The Entangling Power of Spacetime

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Quantum physics and gravity, the two foundational pillars of modern physics, remain incompatible despite substantive efforts by the theoretical physics community to unite them. This proposal is about advancing our understanding of their relationship using a novel approach called entanglement harvesting. It brings together the phenomenon of quantum entanglement, perhaps the strangest aspect of quantum physics, with black holes, the most perplexing objects in gravitational physics. Over the past decade it has become clear that spacetime has the capacity to both entangle qubits and to destroy the information they carry. A subtle relationship exists between quantum entanglement, detector responses, quantum fields, and the structure of spacetime. The goal of this proposal is to investigate this relationship by providing both a firm theoretical foundation for its description, and to obtain predictions that can be tested (or at least simulated) by experiment. Qubits can be regarded as idealized detectors that can have high (excited) or low (ground) internal energy. Entanglement harvesting exploits the property that 2 otherwise independent qubits will become entangled upon exposure to the quantum vacuum. The qubits therefore extract (harvest) this vacuum entanglement, the amount depending on their separation, relative motion, and - most importantly - on the structure of space and time in their vicinity. Here gravity is introduced: it curves space and time, so that the entanglement the detectors extract provides information as to how gravity influences quantum entanglement. This proposal focuses on how black holes impact the entanglement that 2 qubits can harvest and preserve. Black holes curve spacetime more than any other objects, trapping anything venturing too close to their edge. Absorbing everything and emitting nothing, they apparently violate the 2nd law of thermodynamics (that disorder, or entropy, always increases). Quantum physics implies black holes radiate (the radiated particles being entangled with inside partners), increasing disorder and restoring the 2nd law. But strangely, as the hole evaporates away, its radiation becomes `entangled with nothing', a situation impossible in normal quantum physics. This is the black hole information paradox: the quantum physics restoring the 2nd law ultimately predicts a contradictory situation. Entanglement harvesting will be used to see how actual quantum objects (the qubits) become entangled when they are near black holes, when black holes form, and when one (or both) falls into a black hole. By quantifying how entanglement depends on qubit separation, energy, and motion in these various settings, we will learn new information about the quantum structure of spacetime. The impact of this research program will be important new knowledge about the entangling properties of black holes, yield new insight into the information paradox, and advance the reconciliation of gravity with quantum physics.