Abstract

We have established a transportable frequency measurement system using an optical frequency comb linked to a commercial Cs atomic clock, which is in turn linked to international atomic time (TAI) through global positioning system (GPS) time. An iodine-stabilized Nd:YAG laser is used as a flywheel in the frequency measurement system. This system is used to measure the absolute frequency of the clock transition of 87Sr in an optical lattice. We obtained a fractional uncertainty of 2×10-14 in the frequency measurement with a total averaging time of ~105 s over 9 days.

©2005 Optical Society of America

1. Introduction

Optical frequency standards are of great interest in relation to high-resolution spectroscopy, optical communications and testing of fundamental physics. It was recently demonstrated that mode-locked femtosecond lasers can be used to measure the absolute frequency of an optical frequency standard using a Cs atomic clock as a frequency reference [13]. A frequency comb obtained by injecting the light of a Kerr-lens mode-locked Ti:sapphire laser into a photonic crystal fiber (PCF) [4] can cover more than one octave of optical frequencies, enabling the direct measurement of the carrier-envelope offset (CEO) frequency of the femtosecond comb with an f-to-2f interferometer [3]. The frequency comb has been confirmed to be a reliable tool for optical frequency comparisons with an uncertainty of ~10–19 [5]. Furthermore, the achieved frequency instability of the microwave signal converted from an optical frequency standard using the frequency comb has been demonstrated to be ≤3.5×10-15 at an averaging time of 1 s, limited by the optical reference [6]. For high-precision frequency measurements, the combination of a hydrogen-maser (H-maser) and a Cs fountain clock is usually used as a reference source for obtaining both good short-term stability and accuracy.

The rapid development of research on optical frequency measurement based on femtosecond combs has stimulated the field of frequency metrology, especially research on high-performance optical frequency standards. Lasers frequency-stabilized with respect to the transitions of atoms, ions and molecules are recommended by the International Committee for Weights and Measures (CIPM) as optical frequency standards for metrology applications [7]. Among the recommendations, standards based on single trapped ions [810] and ultracold neutral atoms in free fall [11, 12] have shown record high levels of performance that approach those of the best Cs fountain clocks [13]. A different approach called an ‘optical lattice clock’ was proposed recently, in which atoms trapped in an optical lattice serve as quantum references [1416]. The optical lattice clock has achieved [16] a one order of magnitude narrower linewidth than that observed for neutral-atom optical-clocks [11, 12, 17], and its stability is comparable to that of single-ion clocks [8, 9].

In the present paper, we report on the frequency measurement of a Sr lattice clock using a transportable frequency measurement system. This system uses a Ti:sapphire-laser-based frequency comb as a clockwork and a commercial Cs atomic clock as a frequency reference. The Cs clock was calibrated by international atomic time (TAI) through global positioning system (GPS) time. As an alternative experimental scheme, an iodine-stabilized Nd:YAG laser was used as a flywheel in the frequency measurement. The measurement results obtained with different schemes were in agreement and exhibited a fractional measurement uncertainty of 2×10-14 with a total averaging time of ~105 s. The successful absolute frequency measurement of the Sr lattice clock is an important step in research on an ‘optical lattice clock’, and will make a significant contribution to the field of frequency metrology.

 figure: Fig. 1.

Fig. 1. Schematic diagram of the experimental setup. AOM, acousto-optic modulator; SYN, synthesizer; PLL, phase-lock loop; PCF, photonic crystal fiber; DM, dichroic mirror; D, detector; PBS, polarization beam splitter; AMP, amplifier; SMF, single-mode fiber; PZT, piezoelectric transducer; GPS, global positioning system.

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2. Experimental setup

Figure 1 is a schematic diagram of our experimental setup. A clock laser operating at λ 0=698 nm was frequency-stabilized to a high-finesse reference cavity to reduce the laser linewidth. The clock laser light was steered into a Sr optical lattice [1820] through a double-passed acousto-optic modulator (AOM1) for the purpose of frequency shifting and tuning. The drive frequency of the AOM (~184 MHz) was precisely set by a frequency synthesizer. 87Sr atoms were laser-cooled and trapped on the 1 S 0 - 3 P 1 transition and transferred into the optical lattice.

To measure the absolute frequency of the Sr lattice clock, a frequency comb, a Cs clock, and an iodine-stabilized Nd:YAG laser were brought from NMIJ/AIST to The University of Tokyo in Japan. In the frequency comb system, a single-frequency Nd:YVO4 laser with a maximum output power of 5 W (Coherent, Model Verdi-V5) was used as the pump source of a mode-locked Ti:sapphire laser (GigaOptics, Model GigaJet-30). The repetition rate and the width of the laser pulse were 793 MHz and 25 fs, respectively. In the frequency comb, the frequency of the n-th comb component is expressed as fn=n×f rep + f CEO, where f rep is the repetition rate of the laser pulse and f CEO is the carrier-envelope offset frequency. When f rep and f CEO are precisely controlled, the comb works as a “frequency linker” that connects optical and radio frequencies. The control ports of f rep and f CEO in the Ti:sapphire laser are the lengths of the laser cavity and the pump laser power, respectively. An AOM (AOM2) was inserted between the pump laser and the mode-locked laser for the purpose of power modulation.

