  In three-dimensional de Sitter space classical black holes do not exist, and
the Schwarzschild-de Sitter solution instead describes a conical defect with a
single cosmological horizon. We argue that the quantum backreaction of
conformal fields can generate a black hole horizon, leading to a
three-dimensional quantum de Sitter black hole. Its size can be as large as the
cosmological horizon in a Nariai-type limit. We show explicitly how these
solutions arise using braneworld holography, but also compare to a
non-holographic, perturbative analysis of backreaction due to conformally
coupled scalar fields in conical de Sitter space. We analyze the thermodynamics
of this quantum black hole, revealing it behaves similarly to its classical
four-dimensional counterpart, where the generalized entropy replaces the
classical Bekenstein-Hawking entropy. We compute entropy deficits due to
nucleating the three-dimensional black hole and revisit arguments for a
possible matrix model description of dS spacetimes. Finally, we comment on the
holographic dual description for dS spacetimes as seen from the braneworld
perspective.
