Abstract: Understanding gravity in the framework of quantum mechanics is one of the great challenges in modern physics. Along this line, the prime question is to find whether gravity is a quantum entity subject to the rules of quantum mechanics. Here, I will introduce an idea for a test based on the principle that two quantum objects cannot be entangled without a quantum mediator. I will show that despite the weakness of gravity, the phase evolution induced by the gravitational interaction of two micron-size test masses in adjacent matter-wave interferometers can detectably entangle them even when they are placed far apart enough to keep Casimir-Polder forces at bay. I will provide a prescription for witnessing this entanglement, which certifies gravity as a coherent quantum mediator through simple correlation measurements between two spins: one embedded in each test mass. Fundamentally, the above entanglement is shown to certify the presence of non-zero off-diagonal terms in the coherent state basis of the gravitational field modes. I will also propose another experiment where we can probe the spin of the gravitational interaction via entanglement between an intense laser beam in a cavity and an object kept in a quantum ground state. Both experiments are incredibly challenging. However, the import of the outcome will be no less than detecting gravitational waves or discovering the Higgs boson in the LHC.