This PDF file contains the front matter associated with SPIE Proceedings Volume 8132, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

We present a measurement of the 1S-2S transition frequency in atomic hydrogen by two-photon spectroscopy yielding f_1S-2S = 2 466 061 413 187 035 (10) Hz corresponding to a fractional frequency uncertainty of 4.2×10^-15. The result presents a more than three times improvement on the previous best measurement.

We generate a VUV frequency comb centered at 159 nm as the 5th harmonic of Ti:S femto-second pulses by a passive enhancement using an external cavity. Average power up to 1.5 uW is measured by a solar-blind phototube. Stable generation about 10 minutes is obtained by active locking of the Ti:S frequencies to the enhancement cavity. Frequency tunability of our Ti:S comb teeth is also verified, which enables us to continuously sweep its frequency with more than twice a free spectral range. Those features reported here are indispensable to precision spectroscopy of unexplored atomic transitions in the VUV region.

The dynamic intracavity ionization of a dilute gas target can substantially alter the pulse formation inside resonant fs enhancement cavities. We numerically and experimentally study these effects and how they affect intracavity high harmonic generation using fs frequency combs.

Atomic clocks have reached the Quantum Projection Noise (QPN) limit of stability and it has been proposed to use the entangled atomic ensemble in order to overcome the QPN limit. We have proposed a new method to lock the phase of the Local Oscillator (LO) to the atomic phase and call, "atomic phase lock (APL)." This APL could possibly overcome the QPN limit without need of preparing the entangled atomic ensemble. Traditional Ramsey method destroys the coherence of the atomic spin due to the projection measurement at each cycle. This destruction and initialization of the phase at each cycle introduce additional noise and the performance of the atomic clock is limited at the white frequency noise level. APL employs dispersion measurements, i.e. Faraday rotation, in order to measure the phase difference without destroying the atomic phase. By repeating the measurement cycle with sufficiently small dead time, one can suppress the LO noise down to white phase noise level, achieving the τ^-1 dependence of the Allan variance that can eventually overcome the QPN limit in long term. We are preparing a proof-of-principle experiment using the ensemble of trapped 171Yb⁺ ions with hyperfine splitting (12.6 GHz) as a clock transition and report the current status of the experiment as well.

We discuss the novel frequency shift due to inhomogeneous excitations, particularly in the context of optical frequency lattice clocks. We analyze in detail the frequency shifts due to scattering-induced tunneling. These shifts have terms that are linear and non-linear in the atomic density. We show that they are small at moderate to high lattice depths.

An ⁸⁷Sr-based-optical lattice clock in NICT is compared to that of The University of Tokyo using a >50 km fiber link. In this work, we have demonstrated for the first time that two distant Sr lattice clocks generate the same frequency with systematic uncertainty of 0.31 Hz (7.3 × 10^-16 fractionally) for the 429 THz clock frequency.

We demonstrate the frequency comparison of two optical lattice clocks at the relative stabilities close to the quantum projection noise (QPN) limit of optical lattice clocks. This stable frequency comparison is accomplished by synchronous interrogations of two clocks by a common probe laser, which allows us to cancel out the probe laser's frequency noise. We perform the frequency comparison of a one-dimensional (1D) optical lattice clock with spin-polarized fermions ⁸⁷Sr and a three-dimensional (3D) optical lattice clock with unity-occupation bosons ⁸⁸Sr and achieve the Allan standard deviation of σ_γ(τ)=4×10^-16 τ^-1/2, which corresponds to the QPN limited stability for N=1,000 atoms and the spectrum linewidth γ=8 Hz. The relative stability reaches 1×10^-17 with an averaging time of 1,600 s. Finally, we discuss the prospects to realize 10^-18 fractional inaccuracies and the possible application of frequency comparison with synchronous interrogations to remote clocks' comparison for relativistic geodesy.

