  We report the quantum process tomography of a M$\o$lmer-S$\o$rensen
entangling gate. The tomographic protocol relies on a single discriminatory
transition, exploiting excess micromotion in the trap to realize all operations
required to prepare all input states and analyze all output states. Using a
master-slave diode lasers setup, we demonstrate a two-qubit entangling gate,
with a fidelity of Bell state production of 0.985(10). We characterize its
$\chi$-process matrix, the simplest for an entanglement gate on a
separable-states basis, and we observe that the dominant source of error is
accurately modelled by a quantum depolarization channel.
