  We use electromagnetically-induced transparency (EIT) to probe the narrow
electron-spin resonance of nitrogen-vacancy centers in diamond. Working with a
multi-pass diamond chip at temperatures 6-30 K, the zero-phonon absorption line
(637 nm) exhibits an optical depth of 6 and inhomogenous linewidth of ~30 GHz
full-width-at-half-maximum (FWHM). Simultaneous optical excitation at two
frequencies separated by the ground-state zero-field splitting (2.88 GHz),
reveals EIT resonances with a contrast exceeding 6% and FWHM down to 0.4 MHz.
The resonances provide an all-optical probe of external electric and magnetic
fields with a projected photon-shot-noise-limited sensitivity of 0.2
V/cm/sqrt(Hz) and 0.1 nT/sqrt(Hz), respectively. Operation of a prototype
diamond-EIT magnetometer measures a noise floor of less than 1 nT/sqrt(Hz) for
frequencies above 10 Hz and Allan deviation of 1.3 +/- 1.1 nT for 100 s
intervals. The results demonstrate the potential of diamond-EIT devices for
applications ranging from quantum-optical memory to few-photon nonlinear
optics, precision measurement, and tests of fundamental physics.