Various activities of atomic frequency standards studied in National Institute of Information and Communications Technology (NICT) are briefly reviewed. After BIPM accepted the first cesium fountain clock in NICT as a reference to determine International Atomic Time (TAI), efforts to further reduce the uncertainty of collision shifts are ongoing. A second fountain clock using atomic molasses is being built to enable the operation with less atomic density. Single ion clock using calcium has been pursued for several years in NICT. The absolute frequency measured in 2008 has CIPM to adopt the Ca+ clock transition as a part of the list of radiation (LoR) to realize the meter. Sr lattice clock has started its operation last year. The absolute frequency agreed well with those obtained in other institutes. Study of stable cavities to stabilize clock lasers are also introduced.

The recent evaluation of the second cesium fountain NIM5 at National Institute of Metrology(NIM) shows a total relative uncertainty of 2×10^-15. The strontium optical clock project has successfully transferred atoms from broad line cooling to narrow line cooling. About 10^{7 88}Sr atoms have been cooled down to 2uK, ready to be loaded into 1 dimensional (1D) optical lattice. The clock transition interrogation laser is locked to a horizontally oriented high finesse cavity. The linewidth of this laser is reduced to 100Hz level. A fiber based optical frequency comb is being built, which can be used to synthesize ultra-stable microwave frequency for the fountains in the future.

We report the current status of our ytterbium optical lattice clock at the NMIJ, AIST. After the first measurement of the clock transition frequency and the estimation of the uncertainty, we have been improving our clock. For an increased signal to noise ratio of the observed spectrum, we employed an atom number normalization scheme. We stabilized the frequency of the lattice laser using a fiber-based optical frequency comb. We also stabilized the intensity of the lattice laser.

The National Research Council of Canada (NRC) is currently involved in a number of research projects aimed at improving time and frequency realization based on the accurate and precise stabilization of microwave and optical sources on atomic and molecular transitions. Projects described in this summary will focus on the development of a primary standard for the realization of the SI second based on a cesium atomic fountain and a next generation standard based on an optical transition in a single trapped and laser cooled ion of strontium. The cesium fountain is undergoing evaluations of its systematic shifts for an eventual contribution to TAI and for a re-measurement of the absolute frequency of the strontium ion clock transition at the 10^-15 level. The main contribution to the uncertainty budget of the fountain is thought to be caused by the inhomogeneity in the magnitude of the magnetic field in the drift region. The latest measurements of this field are presented. A new strontium ion trap of the endcap design was completed last year. This new system has compensation electrodes and access ports in three orthogonal directions to control the ion position and minimize micromotion. We report preliminary results indicating improved performance of this trap over our previous rf Paul trap. As part of an effort to reduce the systematics shifts to a minimum, the heights of the atomic standards above the geoid were measured with an accuracy of 5 cm, corresponding to a fractional frequency uncertainty of 5 × 10^-18 for the gravitational redshift.

At the Physikalisch-Technische Bundesanstalt (PTB), two caesium fountain primary frequency standards are used for International Atomic Time (TAI) contributions, for the realization of the time scale UTC(PTB), for frequency measurements of the Ytterbium ¹⁷¹Yb⁺ single-ion and the Strontium ⁸⁷Sr lattice optical clocks, as well as for frequency calibration of experiments using Magnesium and Hydrogen atoms. With the help of opticallystabilized microwaves for the caesium fountain primary standards, a quantum projection noise limited operation of the fountain CSF1 was demonstrated. Atom loading using a slow atomic beam source was implemented in CSF2, which improves the fountain stability. The uncertainty of the CSF2 fountain is reduced due to a new evaluation of the distributed cavity phase shifts. The ¹⁷¹Yb⁺ ion offers two narrow optical transitions, both of which can be used as a frequency reference. The two transitions show different sensitivities to the variation of the fine structure constant, and such variations can be studied in the same atomic system. Frequency measurements of the ⁸⁷Sr optical lattice clock were performed against the caesium fountain CSF1. They are in agreement with the values reported by other groups. Investigations of the blackbody radiation shift are in progress.