Most of the output of the mode-locked laser was focused into a 30-cm-long PCF that had a core diameter of ~2 µm. An f-to-2f interferometer [3] was introduced to observe the f CEO of the comb. The green part of the broadened comb (532-nm comb) was separated out by a dichroic mirror and arranged to overlap with the second-harmonic generation (SHG) of the infrared comb (1064-nm comb) at a polarization beam splitter (PBS1). A delay line was introduced in the path of the 532-nm comb to match the timing of the two combs (which were actually fs pulses in the time domain). A 5-mm-long KNbO3 crystal was used for the SHG of the 1064-nm comb. A polarizer was used after the PBS to adjust the power ratio of the two combs in the beat measurement. f CEO was observed with a photo detector followed by a 1/4 prescaler and a digital phase-lock loop (PLL), and was used to servo control the pump laser power to set f CEO=80 MHz.

In the first experimental scheme of the frequency measurement, we phase-locked the whole comb to the Sr clock laser and measured f rep against a Cs atomic clock (Agilent, Model 5071A). A part of the comb around 698 nm was separated out before the SHG part in the f-to-2f interferometer. This part of the comb was combined with the clock laser light at another PBS (PBS2). The beat note, f b, between the clock laser and the 698-nm comb was observed and fed to another digital PLL that controlled the cavity length of the mode-locked laser by using a piezoelectric transducer (PZT).

 figure: Fig. 2.

Fig. 2. Allan standard deviation of the commercial Cs clock (black solid curve), an H-maser (dashed curve), the iodine-stabilized Nd:YAG laser (green solid curve), the Sr lattice clock (dotted line) and the measured beat frequency between the Nd:YAG laser and the clock laser (curve with filled circles).

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Part of the output of the mode-locked laser was picked up by a photo detector to measure f rep. f rep was down converted to ~7 MHz by using a mixer and a synthesizer with a fixed frequency of 800 MHz. The beat frequency was measured and recorded with a universal counter (Counter1, Agilent, Model 53132A). All the synthesizers and counters used in this experiment were phase locked to the Cs clock through a distribution amplifier.

In an alternative experimental scheme, a transportable iodine-stabilized Nd:YAG laser (NMIJ-Y1) [21, 22] was used as a flywheel in the frequency measurement. The short-term instability (Allan standard deviation) σy(τ) of the Sr lattice clock was estimated to be ~ 1×10-14 for τ=1 s [16] (shown as a dotted line in Fig. 2), and this is more than two orders of magnitude smaller than that of a commercially available Cs clock (black solid curve). As also shown in Fig. 2, the instability σ y(τ) of the Nd:YAG laser (green solid curve) was ~2×10-13 for τ=1 s. This value improved after τ=100 s towards < 3×10-14, which was smaller than that of the Cs clock until τ=105 s. The instability of the Nd:YAG laser is also smaller than that of an H-maser (dashed curve) up to 20 s. We also performed a frequency measurement by phase-locking the comb to the Nd:YAG laser to take advantage of the laser’s excellent short-term stability. The IR beam of the Nd:YAG was coupled into a single-mode fiber of a 50:50 fiber coupler for the beat measurement. The 1064-nm comb was also coupled into another port of the fiber coupler. The beat note between the cw IR light and the 1064-nm comb was observed at the output port of the coupler by using an avalanche photo detector. The measured beat signal was fed to a third digital PLL that controlled the cavity length of the mode-locked laser. In this way, the instability of each comb component could be reduced to follow that of the Nd:YAG laser. In this measurement scheme, the beat between the clock laser and the 698-nm comb (f b) was measured and recorded by another universal counter (Counter2).

Our comb system is described in detail elsewhere [23, 24]. In making the transportable comb, some of the optical, mechanical and electronic parts were obtained from Menlo System GmbH.

To calibrate the frequency of the commercial Cs clock, a GPS disciplined oscillator (GPS-DO) and a GPS antenna (Oscilloquartz, Model OSA4530) were employed at The University of Tokyo. The GPS-DO provides both 1 pulse per second (p.p.s.) and 10 MHz outputs referenced to the GPS time [25], by averaging signals obtained from 7~8 GPS satellites all in view. The time intervals between the 1 p.p.s. signal from the Cs clock and that from the GPS-DO were measured and recorded using a third universal counter (Counter3). The relationship between the GPS time and TAI can be found on the web page of the Bureau International des Poids et Mesures (BIPM) [26].

3. Experimental results

3.1 Frequency measurement based on a Cs atomic clock

A clock laser, whose frequency f c was prestabilized to that of a stable high-finesse cavity, was used to connect the Sr lattice clock and the comb measurement, and therefore played an important role in the present experiment.

