Cavity-QED based on collective magnetic dipole coupling: spin ensembles as hybrid two-level systems

TL;DR

This paper proposes a cavity-QED system using collective magnetic dipole coupling of spin ensembles with superconducting circuits, enabling hybrid two-level systems for quantum information processing without single-electron confinement.

Contribution

It introduces a novel hybrid two-level system based on collective spin-wave excitations coupled to a nonlinear microwave cavity, expanding quantum computing platforms.

Findings

Strong coupling regime achieved with spin ensembles and microwave cavities.

Spin ensembles can function as qubits manipulated via cavity nonlinearity.

Potential for quantum protocols in silicon or graphene without single-electron confinement.

Abstract

We analyze the magnetic dipole coupling of an ensemble of spins to a superconducting microwave stripline structure, incorporating a Josephson junction based transmon qubit. We show that this system is described by an embedded Jaynes-Cummings model: in the strong coupling regime, collective spin-wave excitations of the ensemble of electrons pick up the nonlinearity of the cavity mode, such that the two lowest eigenstates of the coupled spin-wave + microwave-cavity + Josephson-junction system define a hybrid two-level system. The proposal described here enables the use of spin ensembles as qubits which can be coherently manipulated and coupled using the same nonlinear-cavity. Possibility of strong-coupling cavity-QED with magnetic-dipole transitions opens up the possibility of extending previously proposed quantum information processing protocols to spins in silicon or graphene, without the need for single-electron confinement.