Quantum Nondemolition Measurement Enables Macroscopic Leggett-Garg Tests

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We show how a test of macroscopic realism based on Leggett-Garg inequalities (LGIs) can be performed in a macroscopic system. Using a continuous-variable approach, we consider quantum non-demolition (QND) measurements applied to atomic ensembles undergoing magnetically-driven coherent oscillation. We identify measurement schemes requiring only Gaussian states as inputs and giving a significant LGI violation with realistic experimental parameters and imperfections. The predicted violation is shown to be due to true quantum effects rather than to a classical invasivity of the measurement. Using QND measurements to tighten the " clumsiness loophole " forces the stubborn macrorealist to re-create quantum back action in his or her account of measurement.




Budroni, C., Vitagliano, G., Colangelo, G., Sewell, R. J., Gühne, O., Tóth, G., & Mitchell, M. W. (2015). Quantum Nondemolition Measurement Enables Macroscopic Leggett-Garg Tests. Physical Review Letters, 115(20). https://doi.org/10.1103/PhysRevLett.115.200403

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