We demonstrate a contradiction of quantum mechanics with local hidden variable theories for continuous quadrature phase amplitude (“position” and “momentum”) measurements. For any quantum state, this contradiction is lost for situations where the quadrature phase amplitude results are always macroscopically distinct. We show that for optical realizations of this experiment, where one uses homodyne detection techniques to perform the quadrature phase amplitude measurement, one has an amplification prior to detection, so that macroscopic fields are incident on photodiode detectors. The high efficiencies of such detectors may open a way for a loophole-free test of local hidden variable theories.

• Received 19 September 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.3169