Quantum electrodynamics (QED) remains one of the most successful and rigorously tested theories in physics, describing the electromagnetic interactions that govern everything from the behavior of light to the forces between charged particles. Despite its extraordinary precision—verified to an accuracy of 0.1 parts per billion—scientists continue to probe its boundaries in search of subtle discrepancies that could hint at new physics beyond the current understanding. Recent groundbreaking work focusing on lithium-like tin ions represents a significant step forward in this endeavor, pushing the limits of both theoretical calculations and experimental precision.
At its core, QED explains electromagnetic interactions as exchanges of virtual photons, the quantum carriers of the electromagnetic force. In atomic systems, electrons “communicate” with each other and the atomic nucleus by exchanging these virtual photons, a process that shapes all their electromagnetic properties. Moreover, electrons interact with themselves through a phenomenon known as self-energy, where an electron emits and subsequently reabsorbs a photon. These complex interactions, while conceptually subtle, have measurable effects on atomic observables that can be exploited to test the theory’s predictions with extraordinary sensitivity.
One particularly illuminating parameter in QED studies is the electron’s g-factor, which quantifies the relationship between its intrinsic angular momentum (spin) and its magnetic moment. Dirac’s relativistic quantum mechanics theory predicts a g-factor of exactly 2 for a free electron, but quantum fluctuations introduce small corrections that deviate from this value. These deviations arise due to QED effects such as vacuum polarization and self-energy, and they are exquisitely sensitive to the electromagnetic environment surrounding the electron, especially the presence of strong electric fields produced by atomic nuclei.
Heavy, highly charged ions provide an ideal testing ground because their immense nuclear charge generates electric fields that are orders of magnitude stronger than those experienced by electrons in lighter atoms. Hydrogen-like ions, which contain a single electron orbiting a heavy nucleus, have long been the focus of both theoretical and experimental investigations. However, lithium-like ions—comprising three electrons—offer a more complex and thus revealing system. The interaction among the outermost electron and the two tightly bound inner electrons introduces intricate interelectronic QED effects that challenge state-of-the-art theoretical models and experimental methods alike.
In an innovative collaboration, researchers at the Max Planck Institute for Nuclear Physics in Heidelberg have embarked on a meticulous study of the g-factor of the outermost bound electron in lithium-like tin ions. This system bridges the simplicity of hydrogen-like ions and the complexity of multi-electron atoms, providing a fertile ground to test theoretical predictions involving electron-electron interactions and QED screening effects, where electrons modify each other’s behavior and also alter their interactions with the quantum vacuum.
Achieving theoretical precision in such a complex system necessitates ab initio QED calculations, a computational approach that includes all electromagnetic interactions from first principles. These calculations account for photon exchanges between electrons, QED screening effects, and self-energy corrections in a highly non-linear regime. The Heidelberg team further refined their predictions by incorporating recent two-loop QED contributions extracted from measurements on hydrogen-like tin ions. This approach yielded an experimentally enhanced theoretical value for the g-factor of lithium-like tin, g_th = 1.980 354 797(12), representing an approximate twenty-five-fold improvement over previous calculations.
Complementing these theoretical advances, the experimental team employed the state-of-the-art ALPHATRAP cryogenic Penning trap, an apparatus designed for ultra-precise measurements of charged particles under extreme conditions. Inside this trap, a strong and stable magnetic field causes the trapped ion to undergo well-defined motions alongside the Larmor precession of the electron’s spin, analogous to the wobbling of a spinning top. By measuring the frequencies of these motions and the spin precession, researchers can extract the g-factor with exceptional accuracy while effectively cancelling out the magnetic field’s value, isolating intrinsic properties of the electron-ion system.
The detection of spin flips—the transition of the electron’s spin between its two quantum states—is a pivotal aspect of the experiment. When the frequency of applied microwave radiation matches the precession frequency of the electron’s spin, spin flips occur at a resonant maximum. Measuring the rate of these flips thus provides a direct and precise probe of the electron’s magnetic properties. The successful implementation of this technique in lithium-like tin ions marks a significant experimental achievement given the ion’s complex electronic structure and the challenges of controlling systematic and statistical uncertainties at the sub-parts-per-billion level.
The experimental determination of the g-factor for lithium-like tin yields a value of g_exp = 1.980 354 799 750(84)_stat(54)_sys(944)_ext, with uncertainties divided into statistical, systematic, and external categories. The dominant source of experimental uncertainty arises from the ion mass, which currently limits the overall precision to approximately 0.5 parts per billion. Encouragingly, there are well-established pathways to improve the mass measurement by more than an order of magnitude, paving the way for even more precise determinations of the g-factor, contingent on concurrent theoretical advancements.
The remarkable agreement between the experimental and theoretical values within combined uncertainties represents a compelling validation of QED in the strong-field regime of highly charged ions. This confirmation not only solidifies confidence in current physical theories but also establishes novel benchmarks for future research endeavors. Looking forward, extending this approach to even heavier lithium-like ions such as Pb^79+ (lead with a charge of +79) promises even more stringent tests of QED under conditions of extreme electromagnetic fields.
Moreover, ongoing improvements in theoretical techniques, particularly in the calculation of two-loop QED processes, will enable deeper insights into interelectronic interactions and vacuum effects in multi-electron systems. The advanced computational frameworks developed in this research have broader implications as they can be adapted to investigate g-factors of more complex ions, including boron- and carbon-like species, as well as explore parity non-conserving transitions in neutral atoms, which are of fundamental interest in the search for physics beyond the Standard Model.
This work exemplifies the symbiosis between cutting-edge theory and state-of-the-art experimentation, pushing the frontiers of precision metrology and quantum physics. In unraveling the nuanced interplay of electrons under extreme conditions, scientists not only bolster the foundational understanding of QED but also sharpen their tools to detect subtle anomalies that might reveal new physics. The lithium-like tin ion thus stands as a new milestone, a quantum laboratory where the fabric of electromagnetic interactions is probed with unparalleled clarity and depth.
As precision increases and theoretical models become more refined, the landscape of quantum electrodynamics testing continues to expand, offering tantalizing possibilities for uncovering novel phenomena. The high resolution achievable in such experiments provides a window into the quantum vacuum itself, a realm teeming with ephemeral particles that influence measurable physical quantities. The ongoing pursuit of quantum accuracy in complex ions thus holds promise for reshaping fundamental physics and potentially opening doors to as yet undiscovered forces or particles.
In sum, the collaborative research on lithium-like tin ions represents a landmark achievement in atomic physics and quantum electrodynamics, bridging theory and experiment in a dance of extraordinary precision. It sets the stage for further explorations of electromagnetic interactions in ever more intricate atomic systems, advancing our grasp of the quantum world and fortifying the edifice of modern physics.
Subject of Research: Quantum electrodynamics and precision measurement of the electron g-factor in lithium-like highly charged ions
Article Title: Testing interelectronic interaction in lithium-like tin
News Publication Date: 29-May-2025
Web References: http://dx.doi.org/10.1126/science.adn5981
Image Credits: MPIK
Keywords
Quantum electrodynamics, QED, lithium-like ions, electron g-factor, highly charged ions, precision measurement, Penning trap, ALPHATRAP, interelectronic interaction, two-loop QED, atomic physics, strong electric fields
