Lu(+) optical frequency references with accuracy verified at the 19th digit.

Two Lu(+) optical frequency references achieve fractional uncertainties near 1×10⁻¹⁹, enabling precise comparison at the 5.7×10⁻¹⁹ level.

• Why it matters: Accurate optical frequency standards are essential for redefining the SI second and testing fundamental physics, but verification at such extreme precision remains challenging.
• What they did: Researchers developed two independent 176 Lu(+) single-ion optical references and directly compared them using correlation spectroscopy, achieving agreement within the 10⁻¹⁹ uncertainty range.
• The result: This high-accuracy, room-temperature system supports improved international timekeeping, geodesy, and fundamental physics tests, including Lorentz invariance and dark matter searches.