A Test of Quantum Electrodynamics at High Fields

量子电动力学（QED）是第一个被公式化的量子场论，提供了电磁力的全新描述。到目前为止，它已成功地通过了各种低、中场的实验测试。在低场（10^9 V/cm量级）下，量子电动力学效应的一个著名例子是氢原子的兰姆位移。在这样的低场，QED效应仍然可以微扰处理，只考虑低阶项。然而，到目前为止，量子电动力学还没有在比这更高的场进行过测试，因为在实验室中产生这样的场有实际的困难。在高场下，微扰QED不再有效，高阶项需要仔细计算。因此，在高场进行的实验测试QED的不同方面，并补充低阶项的高精度测试。由于量子场论是现代物理学的基石，在非微扰极限下检验这些理论是极其重要的。被剥离了几乎所有电子的重原子是在高场下测试QED的理想“实验室”。这些离子在靠近原子核的地方具有10^15 V/cm量级的电场强度。这样的高电荷离子（HCI）现在可以在德国达姆施塔特的GSI实验储存环（ESR）中产生。在GSI正在建造的HITRAP设施中，这些离子将被减慢，捕获和冷却到亚电子伏的能量，并可用于各种实验。我们的团队参与了该设施的规划阶段，并负责设计激光光谱实验以在高场测试QED。这项工作的早期阶段由欧盟FP 5合作（也称为HITRAP）资助。现在，该设施即将完工，所涉各团体必须在国家一级寻求资金，以完成该项目。特别是类氢（一个电子）和类锂（三个电子）高电荷离子是允许在高场下精确研究QED效应的系统的优秀例子。基态超精细分裂（HFS）在这些物种探测的有效性QED在极高的领域发现非常接近的核。由于它们简单的电子结构，可以对这些系统进行精确的QED计算，这可以与我们希望获得的精确实验结果进行比较。唯一提出的将QED效应与核效应（如Bohr-Weisskopf（BW）效应）分离的方法是测量类氢离子和类锂离子的基态HFS。从这两个HFS之间的差异可以有效地消除BW效应。这允许以百分之几的精度来确定QED效应。在中性原子中，超精细跃迁是光谱的微波区域中的弱跃迁。在（HCI）中，所涉及的电场将这些跃迁推入光谱的可见区域并增加其跃迁速率。氢和锂类铋离子是感兴趣的，因为对应于这些超精细跃迁的波长都可以用标准激光器获得。以前在储存环中进行的测量中，一个常见的实验障碍是由于离子的相对论速度而引起的多普勒宽度和跃迁位移。在EBIT（电子束离子阱）中进行的其他测量不那么严重地受到这种效应的影响，但是受到低信号背景比的影响。我们建议捕获高度带电的离子在潘宁陷阱，冷却和压缩的离子成一个小的云，并测量基态超精细分裂通过激光光谱，精度的顺序为10-7。准备工作将在帝国理工学院进行，但最后的实验将在德国的HITRAP设施进行。

Quantum electrodynamics (QED) was the first quantum field theory to be formulated providing a radically new description of the electromagnetic force. So far it has successfully passed every experimental test at low and intermediate fields. A well-known example of QED effects at low fields, of the order of 10^9 V/cm, is the Lamb shift in hydrogen. At such low fields, the QED effects can still be treated perturbatively, only taking into account low order terms. However, up to now QED has never been tested at very much higher fields than this because of the practical difficulties of producing such fields in the laboratory. At high fields, perturbative QED is no longer valid, and higher order terms need to be evaluated carefully. Experiments carried out at high fields therefore test different aspects of QED and are complementary to high precision tests of the low order terms. Since quantum field theories are the cornerstone of modern physics testing these theories in the non-perturbative limit is extremely important. Heavy atoms that have been stripped of almost all their electrons are ideal 'laboratories' for tests of QED at high fields. These ions have electric field strengths of the order of 10^15 V/cm close to the nucleus. Such highly charged ions (HCI) can now be produced at the experimental storage ring (ESR) at GSI in Darmstadt, Germany. In the HITRAP facility being built at GSI, these ions will be slowed, trapped and cooled down to sub-eV energies, and made available to a wide variety of experiments. Our group has been involved in the planning stages of this facility and it has been our responsibility to design the laser spectroscopy experiments to test QED at high fields. The early stages of this work have been funded through a European Union FP5 collaboration (also called HITRAP). Now that the completion of the facility is in sight the various groups involved must seek funding at the national level to complete the project. Hydrogen-like (one electron) and lithium-like (three electrons) highly charged ions in particular are excellent examples of systems that allow for accurate studies of QED effects at high fields. The ground state hyperfine splitting (HFS) in these species probes the validity of QED at the extremely high fields found very close to the nucleus. Due to their simple electronic structure, accurate QED calculations can be performed for these systems, which could be compared for the first time with the accurate experimental results we wish to obtain. The only proposed method of disentangling the QED effects from nuclear effects, such as the Bohr-Weisskopf (BW) effect, is by measuring the ground state HFS in both H-like and Li-like ions. From the difference between these two HFS the BW effect can effectively be eliminated. This allows for a determination of the QED effects with an accuracy of the order of a few percent. In neutral atoms hyperfine transitions are weak transitions in the microwave region of the spectrum. In (HCI) the electric fields involved push these transitions into the visible region of the spectrum and increase their transition rates. H- and Li-like bismuth ions are of interest because the wavelengths corresponding to these hyperfine transitions are both accessible with standard lasers. A common experimental obstacle in previous measurements made in a storage ring was the Doppler width and shift of the transition due to the relativistic velocities of the ions. Other measurements performed in an EBIT (electron beam ion trap) are not as severely subject to this effect, but suffer from a low signal-to-background ratio. We propose to trap highly charged ions in a Penning trap, cool and compress the ions into a small cloud, and measure ground state hyperfine splittings by means of laser spectroscopy, with an accuracy of the order of 10-7. Preparatory work will be performed at Imperial College but the final experiments will be performed at the HITRAP facility in Germany.