Figure 1: A simplified test of macrorealism.

(a) Leggett–Garg (LG) derive their inequality-constraint on macrorealism by considering a measurement of a bivalent observable Q at three consecutive times (on an ensemble ‘G’ of two-level systems). The inequality is tested by gathering two-time correlators from separate experiments, each with a measurement omitted at one of the instants (ensemble ‘’ and so on). The lower pane shows our full, simplified experimental protocol. In our experiment the shuffling operations are pulses induced with resonant microwave radiation that cause a pseudo-spin rotation by a variable angle θ, creating coherent superpositions of |g〉 and |e〉. The expectation value of a final measurement (at t₃) may be influenced by the presence or absence of an earlier operation O (at t₂). (b) Control experiments determine the worst case disturbance when classical states are prepared: |e〉 is prepared from the thermal equilibrium state |g〉 with a θ=π operation. (c) The main experiment is identical, only a maximally coherent superposition is prepared with a θ=π/2 operation. This gives rise to a measurement disturbance not explainable merely by appealing to the clumsiness revealed in the control experiments.