  The exquisite control exhibited over quantum states of individual particles
has revolutionized the field of precision measurement, as exemplified by the
most accurate atomic clock realized in single trapped ions. Whereas many-atom
lattice clocks have shown advantages in measurement precision over trapped-ion
clocks, their accuracy has remained 20 times worse. Here we demonstrate, for
the first time, that a many-atom system achieves accuracy (6x10^{-18}) better
than a single ion-based clock, with vastly reduced averaging times (3000 s).
This is the first time a single clock has achieved the best performance in all
three key ingredients necessary for consideration as a primary standard -
stability, reproducibility, and accuracy. This work paves the way for future
experiments to integrate many-body quantum state engineering into the frontiers
of quantum metrology, creating exciting opportunities to advance precision
beyond the standard quantum limit. Improved frequency standards will have
impact to a wide range of fields from the realization of the SI units, the
development of quantum sensors, to precision tests of the fundamental laws of
nature.

Atomic Physics;Quantum Physics