Testing time order and Leggett-Garg inequalities with noninvasive measurements on public quantum computers

Abstract: We present two protocols, with one and two two-qubit gates, to noninvasively measure two incompatible observables on a qubit. Both protocols are completed with a projective measurement. By gathering sufficiently large statistics, we have been able to violate Leggett-Garg inequality and time-order invariance. The detailed analysis of the data on 10 qubit sets from 5 devices available on IBM Quantum and one on IonQ reveals violations beyond 5 standard deviations in almost all cases. The noninvasiveness is supported by a qualitative and quantitative agreement with the model of weak disturbance. These results show another characteristic feature of quantum measurements.