Strong magnetic coupling of an inhomogeneous NV ensemble to a cavity

TL;DR

This paper demonstrates strong magnetic coupling between an inhomogeneous NV ensemble in diamond and a superconducting cavity, revealing normal mode splitting and potential for a stable maser at low temperatures.

Contribution

It provides experimental and theoretical analysis of the coupling in an inhomogeneous NV ensemble, including modeling of the energy distribution and implications for maser development.

Findings

Normal mode splitting observed in probe spectra

Energy distribution of defects is q-Gaussian and temperature-dependent

Potential for implementing a highly stable maser

Abstract

We study experimentally and theoretically a dense ensemble of negatively charged nitrogen-vacancy centers in diamond coupled to a high $Q$ superconducting coplanar waveguide cavity mode at low temperature. The nitrogen-vacancy centers are modeled as effective spin one defects with inhomogeneous frequency distribution. For a large enough ensemble the effective magnetic coupling of the collective spin dominates the mode losses and inhomogeneous broadening of the ensemble and the system exhibits well resolved normal mode splitting in probe transmission spectra. We use several theoretical approaches to model the probe spectra and the number and frequency distribution of the spins. This analysis reveals an only slowly temperature dependent q-Gaussian energy distribution of the defects with a yet unexplained decrease of effectively coupled spins at very low temperatures below $\unit{100}{\milli\kelvin}$. Based on the system parameters we predict the possibility to implement an extremely stable maser by adding an external pump to the system.