Collective Strong Coupling with Homogeneous Rabi Frequencies using a 3D Lumped Element Microwave Resonator

TL;DR

This paper presents the design of 3D lumped element microwave cavities that achieve homogeneous strong coupling with nitrogen vacancy centers in diamond, enabling coherent control of large spin ensembles for quantum applications.

Contribution

The authors develop a novel 3D cavity design that provides uniform magnetic fields and significantly enhances single spin Rabi frequencies compared to standard cavities.

Findings

Achieved a collective coupling strength of 12 MHz.

Demonstrated a homogeneity RMS deviation of 1.54%.

Entered the strong coupling regime with a cooperativity of 27.

Abstract

We design and implement 3D lumped element microwave cavities for the coherent and uniform coupling to electron spins hosted by nitrogen vacancy centers in diamond. Our design spatially focuses the magnetic field to a small mode volume. We achieve large homogeneous single spin coupling rates, with an enhancement of the single spin Rabi frequency of more than one order of magnitude compared to standard 3D cavities with a fundamental resonance at \SI{3}{GHz}. Finite element simulations confirm that the magnetic field component is homogeneous throughout the entire sample volume, with a RMS deviation of 1.54\%. With a sample containing $10^{17}$ nitrogen vacancy electron spins we achieve a collective coupling strength of $\Omega=\SI{12}{MHz}$, a cooperativity factor $C=27$ and clearly enter the strong coupling regime. This allows to interface a macroscopic spin ensemble with microwave circuits, and the homogeneous Rabi frequency paves the way to manipulate the full ensemble population in a coherent way.