Solid-state quantum sensors are attracting wide interest because of their exceptional sensitivity at room temperature. In particular, the spin properties of individual nitrogen vacancy (NV) color centers in diamond make it an outstanding nanoscale sensor of magnetic fields, electric fields, and temperature, under ambient conditions. Recent work on ensemble NV-based magnetometers, inertial sensors, and clocks have employed $N$ unentangled color centers to realize a factor of up to $sqrt{N}$ improvement in sensitivity. However, to realize fully this signal enhancement, new techniques are required to excite efficiently and to collect fluorescence from large NV ensembles. Here, we introduce a light-trapping diamond waveguide (LTDW) geometry that enables both high fluorescence collection ($sim20%$) and efficient pump absorption achieving an effective path length exceeding $1$ meter in a millimeter-sized device. The LTDW enables in excess of $2%$ conversion efficiency of pump photons into optically detected magnetic resonance (ODMR) fluorescence, a textit{three orders of magnitude} improvement over previous single-pass geometries. This dramatic enhancement of ODMR signal enables broadband measurements of magnetic field and temperature at less than $1$ Hz, a frequency range inaccessible by dynamical decoupling techniques. We demonstrate $sim 1~mbox{nT}/sqrt{mbox{Hz}}$ magnetic field sensitivity for $0.1$ Hz to $10$ Hz and a thermal sensitivity of $sim 400 ~mumbox{K}/sqrt{mbox{Hz}}$ and estimate a spin projection limit at $sim 0.36$ fT/$sqrt{mbox{Hz}}$ and $sim 139~mbox{pK}/sqrt{mbox{Hz}}$, respectively.
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