In a spectroscopic investigation of Sr atoms in an optical lattice, a nearly Fourier-limited linewidth of 27 Hz was observed for the clock transition [16]. f c was frequency-scanned with AOM1 to find the atomic resonance frequency ν 0. The frequency setting of AOM1 δ(t) that corresponds to the resonance center could be determined from the Lorentzian fitting of the spectrum with an uncertainty of 2 Hz every 20 s (time of one scan). Figure 3(a) shows the AOM offset frequency δ(t), which indicates the frequency drift of the reference cavity. A frequency drift of ~6 kHz was observed over ~25000 s, and this gives a drift rate of ~0.2 Hz/s.

Figure 4 shows the spectra of a broadened frequency comb after the PCF. The strong peak near 698 nm permits the observation of the beat frequency f b=|f c-fn| between the clock laser and the n-th comb component at 698 nm. The signal-to-noise ratio (S/N) of f b exceeded 40 dB at a resolution bandwidth of 300 kHz. This was sufficient for the digital PLL that was used to servo-control the f rep of the comb. Consequently, the clock-laser frequency f c(t) was converted to f rep, which was then mixed down to about 7 MHz and recorded by Counter1 with a gate time of 20 s. The clock-laser frequency f c(t) was calculated by using the formula f c=n×f rep + f CEO ± f b with an integer n≈5.4×105. The ambiguous sign was removed by observing the sign of the variation in the beat frequency f b, while f rep was varied. Figure 3(b) shows the clock laser frequency f c(t) measured with the comb referenced to the Cs clock. The observed short-term frequency noise was contributed by the commercial Cs clock through the 800-MHz synthesizer that was used in measuring f rep.

 figure: Fig. 3.

Fig. 3. (a) Measured AOM offset frequency δ(t). (b) Measured clock-laser frequency f c(t) using the comb referenced to the commercially available Cs clock. (c) Absolute frequency of the clock transition ν 0(k) deduced from δ(t) and f c(t). (d) Histogram of ν 0(k) with a Gaussian fitting.

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 figure: Fig. 4.

Fig. 4. Spectra of the original comb of the Ti:sapphire laser (dashed curve) and the broadened comb after the photonic crystal fiber (solid curve). For the frequency measurement, beat note frequencies were observed between the clock laser and the broadened comb at 698 nm, and between the Nd:YAG laser and the broadened comb at 1064 nm.

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We have thus obtained two series of data sets; the offset frequency {δ(t)} and the corresponding clock laser frequency {f c(t)}, both of which were determined every t i=20 s. The atomic resonance frequency was calculated as ν 0(k)=f c(kt i) - δ(kt i) for k=1, 2, … where k max=T max/t i with T max of ~7620 s. Figure 3(c) shows the calculated data set of ν0(k), which gives an averaged value ν 0=429,228,004,229,976 Hz. The standard deviation σ is 578 Hz, which is in agreement with the Cs clock’s stability at τ=20 s. The histogram of ν 0(k) follows a normal distribution [shown in Fig. 3(d)]. The standard deviation of the mean σkmax is 30 Hz.

 figure: Fig. 5.

Fig. 5. (a) Measured AOM offset frequency δ(t). (b) Measured clock laser frequency f c(t) using the comb locked to the iodine-stabilized Nd:YAG laser. (c) Absolute frequency of the clock transition ν 0(k) deduced from δ(t) and f c(t). (d) Histogram of ν 0(k) with a Gaussian fitting.

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3.2 Frequency measurement based on an iodine-stabilized Nd:YAG laser

In the second experimental scheme, the Nd:YAG laser was used as a flywheel in the frequency measurement. Since the comb was locked to the Nd:YAG laser, we were able to observe the slow frequency drift of the clock laser with less short-term frequency noise. The strong peak of the broadened comb near 1064 (shown in Fig. 4) allows us to observe the beat frequency between the comb components and the Nd:YAG laser. We observed a beat frequency f Y=|f YAG - fm| between the Nd:YAG laser and the m-th comb component at 1064 nm and used it to servo-control the f rep of the comb. The S/N of f Y also exceeded 40 dB at a resolution bandwidth of 300 kHz. f rep was then mixed down to about 7 MHz and recorded by Counter1 with a gate time of 1 s. The relationship between the f YAG and f rep is f YAG=m×f rep + f CEO ± f Y. By measuring f rep, the averaged frequency of the Nd:YAG laser was determined with an uncertainty of 26 Hz, which is limited by the measurement time of 12000 s. We simultaneously observed a second beat frequency f b=|f c - fn| between the clock laser and the n-th comb component at 698 nm and this beat frequency was recorded by Counter2. The relationship between f c and f rep is f c=n×f rep + f CEO ± f b. The Allan standard deviation σ y(τ) of the measured f b is shown in Fig. 2 as a curve with filled circles. We notice that the Allan standard deviation of the measured f b basically follows that of the Nd:YAG laser for τ<30 s. When τ>30 s, the Allan standard deviation was limited by the frequency stability of reference cavity.

