This PDF file contains the front matter associated with SPIE Proceedings volume 7431, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

We demonstrated coherent pulse synthesis between the carrier-envelope phase slip (CEPS) locked second-harmonic (SH) pulses from an optical parametric oscillator (OPO) and those from its pump laser. By using a single nonlinear crystal with cascaded gratings for parametric and SH generation, we maximized the common-mode rejection of environmental noise, obtaining a temporal overlap between the pulses as low as 30 attoseconds in an observation time of 20 ms. The CEPS frequencies of the pump laser and the OPO SH signal were locked individually to the same subharmonic of the repetition rate with a coherence time of at least 1.4 ms by using the pump supercontinuum as a common reference. Auto-correlation traces of the combined pulses showed an 8:1 ratio between the peak and the background once the CEPS frequencies were locked, in contrast with a much lower ratio when they were not locked, indicating successful pulse synthesis. This research illustrates the viability of using OPOs for sub-femtosecond optical pulse synthesis. The very low timing jitter and phase coherence between the pulses from this system, which spans from the ultraviolet (SH of the pump) to mid-infrared (idler), also make the system a powerful tool for optical spectroscopy and optical metrology.

Atomic clocks have reached the Standard Quantum Limit (SQL) of precision,1 set by the projection noise inherent in measurements on uncorrelated atoms. It is possible to overcome this limit by entangling the atoms to generate a "squeezed state" of the atomic ensemble. We use the collective interaction of an atomic ensemble with a far-detuned light field in an optical resonator to prepare squeezed states by two different methods: quantum non-demolition (QND) measurement and Hamiltonian evolution. We apply both methods to an ensemble of 5 x 10^{4 87}Rb atoms in a superposition of hyperfine clock states. We measure the suppression of projection noise and compare it to the accompanying reduction in signal, thereby quantifying the net gain in spectroscopic sensitivity. By QND measurement, with resolution up to 9 dB below the projection noise level, we achieve 3.0(8) dB of metrologically relevant squeezing. Whereas the measurement-based approach relies on knowledge of the (randomly distributed) measurement outcome to produce a conditionally squeezed state, the method of Hamiltonian evolution produces a known squeezed state independent of detector performance. We mimic the dynamics of the one-axis twisting Hamiltonian, proposed as a generator of squeezed states by Kitagawa and Ueda,² by using the atom-induced frequency shift of the resonator mode and the corresponding resonator-field-induced shift of the atomic transition frequency to introduce an effective interaction among the atoms. The resulting deterministic squeezing is sufficient to allow a 6.0(4) dB improvement in spectroscopic sensitivity over the SQL.

We developed an optical frequency standard with the 4 ²S_1/2-3 ²D_5/2 electric quadrupole transition of ⁴⁰Ca⁺ ions. Its absolute transition frequency is 411 042 129 776 390(±7) Hz. The accuracy is limited by the electric quadrupole shift and the ambient magnetic field fluctuation. To determine the absolute transition frequency with a better accuracy, we have observed two pairs of the symmetrically-splitting Zeeman components and measured the transition frequency corrected for the electric quadrupole shift. In addition, we are developing a magnetic-shielded ion-trap chamber to suppress the transition-line broadening caused by the magnetic field fluctuation.

Optical clocks largely rely on interrogation lasers with sub-Hz linewidth and low short term instability. The laser stability is mostly determined by the properties of the cavities that are used as short term references. With suitable mounting the influence of vibrations is strongly suppressed and the short term stability is limited by thermal fluctuations to a fractional instability around 1 • 10^-15. Here we give an overview of the present status of our ultrastable lasers used for optical clocks and present possible ways to further reduce their noise levels and to transfer their stability to other wavelengths and to remote lasers.

We have characterized the ²⁴Mg optical frequency standard at the Institute of Quantum Optics (IQ), Hanover, using a clock laser at the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, via a noise compensated 73 km fiber link and present preliminary results for the stability of the Mg standard. The stability of the clock laser (λ = 657 nm) is transferred with a femtosecond frequency comb to a telecommunication laser at λ = 1542 nm. The signal is then transmitted from PTB through the fiber link to IQ. A second comb at IQ (the remote end) is used to compare the transmitted laser frequency with that of the Mg clock laser λ = 914 nm. The frequency ratio of the clock lasers ν_Mg/ν_Ca shows a relative instability < 10^-14 at 1 s. The upper limit for the contribution of the fiber link to the frequency instability is measured by connecting another optical fiber following the same 73 km route at Hanover computer center. The comparison performed at PTB between the local and the transmitted signal after a round trip length of 146 km showed a relative uncertainty below 1 x 10^-19 and a short term instability σy(τ)= 3.3 x 10^-15 / (τ/s).

Fiber optic networks are an attractive means for the remote distribution of highly stable frequencies from optical clocks. The highest performance is achieved by use of the frequency of the optical carrier itself as the transfer frequency. We will review our measurements on the transfer of optical frequency (a stabilized 1550 nm laser) over fiber optic links with lengths ranging from 38 km to 251 km. We discuss experimental details important for optimum performance and relate our measured performance to the theoretical limit on the phase and frequency noise of the transmitted signal as a function of the transmission distance.

Experiments of transmission of sub-Hz cavity-stabilized 1542 nm laser frequency using a pair of 43 km dark fibers in urban environment are reported on successively 86 km and 172 km, with fractional frequency instability in the 10^-19 range. A new approach is then introduced consisting in using part of an optical telecommunications network carrying simultaneously data traffic using a DWDM scheme to multiplex the metrological signal. This method is experimentally implemented using 22 km of fiber linking Université Paris 13 to its internet access point without degradation of the link instability. We finally present a project of large scale link between Paris and the German border using RENATER network which could constitute the first step of the building of a European optical network for ultrastable frequency dissemination and comparison.

Buffer gas induced collision shift for the ⁸⁸Sr ¹S₀-³P₁ transition is investigated by precision saturation spectroscopy of thermal gas in a heat cell. The cell was filled with rare gases of helium, neon, argon, and xenon as buffer gases. Helium showed the largest fractional shift coefficient of 1.6x10^-9 Torr^-1. The disagreement between our experiments and simple impact calculations indicates effective atom losses from zero-velocity class which contributes to saturated absorption spectroscopy. The result could be useful to evaluate the background gas collision shift of Sr lattice clocks. Keywords: collision, saturated absorption spectroscopy, lattice clock, optical clock, density shift

We have developed a one-dimensional optical lattice clock with ultracold ¹⁷¹Yb atoms. The absolute frequency of the ¹S₀(F = 1/2) - ³P₀(F = 1/2) clock transition in ¹⁷¹Yb is determined to be 518 295 836 590 864(28) Hz with respect to the SI second. Details of the experimental setups and atom trapping results are also described.

We discuss two possible configurations for optical lattice clocks; a one-dimensional (1D) lattice loaded with spinpolarized fermions, and a three-dimensional (3D) lattice loaded with bosons. In the former scheme, collisional frequency shifts are suppressed by the quantum statistical property of identical fermions. This Pauli blocking of collisions is critically dependent on the degree of spin polarization of the fermionic atoms, which we carefully investigated in the Rabi excitation process of the clock transition. In the latter scheme, a single occupancy lattice suppresses bunching of bosons and collisional frequency shifts. We demonstrate a frequency comparison of these two optical lattice clocks based on fermionic ⁸⁷Sr and bosonic ⁸⁸Sr. Operating these clocks parallely has yielded a stability approaching 10⁻¹⁷. Such measurements are an important step to ascertain the lattice clocks' uncertainty at the 10⁻¹⁷ level and beyond.