It is well known that the Husimi Q-function of the signal field can actuallybe measured by the eight-port homodyne detection technique, provided that thereference beam (used for homodyne detection) is a very strong coherent field sothat it can be treated classically. Using recent rigorous results on thequantum theory of homodyne detection observables, we show that any phase spaceobservable, and not only the Q-function, can be obtained as a high amplitudelimit of the signal observable actually measured by an eight-port homodynedetector. The proof of this fact does not involve any classicality assumption.
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