  Two grapes irradiated inside a microwave (MW) oven typically produce a series
of sparks and can ignite a violent plasma. The underlying cause of the plasma
has been attributed to the formation of morphological-dependent resonances
(MDRs) in the aqueous dielectric dimers that lead to the generation of a strong
evanescent MW hotspot between them. Previous experiments have focused on the
electric-field component of the field as the driving force behind the plasma
ignition. Here we couple an ensemble of nitrogen-vacancy (NV) spins in
nanodiamonds (NDs) to the magnetic-field component of the dimer MW field. We
demonstrate the efficient coupling of the NV spins to the MW magnetic-field
hotspot formed between the grape dimers using Optically Detected Magnetic
Resonance (ODMR). The ODMR measurements are performed by coupling NV spins in
NDs to the evanescent MW fields of a copper wire. When placing a pair of grapes
around the NDs and matching the ND position with the expected magnetic-field
hotspot, we see an enhancement in the ODMR contrast by more than a factor of
two compared to the measurements without grapes. Using finite-element
modelling, we attribute our experimental observation of the field enhancement
to the MW hotspot formation between the grape dimers. The present study not
only validates previous work on understanding grape-dimer resonator geometries,
but it also opens up a new avenue for exploring novel MW resonator designs for
quantum technologies.
