  Solid-state spin systems including nitrogen-vacancy (NV) centers in diamond
constitute an increasingly favored quantum sensing platform. However, present
NV ensemble devices exhibit sensitivities orders of magnitude away from
theoretical limits. The sensitivity shortfall both handicaps existing
implementations and curtails the envisioned application space. This review
analyzes present and proposed approaches to enhance the sensitivity of
broadband ensemble-NV-diamond magnetometers. Improvements to the spin dephasing
time, the readout fidelity, and the host diamond material properties are
identified as the most promising avenues and are investigated extensively. Our
analysis of sensitivity optimization establishes a foundation to stimulate
development of new techniques for enhancing solid-state sensor performance.
