  We describe an experimentally feasible protocol for performing a variant of
the quantum delayed-choice experiment with massive objects. In this scheme, a
single nitrogen-vacancy (NV) center in diamond driven by microwave fields is
dispersively coupled to a massive mechanical resonator. A double-pulse Ramsey
interferometer can be implemented with the spin-mechanical setup, where the
second Ramsey microwave pulse drives the spin conditioned on the number states
of the resonator. The probability for finding the NV center in definite spin
states exhibits interference fringes when the mechanical resonator is prepared
in a specific number state. On the other hand, the interference is destroyed if
the mechanical resonator stays in some other number states. The wavelike and
particlelike behavior of the NV spin can be superposed by preparing the
mechanical resonator in a superposition of two distinct number states. Thus a
quantum version of Wheeler's delayed-choice experiment could be implemented,
allowing of fundamental tests of quantum mechanics on a macroscopic scale.
