  The Stern-Gerlach (SG) effect, discovered almost a century ago, has become a
paradigm of quantum mechanics. Surprisingly there is little evidence that the
original scheme with freely propagating atoms exposed to gradients from
macroscopic magnets is a fully coherent quantum process. Specifically, no
high-visibility spatial interference pattern has been observed with such a
scheme, and furthermore no full-loop SG interferometer has been realized with
the scheme as envisioned decades ago. On the contrary, numerous theoretical
studies explained why it is a near impossible endeavor. Here we demonstrate for
the first time both a high-visibility spatial SG interference pattern and a
full-loop SG interferometer, based on an accurate magnetic field, originating
from an atom chip, that ensures coherent operation within strict constraints
described by previous theoretical analyses. This also allows us to observe the
gradual emergence of time-irreversibility as the splitting is increased.
Finally, achieving this high level of control over magnetic gradients may
facilitate technological applications such as large-momentum-transfer beam
splitting for metrology with atom interferometry, ultra-sensitive probing of
electron transport down to shot-noise and squeezed currents, as well as nuclear
magnetic resonance and compact accelerators.
