  The preparation of large, low-entropy, highly coherent ensembles of identical
quantum systems is foundational for many studies in quantum metrology,
simulation, and information. Here, we realize these features by leveraging the
favorable properties of tweezer-trapped alkaline-earth atoms while introducing
a new, hybrid approach to tailoring optical potentials that balances
scalability, high-fidelity state preparation, site-resolved readout, and
preservation of atomic coherence. With this approach, we achieve trapping and
optical clock excited-state lifetimes exceeding $ 40 $ seconds in ensembles of
approximately $ 150 $ atoms. This leads to half-minute-scale atomic coherence
on an optical clock transition, corresponding to quality factors well in excess
of $10^{16}$. These coherence times and atom numbers reduce the effect of
quantum projection noise to a level that is on par with leading atomic systems,
yielding a relative fractional frequency stability of
$5.2(3)\times10^{-17}~(\tau/s)^{-1/2}$ for synchronous clock comparisons
between sub-ensembles within the tweezer array. When further combined with the
microscopic control and readout available in this system, these results pave
the way towards long-lived engineered entanglement on an optical clock
transition in tailored atom arrays.
