Session Q02: Advances in Precision Measurement

Search for the atomic EDM of $^{\mathrm{171}}$Yb in an optical dipole trap

We present a search for the atomic electric dipole moment (EDM) of $^{\mathrm{171}}$Yb, a stable isotope with the ground state property of L $=$ 0, S $=$ 0, and I $=$ 1/2. $^{\mathrm{171}}$Yb atoms are captured by a two-stage MOT, transported using a movable optical dipole trap over 65 cm into a science chamber, and transferred to a 1D optical lattice. There, the atoms are allowed to precess under a uniform B field of 10 mG and a strong, reversible E field of 100 kV/cm. The precession frequencies measured under opposite E fields are used to search for the EDM. We describe the progress, challenges, and prospects of the experiment. Through this experiment, we develop atom manipulation techniques and study systematics for a parallel search for the EDM of $^{\mathrm{225}}$Ra, a radioactive isotope expected to possess a much larger Schiff moment due to its nuclear octupole deformation.   

Upgrading the ACME electron EDM search with a molecular lens

The search for an electron electric dipole moment (EDM) sheds light on physics beyond the Standard Model. The most stringent limit on the electron EDM, $\left| {d_{e} } \right|<1.1\times 10^{-29}$e$\cdot $cm, was recently set by the ACME collaboration [Nature~562,~pages355--360 (2018)], constraining new time-reversal-symmetry ($T)$ violating physics for a broad class of proposed models at the $3\sim 30$TeV energy scale. A next generation of ACME is now underway, aimed at improving the sensitivity to d$_{\mathrm{e}}$ by at least another order of magnitude. A major improvement in statistics can be obtained using a molecular lens to focus our cold beam of Thorium Monoxide (ThO) molecules into the EDM measurement region. The Q ${ }^{3}\Delta_{2} $ electronic state of ThO, which should have long lifetime as well as large electric and magnetic polarizability, appears ideal for molecular lensing. Here, we report the first measurements of relevant properties of the Q state. Also, we demonstrate a double-STIRAP procedure that transfers population into and out of the Q state with high efficiency. These results, combined with trajectory simulations on the performance of a molecular lens, lead us to project a signal rate improvement by an order of magnitude relative to our latest result.   

Progress towards an improved electron and positron magnetic moment measurement as a test of the Standard Model and CPT symmetry

The most accurate measurement of the electron magnetic moment is the most precise test of the Standard Model, with precision of 0.28 ppt [1]. A new Penning trap apparatus has been constructed and is being tested with an aspiration of improving the electron magnetic moment measurement precision by a factor of 10. Positrons will now be loaded into the trap to allow for a more precise measurement of the positron magnetic moment which we aspire to improve by a factor of 150 times. A direct comparison of these two measurements (made in the same apparatus) allows for the most precise test of CPT symmetry for the light leptons. In addition, these measurements, combined with the standard model, result in a precise determination of the fine structure constant. A 2.4 standard deviation discrepancy between previous magnetic moment measurements and atom recoil experiments [2] also warrants additional investigation. Recently, the apparatus has been moved to the new Center for Fundamental Physics at Northwestern University where setup is complete. Further developments and progress on the experiment will be presented. 1. D. Hanneke, S. Fogwell, and G. Gabrielse, Physical Review Letters 100 (2008) 120801 2. R. H. Parker, C. Yu, W. Zhong, B. Estey, and H. Müller, Science 360 (2018) 191   

Towards 2S-8D spectroscopy in a cryogenic hydrogen beam

Hydrogen spectroscopy has provided a route to determine fundamental constants and is a testing ground for bound state Quantum Electrodynamic theory. In particular, precisely measured optical transitions in hydrogen have become a core component in the determination of the Rydberg constant and the proton radius. For example, spectroscopy of 2S-nS/D transitions, in conjunction with the precisely measured 1S-2S transition, offers a straightforward route to extract the Rydberg constant and proton charge radius. However, this result is divergent from other determinations of the proton radius. Therefore, we aim to remeasure these transitions with an improved experimental setup. A ~5 K atomic hydrogen beam is optically excited into the 2S state via two-photon absorption. The 2S-8D transition is then excited by two-photon absorption at 780 nm. This method is advantageous compared to previous measurements of the 2S-8D transition by the reduction of velocity effects and greater metastable flux, as well improved frequency metrology via an optical frequency comb.   

Microwave spectroscopy of the positronium n $=$ 2 fine structure

The first measurements of the Ps fine structure were conducted by Mills, Berko, and Canter 1975. These were followed by Hatamian, Conti and Rich in 1987, and then Hagena and co-workers in 1993. Then nothing happened for 25 years, possibly because the methods used by these authors could not easily be improved. We report here some new measurements of the Ps 2$^{\mathrm{3}}$S$_{\mathrm{1}}\to $ 2$^{\mathrm{3}}$P$_{\mathrm{2}}$ transition frequency performed using new techniques not available to the previous experimenters. With near-thermal Ps emitted from a mesoporous silica film excited 2S atoms were generated via single-photon laser excitation and transitions to 2P levels were driven by microwave radiation. This transfer of long-lived 2S atoms to short lived 2P atoms was observed via the time dependence of annihilation radiation. The theoretical value for this interval is 8626.71 \textpm 0.08 MHz, and preliminary measurements are in agreement with theory. Because Ps is essentially a pure QED system, precision measurements of its energy levels can be used to search for forbidden transitions, non-commutative space, or new ultralight interactions. Future developments in this area will be discussed.   

