Black Hole Decay and Quantum Instantons
  We study the analytic structure of the S-matrix which is obtained from the reduced Wheeler-DeWitt wave function describing spherically symmetric gravitational collapse of massless scalar fields. The complex simple poles in the S-matrix lead to the wave functions that satisfy the same boundary condition as quasi-normal modes of a black hole, and correspond to the bounded states of the Euclidean Wheeler-DeWitt equation. These wave function are interpreted as quantum instantons. 
Donsu Bak, Sang Pyo Kim, Sung Ku Kim, Kwang-Sup Soh, Jae Hyung Yee