  In nanoscale metrology applications, measurements are commonly limited by the
performance of the sensor. Here we show that in nuclear magnetic resonance
(NMR) spectroscopy measurements using single nitrogen-vacancy (NV) centers in
diamond, the NV sensor electron spin limits spectral resolution down to a few
hundred Hz, which constraints the characterization and coherent control of
finite spin systems, and furthermore, is insufficient for high resolution NMR
spectroscopy aiming at single molecule recognition and structure analysis of
the latter. To overcome the limitation, we support an NV electron spin sensor
with a nuclear spin qubit acting as quantum and classical memory allowing for
intermediate nonvolatile storage of metrology information, while suppressing
the deleterious back-action of the sensor onto the system under investigation.
We demonstrate quantum and classical memory lifetimes of 8 ms and 4 minutes
respectively under ambient conditions. Furthermore, we design and test
measurement and decoupling protocols, which exploit such memory qubits
efficiently. Using our hybrid quantum-classical sensor device, we achieve high
resolution NMR spectra with linewidths of single spins down to 13 Hz. Our work
is therefore a prerequisite for high resolution NMR spectroscopy on nanoscopic
quantum systems down to the single level.

Quantum Physics;Mesoscale and Nanoscale Physics