Shingo Akama, Chunshan Lin

We demonstrate that quantum entanglement between primordial gravitons in dynamically decoupled gravitational sectors can parametrically enhance the tensor power spectrum during inflation. Unlike standard mechanisms relying on classical dynamics or modified actions, this enhancement originates from the reduced density matrix of the observable sector after tracing over a hidden gravitational reservoir. This framework allows for a sizable tensor-to-scalar ratio r > 0.01 consistent with sub-Planckian inflaton excursions, providing a purely quantum mechanical evasion of the Lyth bound. The resulting mixed state leaves a distinctive "quantum birthmark" in the form of oscillatory features in the power spectrum and a characteristic violation of the single-field consistency relation, manifesting as a scale-dependent enhancement of the squeezed-limit bispectrum. Furthermore, we forecast that this entanglement may manifest as a late-time stochastic noise enhancement in gravitational wave interferometers, offering a novel experimental window into the quantum nature of spacetime.