We again measured δ(t), which indicates the frequency drift of the reference cavity, and it is shown in Fig. 5(a). Figure 5(b) shows the measured clock-laser frequency f c(t) with a gate time of 20 s. In this case, the short-term noise was contributed by the Nd:YAG laser, which was 4×10-14 at τ=20 s, and is much smaller than that shown in Fig. 3(b). We have thus again obtained two series of data sets; the offset frequency {δ(t)} and the corresponding clock laser frequency {fc(t)}, both of which were determined every ti=20 s. The atomic resonance frequency was calculated as ν 0(k)=f c(kt i) - δ(kt i) for k=1, 2, …, k max, where k max=T max/t i with T max=2700 s. Figure 5(c) shows the calculated data set of ν 0(k), which gives an averaged value ν 0=429,228,004,229,958 Hz with a standard deviation σ of 46 Hz. The obtained standard deviation is about three times larger than the instability of the Nd:YAG laser, which was considered to be the main factor limiting the stability. Furthermore, the calculated frequency ν 0(k) shown in Fig. 5(c) exhibits a small time-dependent variation. These unexpected results may be caused by the portable Nd:YAG laser since we have only one laser and were unable to check the performance of the laser after the transportation.

The calculation of ν 0 was based on the averaged absolute frequency of the Nd:YAG laser, which has an uncertainty of 26 Hz. The total uncertainty of ν 0 is calculated by combining this 26 Hz with the uncertainty of the data averaging in Fig. 5(c). Although it may not be fare to calculate the standard deviation of the mean by simply applying σkmax (which is 4 Hz in this case), the histogram of ν 0(k) is not very far from a normal distribution [shown in Fig. 5(d)]. We estimate the standard deviation of the mean of the data in Fig. 5 (c) is much less than 26 Hz. This leads to an overall uncertainty of ν 0 of 26 Hz. By using the Nd:YAG laser as a flywheel, we were able to reduce the operating time of the Sr lattice clock in the measurement.

 figure: Fig. 6.

Fig. 6. Absolute frequency measurement of the 1 S 0 - 3 P 0 transition of 87Sr atoms in an optical lattice. Data shown in this figure do not include systematic corrections.

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3.3 Determination of the absolute frequency of the Sr lattice clock

Figure 6 shows the mean values of {ν 0(k)} measured over nine days with a total averaging time of τ ≈ 9.4×104 s. Each filled square in Fig. 6 represents a mean value of {ν 0(k)} whose measurement was based directly on the Cs atomic clock. The error bars are given by the standard deviation of the mean (σkmax). The frequency value measured directly based on the Cs clock described in Section 3.1 corresponds to measurement #9 in Fig. 6. Each filled circle in Fig. 6 represents a mean value of {ν 0(k)} measured based on the Nd:YAG laser. The error bars are mainly determined by the measurement uncertainty of the Nd:YAG laser frequency, based on the Cs clock. The frequency value measured using the Nd:YAG laser as a flywheel described in Section 3.2 corresponds to measurement #3 in Fig. 6. The weighted average of the nine measurements gives an averaged frequency of 429,228,004,229,998 Hz. The standard deviation of the mean is 9 Hz, which is in agreement with the instability of the Cs clock at τ=9.4×104 s. This gives a fractional uncertainty of 2×10-14 in the frequency measurement.

To reach the final measurement result, systematic corrections need to be applied to the obtained averaged frequency. The frequency measurement was corrected by using the calibration of a commercially available Cs clock. The frequency of the Cs clock was monitored by using the GPS time [25] during the period of the frequency measurement (40 days). Figure 7 shows the recorded timing difference between the 1 p.p.s. signals generated by the Cs clock and a GPS-DO. The frequency offset between the Cs clock and the GPS time was calculated to be -1.04(8)×10-13. We have confirmed that the periodic variation and the spike noise observed for the recorded timing difference was not contributed from the Cs clock, by using the data of long-term frequency comparison between the Nd:YAG laser and the Cs clock. We consider the observed periodic variation and the spike noise to be caused by daily variations in the ionosphere and unexpected reflections (multi paths) during the receipt of the GPS signal, respectively. The spike noise was removed from the data in the fitting of the slope. The daily variation does not affect the calibration result, which was obtained during a measurement period of 40 days. The uncertainty of the fitted slope (8×10-15) was obtained by dividing the data into 4 parts and calculating the standard deviation. This uncertainty indicates the frequency instability of the Cs clock for τ=10 days, which is below the specified flicker floor of 1×10-14 for a commercial clock. The agreement between the GPS time and the TAI was as close as a few parts in 1015 during the period of the frequency measurement [26].

 figure: Fig. 7.

Fig. 7. Time interval between the 1 p.p.s. signals from the Cs clock and the GPS-DO, started with the Cs clock signal and stopped with the GPS-DO signal. The slope α indicates the frequency difference between the two clocks.

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The offset between the Cs clock and the TAI resulted in a correction of -45 Hz in the frequency of the Sr lattice clock. We investigated other corrections from the lattice clock experiment in detail and found them to total 0.7 Hz [16]. Consequently, we determined that the total systematic correction, including the correction of the Cs clock, was -45.7 Hz. We also discussed external perturbations that could affect the absolute frequency of the lattice clock in detail [16]. The combined uncertainty for the measured frequency, including the uncertainty of the frequency measurement, was 15 Hz (fractional 3×10-14). The obtained final absolute frequency of the 1 S 0 - 3 P 0 transition of 87Sr atoms in an optical lattice is 429,228,004,229,952(15) Hz, which is consistent with a measurement reported by Courtillot et al. [27] within their measurement uncertainty of 20 kHz.

