  Vacuum fluctuations of the electromagnetic field set a fundamental limit to
the sensitivity of a variety of measurements, including magnetic resonance
spectroscopy. We report the use of squeezed microwave fields, which are
engineered quantum states of light for which fluctuations in one field
quadrature are reduced below the vacuum level, to enhance the detection
sensitivity of an ensemble of electronic spins at millikelvin temperatures.} By
shining a squeezed vacuum state on the input port of a microwave resonator
containing the spins, we obtain a $1.2$\,dB noise reduction at the spectrometer
output compared to the case of a vacuum input. This result constitutes a proof
of principle of the application of quantum metrology to magnetic resonance
spectroscopy.
