Interplay between dressed and strong-axial-field states in nitrogen vacancy centers for quantum sensing and computation

The nitrogen vacancy (NV) center in diamond is an intriguing electronic spin system with applications in quantum radiometry, sensing, and computation. In those experiments, a bias magnetic field is commonly applied along the NV symmetry axis to eliminate the triplet ground-state manifold's degeneracy ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mi>S</a:mi> <a:mo>=</a:mo> <a:mn>1</a:mn> </a:mrow> </a:math> ). In this configuration, the eigenvectors of the NV spin's projection along its axis are called strong-axial-field states. Conversely, in some experiments, a weak magnetic field is applied orthogonally to the NV symmetry axis, leading to eigenstates that are balanced linear superpositions of strong-axial-field states, referred to as dressed states. The latter are sensitive to environmental magnetic noise at the second order, allowing to perform magnetic field protected measurements while providing increased coherence times. However, if a small axial magnetic field is added in this regime, the linear superposition of strong-axial-field states becomes unbalanced. This paper presents a comprehensive study of free induction decay (FID) measurements performed on an NV center ensemble in the presence of strain and weak orthogonal magnetic field, as a function of a small magnetic field applied along the NV symmetry axis. The simultaneous detection of dressed states and unbalanced superpositions of strong-axial-field states in a single FID measurement is shown, gaining insight into coherence time, nuclear spin, and the interplay between temperature and magnetic field sensitivity. The discussion concludes by describing how the simultaneous presence of magnetically sensitive and insensitive states opens up appealing possibilities for both sensing and quantum computation applications.