  Causal emergence is brought about when a coarse-grained description of a
physical system is more effective (more deterministic and/or less degenerate)
than the fine-grained corresponding model. We show, for the first time to our
knowledge, a causal emergence in a quantum system: a atomic Mach-Zehnder
interferometer with two which-path detectors. The atomic wave-like or
particle-like descriptions are related, respectively, to
\textit{coarse-grained} and \textit{fine-grained} models. The predictions of
the atomic position after the passage through the interferometer when the to
\textit{coarse-grained} description is considered are more effective than the
corresponding \textit{fine-grained} model. We conclude that quantum eraser
measurements yields the causal emergence in this system.
