The preparation of large, low-entropy, highly coherent ensembles of identical quantum systems is fundamental for many studies in quantum metrology(1), simulation(2) and information(3). However, the simultaneous realization of these properties remains a central challenge in quantum science across atomic and condensed-matter systems(2,4-7). Here we leverage the favourable properties of tweezer-trapped alkaline-earth (strontium-88) atoms(8-10), and introduce a 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 comparable with that of leading atomic systems, which use optical lattices to interrogate many thousands of atoms in parallel(11,12). The result is a relative fractional frequency stability of 5.2(3) x 10(-17)tau(-1/2) (where tau is the averaging time in seconds) for synchronous clock comparisons between sub-ensembles within the tweezer array. When further combined with the microscopic control and readout that are available in this system, these results pave the way towards long-lived engineered entanglement on an optical-clock transition(13) in tailored atom arrays.
