Quantum noise in the quantum theory of mixing

The minimum noise temperature obtainable in the quantum theory of mixing, allowing arbitrary terminations at all considered frequencies, is shown to be hω/2k, the same result as for other high-gain linear amplifiers. The conditions for attaining this absolute minimum noise appear to be fulfilled in only two cases, approximated by the super-Schottky mixer with large LO, and by the SIS mixer with small LO. For the ideal SIS mixer, the minimum noise requires one particular value of source impedance, regardless of the image termination. The origin of the quantum noise is discussed.