Quantum electrodynamics (QED) is one of the most fundamental theories of physics and has been shown to be in excellent agreement with experimental results^1,2,3,4,5. In particular, measurements of the electron’s magnetic moment (or g factor) of highly charged ions in Penning traps provide a stringent probe for QED, which allows testing of the standard model in the strongest electromagnetic fields⁶. When studying the differences between isotopes, many common QED contributions cancel owing to the identical electron configuration, making it possible to resolve the intricate effects stemming from the nuclear differences. Experimentally, however, this quickly becomes limited, particularly by the precision of the ion masses or the magnetic field stability⁷. Here we report on a measurement technique that overcomes these limitations by co-trapping two highly charged ions and measuring the difference in their g factors directly. We apply a dual Ramsey-type measurement scheme with the ions locked on a common magnetron orbit⁸, separated by only a few hundred micrometres, to coherently extract the spin precession frequency difference. We have measured the isotopic shift of the bound-electron g factor of the isotopes ²⁰Ne⁹⁺ and ²²Ne⁹⁺ to 0.56-parts-per-trillion (5.6 × 10⁻¹³) precision relative to their g factors, an improvement of about two orders of magnitude compared with state-of-the-art techniques⁷. This resolves the QED contribution to the nuclear recoil, accurately validates the corresponding theory and offers an alternative approach to set constraints on new physics.

量子电动力学 (QED) 是物理学最基本的理论之一，已被证明与实验结果非常吻合^1,2,3,4,5 。特别是，对潘宁陷阱中高电荷离子的电子磁矩（或g因子）的测量为 QED 提供了严格的探针，从而可以在最强的电磁场中测试标准模型⁶ 。在研究同位素之间的差异时，由于相同的电子构型，许多常见的 QED 贡献被抵消，从而可以解决由核差异引起的复杂效应。然而，在实验上，这很快就会受到限制，特别是受到离子质量的精度或磁场稳定性^7的限制。在这里，我们报告了一种测量技术，该技术通过共同捕获两个高电荷离子并直接测量它们的g因子的差异来克服这些限制。我们应用双拉姆齐型测量方案，将离子锁定在公共磁控管轨道⁸上，间隔仅几百微米，以连贯地提取自旋进动频率差。我们测量了同位素²⁰ Ne ⁹⁺和²² Ne ⁹⁺的束缚电子g因子相对于其g因子的同位素位移，精度达到万亿分之 0.56 (5.6 × 10 ^-13 )，提高了与最先进的技术相比大约两个数量级⁷ 。 这解决了 QED 对核反冲的贡献，准确地验证了相应的理论，并提供了一种对新物理设置约束的替代方法。