  Atomicity is a ubiquitous assumption in distributed computing, under which
actions are indivisible and appear sequential. In classical computing, this
assumption has several theoretical and practical guarantees. In quantum
computing, although atomicity is still commonly assumed, it has not been
seriously studied, and a rigorous basis for it is missing. Classical results on
atomicity do not directly carry over to distributed quantum computing, due to
new challenges caused by quantum entanglement and the measurement problem from
the underlying quantum mechanics.
  In this paper, we initiate the study of atomicity in distributed quantum
computing. A formal model of (non-atomic) distributed quantum system is
established. Based on the Dijkstra-Lamport condition, the system dynamics and
observable dynamics of a distributed quantum system are defined, which
correspond to the quantum state of and classically observable events in the
system, respectively. Within this framework, we prove that local actions can be
regarded as if they were atomic, up to the observable dynamics of the system.