4. Conclusion

We have established a transportable frequency measurement system for the absolute frequency measurement of a Sr lattice clock. The coinciding experimental results obtained with two different schemes, namely measurement referenced directly to a Cs clock and measurement using a Nd:YAG laser as a flywheel, indicate the reliability of this measurement system. A fractional uncertainty of 2×10-14 was achieved in the frequency measurement with a total averaging time of τ≈105 s. After applying systematic corrections including the correction of the Cs atomic clock, the transition frequency of the Sr lattice clock was found to be 429,228,004,229,952(15) Hz.

This realization of the absolute frequency measurement of the Sr lattice clock has advanced optical lattice clock research from precision spectroscopy towards a real optical clock. The measurement results should be included in the CIPM recommendation of optical frequency standards [7]. Future research results with further reductions in measurement uncertainty should contribute to the discussion of the redefinition of the ‘second’.

In terms of frequency measurement, if we are to reduce the measurement uncertainty further, we must incorporate a highly stable local frequency reference and a highly precise time transfer system in the frequency measurement system. An H-maser and an improved iodine-stabilized Nd:YAG laser are considered to be candidates for a highly stable local oscillator. A GPS carrier-phase time transfer system [28] and an optical fiber network that delivers optical and microwave frequency standards [29] are effective methods for high-precision time transfer.

Acknowledgments

The authors are grateful to M. Yasuda for his assistance with the experiments and A. Onae, S. Ohshima, H. Matsumoto and M. Imae for helpful discussions. We appreciate the help provided by R. Holzwarth of Menlo Systems GmbH in making the transportable comb. This work received support from the Strategic Information and Communications R&D Promotion Programme (SCOPE) of the Ministry of Internal Affairs and Communications of Japan.

