  In the development of quantum microwave-to-optical (MO) converters, excessive
noise induced by the parametric optical drive remains a major challenge at
milli-Kelvin temperatures. Here we study the extraneous noise added to an
electro-optic transducer in its quantum ground state under an intense pulsed
optical excitation. The integrated electro-optical transducer leverages the
inherent Pockels effect of aluminum nitride microrings, flip-chip bonded to a
superconducting resonator. Applying a pulsed optical drive with peak power
exceeding the cooling power of the dilution refrigerator at its base
temperature, we observe efficient bi-directional MO conversion, with
near-ground state microwave thermal excitation
($\bar{n}_\mathrm{e}=0.09\pm0.06$). Time evolution study reveals that the
residual thermal excitation is dominated by the superconductor absorption of
stray light scattered off the chip-fiber interface. Our results shed light on
suppressing microwave noise in a cavity electro-optic system under intense
optical drive, which is an essential step towards quantum state transduction
between microwave and optical frequencies.
