Gravitationally Mediated Entanglement and Decoherence

Presently contemplated experiments propose to test whether or not gravity itself can serve as a mediator for quantum entanglement. The detection of such gravitationally mediated entanglement would provide the first example of an observed phenomenon that cannot be explained within the framework of classical general relativity, and would constitute the first experimental test of quantum gravity. This dissertation, based on a series of published articles, develops the implications of gravitationally mediated entanglement should it prove to exist in Nature. By analyzing an apparent paradox between causality and complementarity in a gedankenexperiment, it is shown that the experimental discovery of gravitationally mediated entanglement may be viewed as implying the existence of the graviton. A similar gedankenexperiment outside a black hole then shows that a quantum superposition maintained outside a black hole must undergo a constant rate of decoherence, so that a black hole will eventually decohere any quantum superposition in its exterior. This occurs because of an unavoidable accumulation of soft, entangling gravitons on the black hole horizon, in a direct mathematical analog of the gravitational memory effect at null infinity. A similar quantum gravitational decoherence effect is shown to arise in the presence of a cosmological horizon, and more generally in the vicinity of any Killing horizon. The resulting decoherence rates are predicted in each case. A fully local account of this decoherence is developed, and reveals the soft radiation can be viewed as the result of extremely low frequency vacuum fluctuations interacting with the superposition. The decoherence in the presence of black holes is contrasted with the decoherence that would arise in the spacetime of a star, in a thermal bath, or in the presence of an ordinary material body.