new fixed-frequency superconducting qubit and show how it can be scaled up to a grid with strictly local interactions. The circuit QED realization we propose implements ${\sigma }_z$-type coupling between a superconducting qubit and any number of $LC$ resonators. The resulting \textit{longitudinal coupling} is inherently different from the usual ${\sigma }_x$-type \textit{transverse coupling}, which is the one that has been most commonly used for superconducting qubits. In a grid of fixed-frequency qubits and resonators with a particular pattern of always-on interactions, coupling is strictly confined to nearest and next-nearest neighbor resonators; there is never any direct qubit-qubit coupling. We note that just four distinct resonator frequencies, and only a single unique qubit frequency, suffice for the scalability of this scheme. A controlled phase gate between two neighboring qubits can be realized with microwave drives on the qubits, without affecting the other qubits. This fact is a supreme advantage for the scalability of this scheme.

10 pages, 9 figures

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• Charge insensitive qubit design derived from the Cooper pair box
• Quantum information processing with circuit quantum electrodynamics
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• Concentric transmon qubit featuring fast tunability and an anisotropic magnetic dipole moment
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