JT Gravity from Partial Reduction and Defect Extremal Surface

We propose the three-dimensional bulk dual for Jackiw-Teitelboim gravity coupled with CFT${}_2$ bath based on partial reduction. The bulk dual is classical AdS gravity with a defect brane which has small fluctuation in transverse direction. We derive full Jackiw-Teitelboim gravity action by considering the transverse fluctuation as a dilaton field. We demonstrate that the fine grained entropy computed from island formula precisely agrees with that computed from defect extremal surface. Our construction provides a Lorentzian higher dimensional dual for Jackiw-Teitelboim gravity and therefore offers a framework to study problems such as black hole information paradox as well as gravity/ensemble duality.