  Are quantum states real? This most fundamental question in quantum mechanics
has not yet been satisfactorily resolved, although its realistic interpretation
seems to have been rejected by various delayed-choice experiments. Here, to
address this long-standing issue, we present a quantum twisted double-slit
experiment. By exploiting the subluminal feature of twisted photons, the real
nature of a photon during its time in flight is revealed for the first time. We
found that photons' arrival times were inconsistent with the states obtained in
measurements but agreed with the states during propagation. Our results
demonstrate that wavefunctions describe the realistic existence and evolution
of quantum entities rather than a pure mathematical abstraction providing a
probability list of measurement outcomes. This finding clarifies the long-held
misunderstanding of the role of wavefunctions and their collapse in the
evolution of quantum entities.
