  A new simple qubit model of black hole evaporation is proposed. The model
operates on four qubits and is defined in terms of quantum gates and a quantum
circuit. The chief features of the model include explicit unitarity and (most
notably) causality which is understood as the impossibility of the transfer of
information from the interior of the black hole through its horizon. The
corresponding von Neumann entanglement entropy yields a crude (four-qubit)
approximation of the Page curve.
