High Energy Theory Seminar

We show that if a massive (or charged) body is put in a quantum superposition of spatially separated states in the vicinity of a black hole or cosmological horizon, the mere presence of the horizon will eventually destroy the coherence of the superposition. This occurs because, in effect, the long-range fields sourced by the superposition register on the horizon which forces the emission of entangling "soft gravitons/photons" through the horizon. This enables the horizon to harvest "which path" information about the superposition. We provide estimates of the decoherence time for such quantum superpositions in the presence of a black hole and cosmological horizon. Additionally, we show that this decoherence is distinct from—and larger than—the decoherence resulting from the presence of thermal radiation from the horizon. Finally, we further sharpen and generalize this mechanism by recasting the gedankenexperiment in the language of (approximate) quantum error correction. This yields a complementary picture where the decoherence is due to an "eavesdropper" (Eve) inside the black hole attempting to obtain "which path" information by measuring the long-range fields of the superposed body. We compute the quantum fidelity to determine the amount of information such an interior observer can obtain, and use the information-disturbance tradeoff to give a direct relationship between the Eve's information and the decoherence of the superposition in the exterior. In particular, we show that the decoherence of the superposition corresponds to the "optimal" measurement performable in the black hole interior. We comment on how this phenomenon can be interpreted as a low-energy probe of the so-called central dogmas of black hole and cosmological horizons.
Based on arXiv:2112.10798, arXiv:2205.06279, arXiv:2301.00026, and work to appear.

The talk is in 469 Lauritsen Laboratory.

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