prohibits upconversion of a state with negative Wigner function or squeezing, so we refer to it as classical feedforward. Classical feedforward is a resource for recovering classical signals, whose performance — interestingly — depends on system efficiencies rather than temperature, quality factor, or cooperativity. However, classical feedforward is apparently unhelpful for quantum tasks.

A different choice of ancilla would allow quantum tasks to be accomplished, even in the presence of thermal noise. For example if the ancilla is an infinitely squeezed vacuum state, one quadrature can be fed forward noiselessly, permitting noiseless measurement of a single upconverted quadrature or upconversion of a squeezed quadrature without high cooperativity. This quantum protocol does however place limits on measurement and converter efficiencies, as any loss will add noise to the feedforward process. In the limit of perfect measurement efficiencies and squeezing, a converter efficiency $\eta > 50\%$ is still required to upconvert a squeezed quadrature or measure a remotely prepared microwave quadrature with added noise less than vacuum. We therefore refer to $50\%$ as a quantum threshold efficiency for feedforward protocols. More sophisticated tasks can be imagined with quantum feedforward, albeit with more stringent efficiency requirements. In particular, quantum feedforward could be used 