  Quantum sensors based on nitrogen-vacancy centers in diamond have emerged as
a promising detection modality for nuclear magnetic resonance (NMR)
spectroscopy owing to their micron-scale detection volume and non-inductive
based detection. A remaining challenge is to realize sufficiently high spectral
resolution and concentration sensitivity for multidimensional NMR analysis of
picoliter sample volumes. Here, we address this challenge by spatially
separating the polarization and detection phases of the experiment in a
microfluidic platform. We realize a spectral resolution of 0.65 +/- 0.05 Hz, an
order-of-magnitude improvement over previous diamond NMR studies. We use the
platform to perform two-dimensional correlation spectroscopy of liquid analytes
within an effective ~20 picoliter detection volume. The use of diamond quantum
sensors as in-line microfluidic NMR detectors is a significant step towards
applications in mass-limited chemical analysis and single cell biology.
