  Quantum coherence plays a crucial role in manipulating and controlling
quantum systems, leading to breakthroughs in various fields such as quantum
information, quantum sensing, and the detection of gravitational waves. Most
coherence witnesses rely on the assumption of being able to control quantum
states. Here we report a device-independent coherence model by extending the
standard Bell theory to multiple source scenarios. We propose a
Greenberger-Horne-Zeilinger-type paradox to verify the particle and wave
behaviors of a coherent carrier. We experimentally generate generalized
two-photon entangled states that violate the present paradox, witnessing
spatial quantum superposition through local measurements.
