A source term in the quantum Boltzmann equation, which accounts for the spontaneous creation of ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ pairs in external electric fields, is derived from first principles and evaluated numerically. Careful analysis of time scales reveals that this source term is generally non-Markovian. This implies in particular that there may be temporary violations of the H theorem.

• Received 9 February 1994

DOI:https://doi.org/10.1103/PhysRevD.50.6911