A gaseous $^{\mathrm{3}}$He NMR probe at 4.2K for precision measurements of electron and positron magnetic moments

A 4.2 K cold bore magnet is used in the new electron's and positron's magnetic moments experiment. In order to optimize the homogeneity of the magnet at 5 Tesla, a gaseous $^{\mathrm{3}}$He NMR probe based on a two-volume method is developed. A high signal-to-noise ratio comparable to a room temperature water NMR probe is achieved, and the relaxation time constants T$_{\mathrm{1}}$, T$_{\mathrm{2}}$, and T$_{\mathrm{2}}^{\mathrm{\ast }}$ have been measured. We also discuss the stability and the homogeneity of the cold bore magnet studied with the $^{\mathrm{3}}$He probe.   

Laser cooling radium ions

The unstable radium nucleus is appealing for probing new physics due to its large mass, octupole deformation and energy level structure. Ion traps, with long hold times and low particle numbers, are excellent for work with radioactive species, such as radium and radium-based molecular ions, where low activity, and hence low total numbers, is desirable. We laser cooled ${}^{226}\mathrm{Ra}^+$ to form Coulomb crystals in a linear Paul trap. With a single laser-cooled radium ion we measured the $7p\ ^2P_{1/2}$ state branching fractions to the ground state and a metastable excited state. We measured the $7s\ ^2S_{1/2} \rightarrow 7p\ ^2P_{1/2}$ transition frequency of ${}^{226}\mathrm{Ra}^+$ using a nearby tellurium reference line, which provides a convenient frequency reference for this radioactive element. In a chain of laser-cooled radium ions we have produced radium-based molecular ions which may hold promise for precision measurement.   

High-Precision Measurement of Electric Quadrupole Amplitude in Lead using Faraday Rotation Spectroscopy

We have completed a measurement of the $(6s^26p^2)^3P_0-^3P_2$ 939 nm electric quadrupole (E2) transition amplitude in atomic lead. Using a Faraday rotation spectroscopy technique and a sensitive polarimeter, we have measured this very weak E2 transition for the first time, and can compare its amplitude to the predictions of recent ab initio atomic theory work in lead,\footnote{Porsev et al., Phys. Rev. A 93, 012501 (2016)} an element in which highly precise atomic parity nonconservation experiments were completed some years ago.\footnote{Meekhof et al., Phys. Rev. Lett. 71, 3442 (1993)} We heat a sealed lead quartz vapor cell to between 775 and 950C, apply a $\sim$ 10 G longitudinal magnetic field, and use polarization modulation/lock-in detection to measure optical rotation amplitudes of order 1 milliradian with noise near 1 microradian. We compare the Faraday rotation lineshape of the E2 transition to that from the $^3P_0-^3P_1$ 1279 nm magnetic dipole (M1) transition under identical sample conditions. The M1 transition amplitude is precisely calculable without detailed wavefunction knowledge, and thus provides an ideal normalization tool from which to extract the E2 amplitude. Preliminary data analysis indicates precision at the 1\% level. Latest results will be presented.   

Measurement of the ground state tensor polarizability of Cesium atoms

We will describe an in-progress measurement of the ground state tensor polarizability (GSTP) of Cs using laser cooled atoms trapped in optical lattices. Precision measurement of the Cs GSTP will test atomic calculations of the hyperfine interactions between nuclear moments and electrons which are a challenging part of atomic parity-violation calculations. We will directly measure the GSTP by simultaneously driving transitions in two pairs of magnetic sublevels and measuring the populations in individual magnetic sublevels. Ours will be the first GSTP of the F$=$3 hyperfine level, and we anticipate an F$=$4 GSTP measurement that is more than an order of magnitude more precise the best previous measurement [S. Ulzega, et al,~\underline {Phys. Rev. A~75, 042505 (2007)}1].   

Demonstration of a Superluminal Ring Laser for Precision Metrology using Self-Pumped Raman Gain and Depletion in Two Isotopes of Rubidium

A superluminal ring laser (SRL) is one in which the group velocity of light far exceeds the vacuum speed of light, without violating special relativity or causality. Compared to a conventional laser, the sensitivity of such a laser to rotation or a perturbation in the cavity length can be enhanced by a factor as high as a million. Here, we describe our experimental demonstration of an SRL using an atomic vapor containing two isotopes of Rb. In Rb-85, we use the self-pumped Raman gain process to produce a relatively broad gain profile. The self-pumped Raman depletion process in Rb-87 is then used to produce a narrow dip at the center of the gain profile. Comparison of numerical simulation with experimental results show that we have achieved superluminal conditions corresponding to an enhancement in sensitivity by a factor of more than one thousand. Due to the fact that a Raman transition is highly insensitive (sensitive) to the Doppler broadening when the Raman pump and the Raman Probe are co-propagating (counter-propagating), two such counter-propagating SRLs can be realized in the same cavity without any cross talk. Such a pair of SRLs can be used to realize a rotation sensor with sensitivity much higher than that of a conventional ring laser gyroscope.