Positron scattering and annihilation on noble gas atoms

Positron scattering and annihilation on noble gas atoms below the positronium formation threshold is studied ab initio using many-body theory methods. The many-body theory provides a near-complete understanding of the positron-noble-gas-atom system at these energies and yields accurate numerical results. It accounts for positron-atom and electron-positron correlations, e.g., polarization of the atom by the incident positron and the non-perturbative process of virtual positronium formation. These correlations have a large effect on the scattering dynamics and result in a strong enhancement of the annihilation rates compared to the independent-particle mean-field description. Computed elastic scattering cross sections are found to be in good agreement with recent experimental results and Kohn variational and convergent close-coupling calculations. The calculated values of the annihilation rate parameter $Z_{\rm eff}$ (effective number of electrons participating in annihilation) rise steeply along the sequence of noble gas atoms due to the increasing strength of the correlation effects, and agree well with experimental data.Comment: 24 pages, 17 figure

Fluorescence measurements of expanding strongly-coupled neutral plasmas

We report new detailed density profile measurements in expanding strongly-coupled neutral plasmas. Using laser-induced fluorescence techniques, we determine plasma densities in the range of 10^5 to 10^9/cm^3 with a time resolution limit as small as 7 ns. Strong-coupling in the plasma ions is inferred directly from the fluorescence signals. Evidence for strong-coupling at late times is presented, confirming a recent theoretical result.Comment: submitted to PR

Quantum engineering of atomic phase-shifts in optical clocks

Quantum engineering of time-separated Raman laser pulses in three-level systems is presented to produce an ultra-narrow optical transition in bosonic alkali-earth clocks free from light shifts and with a significantly reduced sensitivity to laser parameter fluctuations. Based on a quantum artificial complex-wave-function analytical model, and supported by a full density matrix simulation including a possible residual effect of spontaneous emission from the intermediate state, atomic phase-shifts associated to Ramsey and Hyper-Ramsey two-photon spectroscopy in optical clocks are derived. Various common-mode Raman frequency detunings are found where the frequency shifts from off-resonant states are canceled, while strongly reducing their uncertainties at the 10$^{-18}$ level of accuracy.Comment: accepted for publication in PR

High accuracy measure of atomic polarizability in an optical lattice clock

Despite being a canonical example of quantum mechanical perturbation theory, as well as one of the earliest observed spectroscopic shifts, the Stark effect contributes the largest source of uncertainty in a modern optical atomic clock through blackbody radiation. By employing an ultracold, trapped atomic ensemble and high stability optical clock, we characterize the quadratic Stark effect with unprecedented precision. We report the ytterbium optical clock's sensitivity to electric fields (such as blackbody radiation) as the differential static polarizability of the ground and excited clock levels: 36.2612(7) kHz (kV/cm)^{-2}. The clock's fractional uncertainty due to room temperature blackbody radiation is reduced an order of magnitude to 3 \times 10^{-17}.Comment: 5 pages, 3 figures, 2 table

Two-photon absorption in potassium niobate

We report measurements of thermal self-locking of a Fabry-Perot cavity containing a potassium niobate (KNbO3) crystal. We develop a method to determine linear and nonlinear optical absorption coefficients in intracavity crystals by detailed analysis of the transmission lineshapes. These lineshapes are typical of optical bistability in thermally loaded cavities. For our crystal, we determine the one-photon absorption coefficient at 846 nm to be (0.0034 \pm 0.0022) per m and the two-photon absorption coefficient at 846 nm to be (3.2 \pm 0.5) \times 10^{-11} m/W and the one-photon absorption coefficient at 423 nm to be (13 \pm 2) per m. We also address the issue of blue-light-induced-infrared-absorption (BLIIRA), and determine a coefficient for this excited state absorption process. Our method is particularly well suited to bulk absorption measurements where absorption is small compared to scattering. We also report new measurements of the temperature dependence of the index of refraction at 846 nm, and compare to values in the literature.Comment: 8 pages. To appear in J. Opt. Soc. Am.

An atomic clock with 10−1810^{-18} instability

Atomic clocks have been transformational in science and technology, leading to innovations such as global positioning, advanced communications, and tests of fundamental constant variation. Next-generation optical atomic clocks can extend the capability of these timekeepers, where researchers have long aspired toward measurement precision at 1 part in $\bm{10^{18}}$. This milestone will enable a second revolution of new timing applications such as relativistic geodesy, enhanced Earth- and space-based navigation and telescopy, and new tests on physics beyond the Standard Model. Here, we describe the development and operation of two optical lattice clocks, both utilizing spin-polarized, ultracold atomic ytterbium. A measurement comparing these systems demonstrates an unprecedented atomic clock instability of $\bm{1.6\times 10^{-18}}$ after only $\bm{7}$ hours of averaging