  The double-slit experiment has become a classic thought experiment, for its
clarity in expressing the central puzzle of quantum mechanics -- wave-particle
complementarity. Such wave-particle duality continues to be challenged and
investigated in a broad range of entities with electrons, neutrons, helium
atoms, C$_{60}$ fullerenes, Bose-Einstein condensates and biological molecules.
All existing experiments are performed with the scale larger than angstrom. In
this article, we present a double-slit scenario at fermi scale with new
entities -- coherent photon products in heavy-ion collisions. Virtual photons
from the electromagnetic fields of relativistic heavy ions can fluctuate to
quark-antiquark pairs, scatter off a target nucleus and emerge as vector
mesons. The two colliding nuclei can take turns to act as targets, forming a
double-slit interference pattern. Furthermore, the `which-way' information can
be partially solved by the violent strong interactions in the proposed
scenario, which demonstrates a key concept of quantum mechanics --
complementary principle.
