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理论，为电磁力提供了全新的描述。到目前为止，它已经成功地通过了中低场的所有实验测试。在10^9V/cm量级的低场下QED效应的一个著名例子是氢的兰姆位移。在这样的低场下，QED效应仍然可以微扰处理，只考虑低阶项。然而，到目前为止，QED从未在比这更高的领域进行过测试，因为在实验室生产这样的领域存在实际困难。在高场下，微扰QED不再有效，需要仔细计算高阶项。因此，在高场下进行的实验测试了QED的不同方面，并且是对低阶项的高精度测试的补充。由于量子场论是现代物理学的基石，因此在非微扰极限下检验这些理论是非常重要的。几乎所有电子都被剥离的重原子是在高场下进行QED测试的理想实验室。这些离子在原子核附近的电场强度约为10^15V/cm。这种高电荷态离子(HCI)现在可以在德国达姆施塔特GSI的实验存储环(ESR)上产生。在GSI正在建造的HITRAP设施中，这些离子将被减慢、捕获并冷却到亚电子伏能量，并可用于各种实验。我们小组已经参与了这个设施的规划阶段，我们的责任是设计激光光谱学实验，以在高场下测试QED。这项工作的早期阶段是通过欧洲联盟FP5合作(也称为HITRAP)提供资金的。现在，该设施即将完工，各有关团体必须在国家一级寻求资金，以完成该项目。尤其是类氢(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效应的系统的极好例子。这些物种中的基态超精细分裂(HFS)在离原子核非常近的极高场中探测QED的有效性。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，这可以第一次与我们希望获得的准确的实验结果进行比较。将QED效应与玻尔-魏斯柯夫(BW)效应等核效应分离的唯一方法是通过测量类H离子和类Li离子中的基态HFS。从这两个HF之间的差异可以有效地消除BW效应。这使得对QED效应的确定具有数量级的几个百分点的精度。在中性原子中，超精细跃迁是光谱中微波区域的微弱跃迁。在(HCI)中，所涉及的电场将这些跃迁推进到光谱的可见光区域，并增加了它们的跃迁速率。类氢和类锂铋离子之所以令人感兴趣，是因为与这些超精细跃迁相对应的波长都可以用标准激光访问。在以前的存储环测量中，一个常见的实验障碍是由于离子的相对论速度导致的多普勒宽度和跃迁。在EBIT(电子束离子陷阱)中进行的其他测量不会受到这种影响的严重影响，但会受到低信号背景比的影响。我们建议将高电荷态离子囚禁在Penning陷阱中，冷却并压缩成小云，并利用激光光谱测量基态超精细分裂，精度为10-7数量级。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 At high fields, perturbative QED is no longer valid, and higher order terms need to be evaluated carefully.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.