每个电子都携带一个内在的磁偶极矩， 所以任意两个电子都应因此而对彼此施加磁力。所涉及的磁力非常小，而且在原子尺度上库仑相互作用是主导性的，所以观察这种磁相互作用极为困难。然而，Shlomi Kotler等人现在正好做到了这一点：对在被单独约束的锶-88离子中的两个电子之间的相互作用进行测量。这两个电子在超过15秒的相干演变过程中发生自旋纠缠，而且通过改变电子间的间隔，本文作者还演示了与已知的“立方反比律”(inverse-cube law)相一致的距离依赖性。
Electrons have an intrinsic, indivisible, magnetic dipole aligned with their internal angular momentum (spin). The magnetic interaction between two electronic spins can therefore impose a change in their orientation. Similar dipolar magnetic interactions exist between other spin systems and have been studied experimentally. Examples include the interaction between an electron and its nucleus and the interaction between several multi-electron spin complexes. The challenge in observing such interactions for two electrons is twofold. First, at the atomic scale, where the coupling is relatively large, it is often dominated by the much larger Coulomb exchange counterpart1. Second, on scales that are substantially larger than the atomic, the magnetic coupling is very weak and can be well below the ambient magnetic noise. Here we report the measurement of the magnetic interaction between the two ground-state spin-1/2 valence electrons of two 88Sr+ ions, co-trapped in an electric Paul trap. We varied the ion separation, d, between 2.18 and 2.76 micrometres and measured the electrons’ weak, millihertz-scale, magnetic interaction as a function of distance, in the presence of magnetic noise that was six orders of magnitude larger than the magnetic fields the electrons apply on each other. The cooperative spin dynamics was kept coherent for 15 seconds, during which spin entanglement was generated, as verified by a negative measured value of −0.16 for the swap entanglement witness. The sensitivity necessary for this measurement was provided by restricting the spin evolution to a decoherence-free subspace that is immune to collective magnetic field noise. Our measurements show a d−3.0(4) distance dependence for the coupling, consistent with the inverse-cube law.