PM: Precision Laser Spectroscopy of 2S-nS Two-Photon Transitions in Hydrogen

在这个项目中，激光将被用来探测原子氢的内部量子态。氢是最简单的原子，它只由一个电子和一个质子组成。由于这种简单性，描述氢的理论非常完善，对氢的精确测量可以为我们最好的物理理论提供严格的测试，并确定自然的关键基本常数。可以通过测量氢来确定的一个常数是质子的大小，这也可以在高能物理设施中测量，在那里电子从质子中散射出来，或者通过测量一种奇异形式的氢，在那里电子被一种更重的基本粒子取代，称为μ子。对这些不同物理系统中质子大小的测定产生了不一致，这可能表明我们当前的理论存在问题，或者需要其他理论。这个项目将通过提供比以前测量相同氢状态的测量少一个数量级的不确定性来帮助带来清晰度。计划中的实验将培养学生在原子、分子、光学物理、精密激光科学和精密测量等领域的知识。最终，这个项目可以为我们的基本常数提供更可靠的值，建立来自不同物理系统和不同科学学科的物理常数的一致性，或者可能提供新的物理定律的指示。本项目的目标是对原子氢进行高精度激光光谱分析。要测量的具体跃迁将是氢的2S-nS双光子跃迁（n在8和12之间）。结合发达的氢理论，这些测量可以用来提取里德伯常数和质子电荷半径。以前对不同物理系统中这些常数的测定——比如其他氢跃迁、介子氢的测量和电子散射——产生了不一致的结果，这可能预示着新的物理学。光谱将在低温和速度表征的氢光束上进行，光谱激光器将是参考相干光学频率梳的腔增强连续波Ti：蓝宝石激光器。在以前的测量中，交流电斯塔克位移使光谱共振变宽和扭曲，从而引入了系统的不确定性。然而，在这些实验中，将使用一个波长约为650纳米的额外激光来抵消交流斯塔克位移。这种取消将允许恢复宽度接近自然线宽的窄洛伦兹共振-在50千赫和144千赫之间-以及相对不确定度小于万亿分之一（十二位精度）的绝对频率测量，这代表了不确定度比以前测量相同转换的不确定度降低了一个数量级。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

For this project, a laser will be used to probe the internal quantum states of atomic hydrogen. Hydrogen is the simplest atom and is comprised of only one electron and one proton. Due to this simplicity, the theory describing hydrogen is very well-developed and precise measurements in hydrogen can provide a stringent test of our best physical theories and determine key fundamental constants of nature. One constant that can be determined through hydrogen measurements is the proton size, which can also be measured at high-energy physics facilities where electrons are scattered off of protons, or through measurements of an exotic form of hydrogen where the electron is replaced by a heavier fundamental particle called a muon. Determinations of the proton size in these different physical systems have produced inconsistencies, which may be indicating a problem with our current theories, or the need for additional theories. This project will help bring clarity by providing measurements with an order of magnitude less uncertainty over previous measurements of the same hydrogen states. The planned experiments will train students in the fields of atomic, molecular, and optical physics, precision laser science, and precision measurement. Ultimately, this project could provide more reliable values for our fundamental constants, establish consistency in the physical constants derived from different physical systems and within different scientific disciplines, or possibly provide indications of new physical laws. The goal of this project is to perform high-precision laser spectroscopy of atomic hydrogen. The specific transitions to be measured will be the hydrogen 2S-nS two-photon transitions (with n between 8 and 12). In conjunction with well-developed hydrogen theory, these measurements can be used to extract the Rydberg constant and proton charge radius. Previous determinations of these constants in different physical systems – such as other hydrogen transitions, measurements in muonic hydrogen, and electron scattering – have produced inconsistencies which may indicate new physics. The spectroscopy will be performed on a cryogenic and velocity-characterized hydrogen beam, and the spectroscopy laser will be a cavity-enhanced continuous-wave Ti:sapphire laser referenced to a coherent optical frequency comb. In previous measurements, the AC Stark shift broadened and distorted spectroscopic resonances which introduced systematic uncertainty. However, for these experiments, an additional laser with a wavelength of approximately 650 nm will be used to cancel the AC Stark shift. This cancellation will allow for the recovery of narrow Lorentzian resonances with widths approaching the natural linewidths – between 50 kHz and 144 kHz – and absolute frequency measurements with relative uncertainties of less than 1 part in a trillion (twelve digits of accuracy), which represents a one order of magnitude decrease in uncertainty over previous measurements of those same transitions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.