  Microwave delivery to samples in a cryogenic environment can pose
experimental challenges such as restricting optical access, space constraints
and heat generation. Moreover, existing solutions that overcome various
experimental restrictions do not necessarily provide a large, homogeneous
oscillating magnetic field over macroscopic lengthscales, which is required for
control of spin ensembles or fast gate operations in scaled-up quantum
computing implementations. Here we show fast and coherent control of a
negatively charged nitrogen vacancy spin ensemble by taking advantage of the
high permittivity of a KTaO3 dielectric resonator at cryogenic temperatures. We
achieve Rabi frequencies of up to 48 MHz, with the total field-to-power
conversion ratio $C_{\rm P} = $ 9.66 mT/$\sqrt{\rm W}$ ($\approx191$
MHz/$\sqrt{\rm W}$). We use the nitrogen vacancy center spin ensemble to probe
the quality factor, the coherent enhancement, and the spatial distribution of
the magnetic field inside the diamond sample. The key advantages of the
dielectric resonator utilised in this work are: ease of assembly, in-situ
tuneability, a high magnetic field conversion efficiency, a low volume
footprint, and optical transparency. This makes KTaO3 dielectric resonators a
promising platform for the delivery of microwave fields for the control of
spins in various materials at cryogenic temperatures.
