  Quantum effects, prevalent in the microscopic scale, generally elusive in
macroscopic systems due to dissipation and decoherence. Quantum phenomena in
large systems emerge only when particles are strongly correlated as in
superconductors and superfluids. Cooperative interaction of correlated atoms
with electromagnetic fields leads to superradiance, the enhanced quantum
radiation phenomenon, exhibiting novel physics such as quantum Dicke phase and
ultranarrow linewidth for optical clocks. Recent researches to imprint atomic
correlation directly demonstrated controllable collective atom-field
interactions. Here, we report cavity-mediated coherent single-atom
superradiance. Single atoms with predefined correlation traverse a high-Q
cavity one by one, emitting photons cooperatively with the atoms already gone
through the cavity. Such collective behavior of time-separated atoms is
mediated by the long-lived cavity field. As a result, a coherent field is
generated in the steady state, whose intensity varies as the square of the
number of traversing atoms during the cavity decay time, exhibiting more than
ten-fold enhancement from noncollective cases. The correlation among single
atoms is prepared with the aligned atomic phase achieved by nanometer-precision
position control of atoms with a nanohole-array aperture. The present work
deepens our understanding of the collective matter-light interaction and
provides an advanced platform for phase-controlled atom-field interactions.
