Measurements of the fine-structure constant α require methods from across subfields and are thus powerful tests of the consistency of theory and experiment in physics. Using the recoil frequency of cesium-133 atoms in a matter-wave interferometer, we recorded the most accurate measurement of the fine-structure constant to date: α = 1/137.035999046(27) at 2.0 × 10 accuracy. Using multiphoton interactions (Bragg diffraction and Bloch oscillations), we demonstrate the largest phase (12 million radians) of any Ramsey-Bordé interferometer and control systematic effects at a level of 0.12 part per billion. Comparison with Penning trap measurements of the electron gyromagnetic anomaly - 2 via the Standard Model of particle physics is now limited by the uncertainty in - 2; a 2.5σ tension rejects dark photons as the reason for the unexplained part of the muon's magnetic moment at a 99% confidence level. Implications for dark-sector candidates and electron substructure may be a sign of physics beyond the Standard Model that warrants further investigation.

Bragg diffraction has been used in atom interferometers because it allows signal enhancement through multiphoton momentum transfer and suppression of systematics by not changing the internal state of atoms. Its multi-port nature, however, can lead to parasitic interferometers, allows for intensity-dependent phase shifts in the primary interferometers, and distorts the ellipses used for phase extraction. We study and suppress these unwanted effects. Specifically, phase extraction by ellipse fitting and the resulting systematic phase shifts are calculated by Monte Carlo simulations. Phase shifts arising from the thermal motion of the atoms are controlled by spatial selection of atoms and an appropriate choice of Bragg intensity. In these simulations, we found that Gaussian Bragg pulse shapes yield the smallest systematic shifts. Parasitic interferometers are suppressed by a "magic" Bragg pulse duration. The sensitivity of the apparatus was improved by the addition of AC Stark shift compensation, which permits direct experimental study of sub-part-per-billion (ppb) systematics. This upgrade allows for a 310 k momentum transfer, giving an unprecedented 6.6 Mrad measured in a Ramsey-Bordé interferometer.Atom interferometers have been used for tests of fundamental physics such as the isotropy of gravity [1], the equivalence principle [2][3][4][5], the search for dark-sector particles [6,7], and measurements of the fine structure constant α [8,9], which characterizes the strength of the electromagnetic interaction. This constant can be obtained from the electron's gyromagnetic anomaly g e − 2. At the current accuracy, this involves > 10,000 Feynman diagrams, as well as muonic and hadronic physics [10]. At increased accuracy, the tauon and the weak interaction will also be included. Since this path leads to 0.24 ppb accuracy [11], an independent measurement of α would create a unique test for the standard model. The best such measurements of α are currently based on the recoil energy 2 k 2 /2m At of an atom of mass m At that has scattered a photon of momentum k [12,13]. This measurement yields /m At , and yields α to 0.66 ppb [8] via the relationThe Rydberg constant R ∞ is known to 0.005 ppb accuracy, and the atom-to-electron mass ratio is known to better than 0.1 ppb for many species [14]. In this paper, we improve the accuracy of a measurement of the fine structure constant using Bragg diffraction, by both increasing the sensitivity of the experiment and a thorough theoretical analysis of important systematic effects. In Section I, we present an enhancement in the sensitivity of an atom interferometer (AI) by AC Stark compensation, which allows faster integration. In Section II, we investigate aberrations to the elliptical shape used for phase extraction which arise from the diffraction phase. Section III shows how this leads to phase shifts due to thermal motion, and Section IV describes how spatial filtering can be used to suppress those shifts. In Section V, we consider the influence of the Bragg pulse shape, and i...

Using an atom interferometer to measure the quotient of the reduced Planck's constant and the mass of a cesium-133 atom /m Cs , the most accurate measurement of the fine structure constant α = 1/137.035999046(27) is recorded, at an accuracy of 0.20 parts per billion (ppb). Using multiphoton interactions (Bragg diffraction and Bloch oscillations), the largest phase (12 million radians) of any Ramsey-Bordé interferometer and controlled systematic effects at a level of 0.12 ppb are demonstrated. Comparing the Penning trap measurements with the Standard Model prediction of the electron gyromagnetic anomaly a e based on the α measurement, a 2.5 σ tension is observed, rejecting dark photons as the reason for the unexplained part of the muon's gyromagnetic moment discrepancy at a 99% confidence level according to frequentist statistics. Implications for dark-sector candidates (e.g., scalar and pseudoscalar bosons, vector bosons, and axial-vector bosons) may be a sign of physics beyond the Standard Model. A future upgrade of the cesium fountain atom interferometer is also proposed to increase the accuracy of /m Cs by 1 to 2 orders of magnitude, which would help resolve the tension.

Yb 14 MnSb 11 is a magnetic Zintl compound as well as being one of the best high temperature p-type thermoelectric materials. According to the Zintl formalism, which defines intermetallic phases where cations and anions are valence satisfied, this structure type is nominally made up of 14 Yb 2+ , 1 MnSb 9− 4 , 1 Sb 7− 3 , and 4 Sb 3− atoms. When Mn is replaced by Mg or Zn, the Zintl defined motifs become 13 Yb 2+ , 1 Yb 3+ , 1 (Mg, Zn)Sb 10− 4 , 1 Sb 7− 3 , and 4 Sb 3− . The predicted existence of Yb 3+ based on simple electron counting rules of the Zintl formalism calls the Yb valence of these compounds into question. X-ray absorption near-edge structure, magnetic susceptibility, and specific heat measurements on single crystals of the three analogs show signatures of intermediate valence Yb behavior and in particular, reveal the heavy fermion nature of Yb 14 MgSb 11 .Inthese isostructural compounds, Yb can exhibit a variety of electronic configurations from intermediate (M = Zn), mostly 2+ (M = Mn), to 3+ (M = Mg). In all cases, there is a small amount of intermediate valency at the lowest temperatures. The amount of intermediate valency is constant for M = Mn, Mg and temperature dependent for M = Zn. The evolution of the Yb valence correlated to the transport properties of these phases is highlighted. The presence of Yb in this structure type allows for fine tuning of the carrier concentration and thereby the possibility of optimized thermoelectric properties along with unique magnetic phenomena.

We present a hybrid laser frequency stabilization method combining modulation transfer spectroscopy (MTS) and frequency modulation spectroscopy (FMS) for the cesium D₂ transition. In a typical pump-probe setup, the error signal is a combination of the DC-coupled MTS error signal and the AC-coupled FMS error signal. This combines the long-term stability of the former with the high signal-to-noise ratio of the latter. In addition, we enhance the long-term frequency stability with laser intensity stabilization. By measuring the frequency difference between two independent hybrid spectroscopies, we investigate the short-and long-term stability. We find a long-term stability of 7.8 kHz characterized by a standard deviation of the beating frequency drift over the course of 10 h and a short-term stability of 1.9 kHz characterized by an Allan deviation of that at 2 s of integration time.