  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 $\sim 400 ~\mu\mbox{K}/\sqrt{\mbox{Hz}}$
and estimate a spin projection limit at $\sim 0.36$ fT/$\sqrt{\mbox{Hz}}$ and
$\sim 139~\mbox{pK}/\sqrt{\mbox{Hz}}$, respectively.
