Bulletin of the American Physical Society
43rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 57, Number 5
Monday–Friday, June 4–8, 2012; Orange County, California
Session B6: Invited Session: The Fine Structure Constant |
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Sponsoring Units: GPMFC Chair: Gerald Gabrielse, Harvard University Room: Garden 4 |
Tuesday, June 5, 2012 10:30AM - 11:00AM |
B6.00001: Determination of the fine structure constant from atom interferometry Invited Speaker: Saida Guellati We report a new measurement of the atomic recoil using atom interferometry and Bloch oscillations (BO) in a vertical accelerated optical lattice. Such a measurement yields to a determination of $h/m_{\mathrm{Rb}}$ ($m_{\mathrm{Rb}}$ is the mass of Rubidium atom) which can be used to obtain a value of the fine structure constant following the equation: \begin{equation} \alpha^2=\frac{2R_\infty}{c}\frac{m_{\mathrm{Rb}}}{m_{\mathrm{e}}}\frac{h}{m_{\mathrm{Rb}}} \end{equation} where the Rydberg constant $R_\infty$ and the mass ratio $m_{\mathrm{Rb}}/m_{\mathrm{e}}$ are precisely known. The key idea to precisely determine the recoil velocity, is to transfer to the atoms as many recoils as possible and to measure their velocity variation. For this purpose we use an atomic interferometer consisting in two pairs of $\pi/2$ pulses combined with Bloch oscillations. The first pair selects an atomic sub recoil velocity Ramsey pattern from an ultra cold Rb atoms sample. We then accelerate the atoms and give to the selected atoms up to 1000 recoils by means of Bloch oscillations. The final velocity distribution is measured by scanning the frequency of the second pair of $\pi /2$ pulses. Following this scheme, we have performed in 2010 a measurement of $\alpha$ with an uncertainty of $6.6 \times 10^{-10}$. Our final result is: \begin{equation} 1/\alpha = 137.035~999~037 (91). \end{equation} Using this determination, we obtain a theoretical value of the electron anomaly $a_{\mathrm{e}}=0.001~159~652~181~13(84)$ which is in agreement with the experimental measurement of Gabrielse ($a_{\mathrm{e}}=0.001~159~652~180~73(28)$). The comparison of these values provides the most stringent test of the QED. Moreover, the precision is large enough to verify for the first time the muonic and hadronic contributions to this anomaly. [Preview Abstract] |
Tuesday, June 5, 2012 11:00AM - 11:30AM |
B6.00002: Most Precise Determination of the Fine Structure Constant: Electron g and QED Invited Speaker: Shannon Fogwell Hoogerheide Currently, the most accurate determination of the fine structure constant comes from our measurement of the electron magnetic moment in Bohr magnetons, $g/2$ = 1.001 159 652 180 73(28) [0.28 ppt]\footnote{D. Hanneke, S. Fogwell, and G. Gabrielse, Phys. Rev. Lett. 100, 120801 (2008)}. This measurement utilized quantum jump spectroscopy of transitions between the lowest quantum levels of a one-electron quantum cyclotron. The single trapped electron itself served as a precise magnetometer to allow for greater accumulation of quantum-jump line shape statistics. The spontaneous emission rate of the single electron at multiple values of the magnetic field was used to correct for the interaction between the electron and a cylindrical cavity, used to inhibit spontaneous emission by about a factor of 200. This measurement, combined with QED theory and small additional standard model corrections yields $\alpha^{-1}$ = 137.035 999 084(51) [0.37 ppb]. Improvements to the QED theory, to be reported in this session, will allow us to report a slightly shifted value of the fine structure constant with a slightly reduced uncertainty.\\ A new trap, now in operation, is designed to use cavity-sideband cooling to cool the axial motion of a single electron in a Penning trap to near its quantum ground state. The new apparatus also contains a positron source which should allow a greatly improved comparison of the magnetic moments of the positron and electron as a test of CPT invariance. Additionally, the completely new apparatus is designed to reduce the effect of vibrations and thermal fluctuations. Progress toward improved measurements of both the electron and positron magnetic moments will be summarized. [Preview Abstract] |
Tuesday, June 5, 2012 11:30AM - 12:00PM |
B6.00003: A tenth-order QED coontribution to the lepton g-2 Invited Speaker: Makiko Nio The anomalous magnetic moment of the electron $a_e$ has played the central role in testing the validity of quantum electrodynamics (QED) as well as the standard model of the elementary-particle physics. In this talk we report further improvement of the test which is made possible by the newly evaluated tenth-order QED contribution to $a_e$. Altogether 12672 Feynman diagrams contribute to the tenth-order term. To handle them, we have developed the computer algorithm that generates FORTRAN programs automatically for individual diagrams. The resulting programs have been numerically evaluated on the supercomputer systems at RIKEN. Our preliminary result of the mass-independent tenth-order term, which is universal for all spices of leptons, is $A_1^{(10)} = 9.1 \pm 0.6 $ in units of $(\alpha/pi)^5$. As a byproduct, the muon contribution to the tenth order $a_e$ is obtained. It is far smaller than the uncertainty of the mass-independent term. We have also improved the eighth-order term by an intense numerical work. The uncertainty in the mass-independent term $A_1^{(8)} =-1.9109 \pm 0.0021$ in units of $(\alpha/\pi)^4$ has been reduced by about 2/3. The muon contribution to $a_e$ at the eighth order, $A_2^{(8)}(m_e/m_\mu) = 0.000 922 2~(66) $, is newly obtained. The improvement in the QED theory leads to about 10\% improvement in the theoretical prediction of $a_e$ and about 30\% improvement in the fine-structure constant $\alpha$ derived from the measured value of $a_e$ and the QED theory. [Preview Abstract] |
Tuesday, June 5, 2012 12:00PM - 12:30PM |
B6.00004: A matter wave clock and new measurement of the fine structure constant Invited Speaker: Shau-Yu Lan The rest mass of a particle defines its Compton frequency, mc$^{2}$/h and thereby sets a fundamental timescale. However, the Compton frequency of a single, non-interacting particle is too high to be harnessed as a clock (about 3x10$^{25}$ Hz for a cesium atom) and does not directly give rise to observable effects. Here, we demonstrate a clock that stabilizes a radio-frequency signal to a certain fraction of the Cs Compton frequency, using a Ramsey-Bord\'{e} matter-wave interferometer combined with an optical frequency comb. The relative phase accumulated between matter-waves travelling along different paths provides us with an indirect way to access the Compton frequency. The paths are defined by atom-laser interactions, and the frequency comb relates the laser frequency to the clock's own output. In principle, the experiment could still function even if all other standards of measurement were lost. This demonstrates that a single, massive particle indeed defines a timescale, even in practice. This clock relates mass directly to time, which may find application in a new definition of the kilogram with competitive accuracy, by fixing the Planck constant. Moreover, I will report our recent progress on the measurement of the photon recoil frequency using a pair of conjugate Ramsey-Bord\'{e} interferometers with large momentum transfer beam splitters. The sensitivity of the interferometers scale quadratically with the momentum transfers on the beam splitters while the common mode noise can be removed by running two interferometers simultaneously. Such a measurement can be used to obtain a new determination of the fine structure constant. [Preview Abstract] |
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