Phases beyond conventional paradigms. Here, we demonstrate that symmetry control via epitaxial strain and heterostructure design can resolve recently intensified debates on spin ordering in RuO2, while simultaneously inducing unconventional magnetic states and additional emergent quantum phases. Using hybrid molecular beam epitaxy for precise thin-film synthesis1-9, combined with multimodal characterization, including optical second harmonic generation1, spin- and angle-resolved photoemission spectroscopy4, magneto-optical effect1, polarized neutron reflectometry2, anomalous Hall measurements3, multislice electron ptychography1,6, group-theoretical analysis1, and density functional theory1-4,9, we uncover a strain-driven altermagnetic order emerging within a polar metallic state. Our results show that tuning lattice symmetry not only stabilizes previously debated magnetic ground states but also enables access to coupled electronic and magnetic phases absent in bulk counterparts. This work establishes symmetry engineering as a unifying design principle for realizing altermagnetism and other unconventional quantum states in epitaxial materials, opening new opportunities for symmetry-driven functionalities in quantum oxide platforms.