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References

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  1. Th. Udem, J. Reichert, R. Holzwarth, and T. W. Hänsch, “Absolute optical frequency measurement of the cesium D1 line with a mode-locked laser,” Phys. Rev. Lett. 82, 3568–3571 (1999).
    [Crossref]
  2. S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, Th. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84, 5102–5105 (2000).
    [Crossref] [PubMed]
  3. D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, and S. T. Cundiff, “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,” Science 288, 635–639 (2000).
    [Crossref] [PubMed]
  4. J. C. Knight, “Photonic crystal fiber,” Nature 424, 847–851 (2003).
    [Crossref] [PubMed]
  5. L.-S. Ma, Z. Bi, A. Bartels, L. Robertsson, M. Zucco, R. S. Windeler, G. Wilpers, C. Oates, L. Hollberg, and S. A. Diddams, “Optical frequency synthesis and comparison with uncertainty at the 10-19 level,” Science 303, 1843–1845 (2004).
    [Crossref] [PubMed]
  6. A. Bartels, S. A. Diddams, C. W. Oates, G. Wilpers, J. C. Bergquist, W. H. Oskay, and L. Hollberg, “Femtosecond-laser-based synthesis of ultrastable microwave signals from optical frequency references,” Opt. Lett. 30, 667–669 (2005).
    [Crossref] [PubMed]
  7. T. J. Quinn, “Practical realization of the definition of the metre, including recommended radiations of the optical frequency standards (2001),” Metrologia 40, 103–133 (2003).
    [Crossref]
  8. S. A. Diddams, Th. Udem, J. C. Bergquist, E. A. Curtis, R. E. Drullinger, L. Hollberg, W. M. Itano, W. D. Lee, C. W. Oates, K. R. Vogel, and D. J. Wineland, “An optical clock based on a single trapped 199Hg+ ion,” Science 293, 825–828 (2001).
    [Crossref] [PubMed]
  9. E. Peik, B. Lipphardt, H. Schnatz, T. Schneider, Chr. Tamm, and S. G. Karshenboim, “Limit on the present temporal variation of the fine structure constant,” Phys. Rev. Lett. 93, 170801 (2004).
    [Crossref] [PubMed]
  10. H. S. Margolis, G. P. Barwood, G. Huang, H. A. Klein, S. N. Lea, K. Szymaniec, and P. Gill, “Hertz-level measurement of the optical clock frequency in a single ion” Science 306, 1355–1358 (2004).
    [Crossref] [PubMed]
  11. G. Wilpers, T. Binnewies, C. Degenhardt, U. Sterr, J. Helmcke, and F. Riehle, “Optical clock with ultracold neutral atoms,” Phys. Rev. Lett. 89, 230801 (2002).
    [Crossref] [PubMed]
  12. C. W. Oates, E. A. Curtis, and L. Hollberg, “Improved short-term stability of optical frequency standards: approaching 1Hz in 1s with the Ca standard at 657 nm,” Opt. Lett. 25, 1603–1605 (2000).
    [Crossref]
  13. F. Pereira Dos Santos, H. Marion, S. Bize, Y. Sortais, A. Clairon, and C. Salomon, “Controlling the cold collision shift in high precision atomic interferometry,” Phys. Rev. Lett. 89, 233004 (2002).
    [Crossref] [PubMed]
  14. H. Katori, “Spectroscopy of strontium atoms in the Lamb-Dicke confinement,” in Proceedings of the 6th Symposium on Frequency Standards and Metrology, P. Gill, ed. (World Scientific, Singapore, 2002), pp. 323–330.
    [Crossref]
  15. H. Katori, M. Takamoto, V. G. Pal’chikov, and V. D. Ovsiannikov, “Ultrastable optical clock with neutral atoms in an engineered light shift trap,” Phys. Rev. Lett. 91, 173005 (2003).
    [Crossref] [PubMed]
  16. M. Takamoto, F.-L. Hong, R. Higashi, and H. Katori, “An optical lattice clock,” Nature 435, 321–324 (2005).
    [Crossref] [PubMed]
  17. F. Ruschewitz, J. L. Peng, H. Hinderthur, N. Schaffrath, K. Sengstock, and W. Ertmer, “Sub-kilohertz optical spectroscopy with a time domain atom interferometer,” Phys. Rev. Lett. 80, 3173–3176 (1998).
    [Crossref]
  18. H. Katori, T. Ido, and M. Kuwata-Gonokami, “Optimal design of dipole potentials for efficient loading of Sr atoms,” J. Phys. Soc. Jpn. 68, 2479–2482 (1999).
    [Crossref]
  19. T. Ido and H. Katori, “Recoil-free spectroscopy of neutral Sr atoms in the Lamb-Dicke regime,” Phys. Rev. Lett. 91, 053001 (2003).
    [Crossref] [PubMed]
  20. M. Takamoto and H. Katori, “Spectroscopy of the 1S0 - 3P0 clock transition of 87Sr in an optical lattice,” Phys. Rev. Lett. 91, 223001 (2003).
    [Crossref] [PubMed]
  21. F. -L. Hong, J. Ishikawa, J. Yoda, J. Ye, L.-S. Ma, and J. L. Hall, “Frequency comparison of 127I2-stabilized Nd:YAG lasers,” IEEE Trans. Instrum. Meas. 48, 532–536 (1999).
    [Crossref]
  22. F.-L. Hong, J. Ishikawa, Y. Zhang, R. Guo, A. Onae, and H. Matsumoto, “Frequency reproducibility of an iodine-stabilized Nd:YAG laser at 532 nm,” Opt. Commun. 235, 377–385 (2004).
    [Crossref]
  23. F.-L. Hong, S. A. Diddams, R. Guo, Z.-Y. Bi, A. Onae, H. Inaba, J. Ishikawa, K. Okumura, D. Katsuragi, J. Hirata, T. Shimizu, T. Kurosu, Y. Koga, and H. Matsumoto, “Frequency measurements and hyperfine structure of the R(85)33-0 transition of molecular iodine with a femtosecond optical comb,” J. Opt. Soc. Am. B 21, 88–95 (2004).
    [Crossref]
  24. F. -L. Hong, A. Onae, J. Jiang, R. Guo, H. Inaba, K. Minoshima, T. R. Schibli, H. Matsumoto, and K. Nakagawa, “Absolute frequency measurement of an acetylene-stabilized laser at 1542 nm,” Opt. Lett. 28, 2324–2326 (2003).
    [Crossref] [PubMed]
  25. W. Lewandowski, J. Azoubib, and W. J. Klepczynski, “GPS: Primary tool for time transfer,” Proc. IEEE 87, 163–172 (1999).
    [Crossref]
  26. Bureau International des Poids et Mesures (BIPM), Circular T, No. 200, (August 2004), http://www1.bipm.org/en/scientific/tai/time_ftp.html.
  27. I. Courtillot, a. Quessada, R. P. Kovacich, A. Brusch, D. Kolker, J.-J. Zondy, G. D. Rovera, and R. Lemonde, “Clock transition for a future optical frequency standard with trapped atoms” Phys. Rev. A 68, 030501 (2003).
    [Crossref]
  28. K. M. Larson and J. Levine, “Carrier-phase time transfer,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr. 46, 1001–1012 (1999).
    [Crossref]
  29. J. Ye, J.-L. Peng, R. J. Jones, K. W. Holman, J. L. Hall, D. J. Jones, S. A. Diddams, J. Kitching, S. Bize, J. C. Bergquist, L. W. Hollberg, L. Robertsson, and L.-S. Ma, “Delivery of high-stability optical and microwave frequency standards over an optical fiber network,” J. Opt. Soc. Am. B 20, 1459–1467 (2003).
    [Crossref]

2005 (2)

2004 (5)

L.-S. Ma, Z. Bi, A. Bartels, L. Robertsson, M. Zucco, R. S. Windeler, G. Wilpers, C. Oates, L. Hollberg, and S. A. Diddams, “Optical frequency synthesis and comparison with uncertainty at the 10-19 level,” Science 303, 1843–1845 (2004).
[Crossref] [PubMed]

E. Peik, B. Lipphardt, H. Schnatz, T. Schneider, Chr. Tamm, and S. G. Karshenboim, “Limit on the present temporal variation of the fine structure constant,” Phys. Rev. Lett. 93, 170801 (2004).
[Crossref] [PubMed]

