MRI: Development of an Instrument for Ultra-High Resolution 1S-2S Spectrosopy of Exotic Hydrogenic Atoms

我们关于原子的结构和相互作用的最精确的信息最终是基于对各种原子和分子发射和吸收的光的精确测量。这种光的光谱（或“共振频率”）提供了用于化学鉴定的计算和测量的基本输入，用于估计反应速率，以及在从医学到国防和制造业质量控制的广泛应用中预测新结构的稳定性。最近，对不同类型氢原子（由中心重质子和类行星电子或介子组成）的测量在质子半径上存在分歧。这种差异表明了以下三种可能性之一：(1)带电粒子相互作用的理论，即量子电动力学（QED）是不正确的；(2)我们对质子结构的认识不正确；或者(3)某种新的相互作用或自然力量尚未被发现。为了确定这个谜团是否是由于QED的问题，来自加州大学河滨分校（UCR）的两位科学家提议开发一种新型仪器，该仪器将能够测量正电子的谐振频率，最终精度比目前的技术水平提高1000倍。因为正电子是最简单的原子，由一个电子和一个反电子组成，所以它应该被QED理论完美地描述。新仪器的测量结果将决定QED理论是否是问题所在，并为我们对质子结构的理解差异设定更高的限制。这种新仪器将使不稳定原子的光谱学在精密光谱学领域的前沿得到改进。纯轻子原子正电子（Ps）非常适合于测试束缚态量子电动力学（QED），并提供了理解和背景，我们可以从较重的轻子和强子的精确原子测量中提取非QED物理。很少有人敢尝试在万亿分之一的水平上对正电子进行测量，这将使人们能够深入了解物理学，比如质子电荷半径和介子中更高水平的反冲效应修正，这可能会显示出轻子和重子之间的差异。精确测量正电子离子的理想电平间距是1S-2S间隔，频率约为1 233 607 216 MHz。对这个间隔的前10位的了解已经持续了20年，不确定度为±3mhz。该仪器将采用几种新技术，通过高精度的单个原子轨迹分析，使线中心测量精度达到kHz，从而大大提高Ps原子光谱和其他高分辨率光谱实验的精度。在原子通过激光场的传输过程中，激光频率测量将以亚khz的相对和绝对精度完成，通过记录热稳定超低膨胀玻璃参考腔的线中心的瞬时偏差，使用gps规范的rf参考频率梳进行校准。通过提供超稳定的环境，机械和振动隔离，仪器的总体规格将足以产生窄线宽（~2 MHz），减少系统误差，使精度提高1000倍，计数率提高10倍，以便在可管理的测量持续时间（数月vs年）内实现精确测量。

Our most precise information about the structure and interactions of atoms is ultimately based on precise measurements of the light that is emitted and absorbed by various atoms and molecules. The spectrum of colors (or "resonant frequencies") of this light provides the basic input to calculations and measurements that are used for chemical identification, for reaction rate estimates, and for predictions of stability of new structures in a broad range of applications from medicine to defense and quality control in manufacturing. Recently, measurements on different types of hydrogen atoms (consisting of a central heavy proton and a planet-like electron or muon) are in disagreement about the radius of the proton. The discrepancy suggests one of 3 possibilities: (1) the theory of how charged particles interact, known as quantum electrodynamics or QED, is incorrect; (2) our knowledge of the structure of the proton is incorrect; or (3) there is some new kind of interaction or force of nature yet to be discovered. To decide whether the mystery is due to a problem with QED or not, two scientists from the University of California Riverside (UCR) propose to develop a novel instrument that will enable a measurement of the resonant frequency of positronium at ultimately 1000X more accuracy than the current state of the art. Because positronium is the simplest possible atom, consisting of an electron bound to an anti-electron, it should be described perfectly by QED theory. New measurements with the proposed instrument will decide whether QED theory is the problem and set significantly higher limits on the discrepancy in our understanding of the structure of the proton. The new instrument will lead to an improvement in the spectroscopy of unstable atoms at the frontier of the field of precision spectroscopy. The purely leptonic atom positronium (Ps) is uniquely well-suited for testing bound-state quantum electrodynamics (QED) and provides the understanding and background by which we may extract non-QED physics out of precision atomic measurements on heavier leptons and hadrons. Few have dared to try measurements on positronium at the few parts per trillion level that would allow insight into physics such as the proton charge radius and higher level recoil effect corrections in muonium, and that might show differences between light and heavy leptons. The ideal level spacing for a precision measurement on positronium is the 1S-2S interval at approximately 1 233 607 216 MHz. Knowledge of the first 10 digits of this interval has stood for 20 years with an uncertainty of ±3 MHz. The proposed instrument will implement several new techniques that would dramatically improve the accuracy of Ps atom spectroscopy and potentially other high resolution spectroscopy experiments by high precision individual atom trajectory analysis allowing line-centers to be measured to kHz precision. The laser frequency metrology during the transit of the atoms through the laser field will be accomplished with sub-kHz relative and absolute accuracies by recording the instantaneous deviations from the line-center of a thermally-stabilized ultra-low expansion glass reference cavity that is calibrated using a GPS-disciplined RF-referenced frequency comb. By providing ultrastable environmental, mechanical, and vibrational isolation, the overall specifications of the instrument will be sufficient to produce a narrow linewidth (~2 MHz), a reduction in systematic errors to enable up to 1000X increase in accuracy, and a 10X higher count rate to enable precision measurements in manageable measurement durations (months vs years).