We demonstrate the capability of accurate time transfer using optical fibers over long distances utilizing a dark fiber and hardware which is usually employed in two-way satellite time and frequency transfer (TWSTFT). Our time transfer through optical fiber (TTTOF) system is a variant of the standard TWSTFT by employing an optical fiber in the transmission path instead of free-space transmission of signals between two ground stations through geostationary satellites. As we use a dark fiber there are practically no limitations to the bandwidth of the transmitted signals so that we can use the highest chip-rate of the binary phase-shift modulation available from the commercial equipment. This leads to an enhanced precision compared to satellite time transfer where the occupied bandwidth is limited for cost reasons. The TTTOF system has been characterized and calibrated in a common clock experiment at PTB, and the combined calibration uncertainty is estimated as 74 ps. In a second step the remote part of the system was operated at Leibniz Universität Hannover, Institut für Quantenoptik (IQ) separated by 73 km from PTB in Braunschweig. In parallel, a GPS time transfer link between Braunschweig and Hannover was established, and both links connected a passive hydrogen maser at IQ with the reference time scale UTC(PTB) maintained in PTB. The results obtained with both links agree within the 1-σ uncertainty of the GPS link results, which is estimated as 0.72 ns. The fiber link exhibits a nearly 10-fold improved stability compared to the GPS link, and assessment of its performance has been limited by the properties of the passive maser.

We present and investigate different external cavity diode laser (ECDL) configurations for the manipulation of neutral atoms, wavelength-stabilized by a narrow-band high transmission interference filter. A novel diode laser, providing high output power of more than 1 W, with a linewidth of less than 85 kHz, based on a self-seeded tapered amplifier chip has been developed. Additionally, we compare the optical and spectral properties of two laser systems based on common laser diodes, differing in their coating, as well as one, based on a distributed-feedback (DFB) diode. The linear cavity setup in all these systems combines a robust and compact design with a high wavelength tunability and an improved stability of the optical feedback compared to diode laser setups using diffraction gratings for wavelength discrimination.

We load 10 5 magnesium atoms in a dipole trap from a millikelvin-hot magneto-optical trap (MOT) using a continuous-loading scheme. Light-assisted two-body processes limit the maximum achievable density in a MOT, resulting in a reduced transfer efficiency into a dipole trap when using the conventional sequential scheme. It is overcome in a continuous-loading scheme where a loss channel is opened in the MOT. This allows the accumulation of atoms in the dipole trap over the trap lifetime, determined by collisions with the background gas. This results in a significantly higher number of trapped atoms even at a lower steady-state peak density in the MOT.

We determine the frequency of the Yb I 1 S0 − 1 P1 transition at 399 nm using an optical frequency comb. Although this transition was measured previously using an optical transfer cavity [D. Das et al., Phys. Rev. A 72, 032506 (2005)], recent work has uncovered significant errors in that method. We compare our result of 751 526 533.49 ± 0.33 MHz for the Yb-174 isotope with those from the literature and discuss observed differences. We verify the correctness of our method by measuring the frequencies of well-known transitions in Rb and Cs, and by demonstrating proper control of systematic errors in both laser metrology and atomic spectroscopy. We also demonstrate the effect of quantum interference due to hyperfine structure in a divalent atomic system and present isotope shift measurements for all stable isotopes.

We demonstrate a fully optical, long-distance remote comparison of independent ultrastable optical frequencies reaching a short term stability that is superior to any reported remote comparison of optical frequencies. We use two ultrastable lasers, which are separated by a geographical distance of more than 50 km, and compare them via a 73 km long phase-stabilized fiber in a commercial telecommunication network. The remote characterization spans more than one optical octave and reaches a fractional frequency instability between the independent ultrastable laser systems of 3 x 10 (-15) in 0.1 s. The achieved performance at 100 ms represents an improvement by one order of magnitude to any previously reported remote comparison of optical frequencies and enables future remote dissemination of the stability of 100 mHz linewidth lasers within seconds.

We have explored the performance of two "dark fibers" of a commercial telecommunication fiber link for a remote comparison of optical clocks. These fibers establish a network in Germany that will eventually link optical frequency standards at PTB with those at the Institute of Quantum Optics (IQ) at the Leibniz University of Hanover, and the Max Planck Institutes in Erlangen (MPL) and Garching (MPQ). We demonstrate for the first time that within several minutes a phase coherent comparison of clock lasers at the few 10 -15 level can also be accomplished when the lasers are more than 100 km apart. Based on the performance of the fiber link to the IQ we estimate the expected stability for the link from PTB to MPQ via MPL that bridges a distance of approximately 900 km.

We have characterized the 24 Mg 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 σ y (τ)= 3.3 x 10 -15 / (τ/s).

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