Converts all incoming radiation into an electric signal
no photons “lost”
has no own noise contribution
However: Heisenberg’s uncertainty principle (ΔE x Δt ≥ h/2π) makes such a noiseless mixer impossible.
Why ? – A heterodyne mixer measures signal amplitude andphase. This corresponds to number of photons and time in the photon picture which – according to the uncertainty principle – cannot be measured simultaneously with infinite precision. This uncertainty results in a minimum noise of a heterodyne mixer, the “quantum limit”.
Current best mixers are ~few times worse than the quantum limit.