H. S. Margolis, G. P. Barwood, G. Huang, H. A. Klein, S. N. Lea, K. Szymaniec, and P. Gill, “Hertz-level measurement of the optical clock frequency in a single ion” Science 306, 1355–1358 (2004).
[Crossref] [PubMed]

F.-L. Hong, J. Ishikawa, Y. Zhang, R. Guo, A. Onae, and H. Matsumoto, “Frequency reproducibility of an iodine-stabilized Nd:YAG laser at 532 nm,” Opt. Commun. 235, 377–385 (2004).
[Crossref]

F.-L. Hong, S. A. Diddams, R. Guo, Z.-Y. Bi, A. Onae, H. Inaba, J. Ishikawa, K. Okumura, D. Katsuragi, J. Hirata, T. Shimizu, T. Kurosu, Y. Koga, and H. Matsumoto, “Frequency measurements and hyperfine structure of the R(85)33-0 transition of molecular iodine with a femtosecond optical comb,” J. Opt. Soc. Am. B 21, 88–95 (2004).
[Crossref]

2003 (8)

F. -L. Hong, A. Onae, J. Jiang, R. Guo, H. Inaba, K. Minoshima, T. R. Schibli, H. Matsumoto, and K. Nakagawa, “Absolute frequency measurement of an acetylene-stabilized laser at 1542 nm,” Opt. Lett. 28, 2324–2326 (2003).
[Crossref] [PubMed]

T. Ido and H. Katori, “Recoil-free spectroscopy of neutral Sr atoms in the Lamb-Dicke regime,” Phys. Rev. Lett. 91, 053001 (2003).
[Crossref] [PubMed]

M. Takamoto and H. Katori, “Spectroscopy of the 1S0 - 3P0 clock transition of 87Sr in an optical lattice,” Phys. Rev. Lett. 91, 223001 (2003).
[Crossref] [PubMed]

I. Courtillot, a. Quessada, R. P. Kovacich, A. Brusch, D. Kolker, J.-J. Zondy, G. D. Rovera, and R. Lemonde, “Clock transition for a future optical frequency standard with trapped atoms” Phys. Rev. A 68, 030501 (2003).
[Crossref]

J. Ye, J.-L. Peng, R. J. Jones, K. W. Holman, J. L. Hall, D. J. Jones, S. A. Diddams, J. Kitching, S. Bize, J. C. Bergquist, L. W. Hollberg, L. Robertsson, and L.-S. Ma, “Delivery of high-stability optical and microwave frequency standards over an optical fiber network,” J. Opt. Soc. Am. B 20, 1459–1467 (2003).
[Crossref]

J. C. Knight, “Photonic crystal fiber,” Nature 424, 847–851 (2003).
[Crossref] [PubMed]

T. J. Quinn, “Practical realization of the definition of the metre, including recommended radiations of the optical frequency standards (2001),” Metrologia 40, 103–133 (2003).
[Crossref]

H. Katori, M. Takamoto, V. G. Pal’chikov, and V. D. Ovsiannikov, “Ultrastable optical clock with neutral atoms in an engineered light shift trap,” Phys. Rev. Lett. 91, 173005 (2003).
[Crossref] [PubMed]

2002 (2)

F. Pereira Dos Santos, H. Marion, S. Bize, Y. Sortais, A. Clairon, and C. Salomon, “Controlling the cold collision shift in high precision atomic interferometry,” Phys. Rev. Lett. 89, 233004 (2002).
[Crossref] [PubMed]

G. Wilpers, T. Binnewies, C. Degenhardt, U. Sterr, J. Helmcke, and F. Riehle, “Optical clock with ultracold neutral atoms,” Phys. Rev. Lett. 89, 230801 (2002).
[Crossref] [PubMed]

2001 (1)

S. A. Diddams, Th. Udem, J. C. Bergquist, E. A. Curtis, R. E. Drullinger, L. Hollberg, W. M. Itano, W. D. Lee, C. W. Oates, K. R. Vogel, and D. J. Wineland, “An optical clock based on a single trapped 199Hg+ ion,” Science 293, 825–828 (2001).
[Crossref] [PubMed]

2000 (3)

C. W. Oates, E. A. Curtis, and L. Hollberg, “Improved short-term stability of optical frequency standards: approaching 1Hz in 1s with the Ca standard at 657 nm,” Opt. Lett. 25, 1603–1605 (2000).
[Crossref]

S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, Th. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84, 5102–5105 (2000).
[Crossref] [PubMed]

D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, and S. T. Cundiff, “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,” Science 288, 635–639 (2000).
[Crossref] [PubMed]

1999 (5)

Th. Udem, J. Reichert, R. Holzwarth, and T. W. Hänsch, “Absolute optical frequency measurement of the cesium D1 line with a mode-locked laser,” Phys. Rev. Lett. 82, 3568–3571 (1999).
[Crossref]

H. Katori, T. Ido, and M. Kuwata-Gonokami, “Optimal design of dipole potentials for efficient loading of Sr atoms,” J. Phys. Soc. Jpn. 68, 2479–2482 (1999).
[Crossref]

