Photon-graviton conversion in a magnetic field produces a Leggett-Garg inequality violation, but the violation follows from the generic two-level oscillation formula and an invasive measurement protocol, so it is not a valid probe of quantum gravity.
Leggett-Garg inequalities for testing quantumness of gravity
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abstract
In this study, we determine a violation of the Leggett-Garg inequalities due to gravitational interaction in a hybrid system consisting of a harmonic oscillator and a spatially localized superposed particle. The violation of the Leggett-Garg inequalities is discussed using the two-time quasiprobability in connection with the entanglement negativity generated by gravitational interaction. It is demonstrated that the entanglement suppresses the violation of the Leggett-Garg inequalities when one of the two times of the quasiprobability t_1 is chosen as the initial time. Further, it is shown that the Leggett-Garg inequalities are generally violated due to gravitational interaction by properly choosing the configuration of the parameters, including t_1 and t_2, which are the times of the two-time quasiprobability. The feasibility of detecting violations of the Leggett-Garg inequalities in hybrid systems is also discussed.
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Violation of the Leggett-Garg inequality in photon-graviton conversion
Photon-graviton conversion in a magnetic field produces a Leggett-Garg inequality violation, but the violation follows from the generic two-level oscillation formula and an invasive measurement protocol, so it is not a valid probe of quantum gravity.