  We study how decoherence increases the efficiency with which we can simulate
the quantum dynamics of an anharmonic oscillator, governed by the Kerr effect.
As decoherence washes out the fine-grained subPlanck structure associated with
phase-space quantum interference in the closed quantum system, open quantum
dynamics can be more efficiently simulated using a coarse-grained
finite-difference numerical integration. We tie this to the way in which
decoherence recovers the semiclassical truncated Wigner approximation (TWA),
which strongly differs from the exact closed-system dynamics at times when
quantum interference leads to cat states and more general superpositions of
coherent states. The regression in quadrature measurement statistics to
semiclassical dynamics becomes more pronounced as the initial amplitude of the
oscillator grows, with implications for the quantum advantage that might be
accessible as system size grows in noisy quantum devices. Lastly, we show that
this regression does not have the form of a convex noise model, such as for a
depolarizing noise channel. Instead, closed quantum system effects interact
with the open system effects, giving rise to distinct open system behavior.
