to upconvert a qubit state given a qubit encoding robust to noise in one quadrature. One example of such a feedforward-based qubit upconversion scheme is explored in the supplement.

In the present work we focus on the experimental identification of correlations that allow feedforward, and perform classical feedforward with a vacuum ancilla. For this demonstration we measure both converter outputs in heterodyne detection and perform subtraction in post-processing to remove the correlated noise (Fig. \[schematic1\]b). A quantum protocol would instead entail homodyne measurement of the ancilla at the microwave port, and conditional unitary displacement at the optical port.[@braunstein_teleportation_1998] Our electro-optic converter is housed in a cryostat with base temperature $T=35~\mathrm{mK}$, and comprises microwave and optical cavities simultaneously coupled to a single vibrational mode of a suspended dielectric membrane with resonant frequency $\omega_\mathrm{m} / 2 \pi = f_\mathrm{m} = 1.473~\mathrm{MHz}$ (Fig. \[schematic1\]b). Strong red-detuned pump tones incident on both cavities parametrically couple mechanical motion to propagating microwave and optical fields at rates $\Gamma_\mathrm{e,o}$ that greatly exceed the intrinsic mechanical damping rate $\gamma_\mathrm{m} = 2\pi\times11~\mathrm{Hz}$ (see methods). Along with the two pump tones, a weak probe tone, incident on either the microwave or optical cavity, is measured in heterodyne detection, and used 