  Entanglement is a genuine quantum mechanical property and the key resource in
currently developed quantum technologies. Sharing this fragile property between
superconducting microwave circuits and optical or atomic systems would enable
new functionalities but has been hindered by the tremendous energy mismatch of
$\sim10^5$ and the resulting mutually imposed loss and noise. In this work we
create and verify entanglement between microwave and optical fields in a
millikelvin environment. Using an optically pulsed superconducting
electro-optical device, we deterministically prepare an itinerant
microwave-optical state that is squeezed by $0.72^{+0.31}_{-0.25}$\,dB and
violates the Duan-Simon separability criterion by $>5$ standard deviations.
This establishes the long-sought non-classical correlations between
superconducting circuits and telecom wavelength light with wide-ranging
implications for hybrid quantum networks in the context of modularization,
scaling, sensing and cross-platform verification.