K. M. Larson and J. Levine, “Carrier-phase time transfer,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr. 46, 1001–1012 (1999).
[Crossref]

F. -L. Hong, J. Ishikawa, J. Yoda, J. Ye, L.-S. Ma, and J. L. Hall, “Frequency comparison of 127I2-stabilized Nd:YAG lasers,” IEEE Trans. Instrum. Meas. 48, 532–536 (1999).
[Crossref]

W. Lewandowski, J. Azoubib, and W. J. Klepczynski, “GPS: Primary tool for time transfer,” Proc. IEEE 87, 163–172 (1999).
[Crossref]

1998 (1)

F. Ruschewitz, J. L. Peng, H. Hinderthur, N. Schaffrath, K. Sengstock, and W. Ertmer, “Sub-kilohertz optical spectroscopy with a time domain atom interferometer,” Phys. Rev. Lett. 80, 3173–3176 (1998).
[Crossref]

Azoubib, J.

W. Lewandowski, J. Azoubib, and W. J. Klepczynski, “GPS: Primary tool for time transfer,” Proc. IEEE 87, 163–172 (1999).
[Crossref]

Bartels, A.

A. Bartels, S. A. Diddams, C. W. Oates, G. Wilpers, J. C. Bergquist, W. H. Oskay, and L. Hollberg, “Femtosecond-laser-based synthesis of ultrastable microwave signals from optical frequency references,” Opt. Lett. 30, 667–669 (2005).
[Crossref] [PubMed]

L.-S. Ma, Z. Bi, A. Bartels, L. Robertsson, M. Zucco, R. S. Windeler, G. Wilpers, C. Oates, L. Hollberg, and S. A. Diddams, “Optical frequency synthesis and comparison with uncertainty at the 10-19 level,” Science 303, 1843–1845 (2004).
[Crossref] [PubMed]

Barwood, G. P.

H. S. Margolis, G. P. Barwood, G. Huang, H. A. Klein, S. N. Lea, K. Szymaniec, and P. Gill, “Hertz-level measurement of the optical clock frequency in a single ion” Science 306, 1355–1358 (2004).
[Crossref] [PubMed]

Bergquist, J. C.

Bi, Z.

L.-S. Ma, Z. Bi, A. Bartels, L. Robertsson, M. Zucco, R. S. Windeler, G. Wilpers, C. Oates, L. Hollberg, and S. A. Diddams, “Optical frequency synthesis and comparison with uncertainty at the 10-19 level,” Science 303, 1843–1845 (2004).
[Crossref] [PubMed]

Bi, Z.-Y.

Binnewies, T.

G. Wilpers, T. Binnewies, C. Degenhardt, U. Sterr, J. Helmcke, and F. Riehle, “Optical clock with ultracold neutral atoms,” Phys. Rev. Lett. 89, 230801 (2002).
[Crossref] [PubMed]

Bize, S.

Brusch, A.

I. Courtillot, a. Quessada, R. P. Kovacich, A. Brusch, D. Kolker, J.-J. Zondy, G. D. Rovera, and R. Lemonde, “Clock transition for a future optical frequency standard with trapped atoms” Phys. Rev. A 68, 030501 (2003).
[Crossref]

Clairon, A.

F. Pereira Dos Santos, H. Marion, S. Bize, Y. Sortais, A. Clairon, and C. Salomon, “Controlling the cold collision shift in high precision atomic interferometry,” Phys. Rev. Lett. 89, 233004 (2002).
[Crossref] [PubMed]

Courtillot, I.

I. Courtillot, a. Quessada, R. P. Kovacich, A. Brusch, D. Kolker, J.-J. Zondy, G. D. Rovera, and R. Lemonde, “Clock transition for a future optical frequency standard with trapped atoms” Phys. Rev. A 68, 030501 (2003).
[Crossref]

Cundiff, S. T.

D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, and S. T. Cundiff, “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,” Science 288, 635–639 (2000).
[Crossref] [PubMed]

S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, Th. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84, 5102–5105 (2000).
[Crossref] [PubMed]

Curtis, E. A.

S. A. Diddams, Th. Udem, J. C. Bergquist, E. A. Curtis, R. E. Drullinger, L. Hollberg, W. M. Itano, W. D. Lee, C. W. Oates, K. R. Vogel, and D. J. Wineland, “An optical clock based on a single trapped 199Hg+ ion,” Science 293, 825–828 (2001).
[Crossref] [PubMed]

C. W. Oates, E. A. Curtis, and L. Hollberg, “Improved short-term stability of optical frequency standards: approaching 1Hz in 1s with the Ca standard at 657 nm,” Opt. Lett. 25, 1603–1605 (2000).
[Crossref]

Degenhardt, C.

G. Wilpers, T. Binnewies, C. Degenhardt, U. Sterr, J. Helmcke, and F. Riehle, “Optical clock with ultracold neutral atoms,” Phys. Rev. Lett. 89, 230801 (2002).
[Crossref] [PubMed]

Diddams, S. A.

A. Bartels, S. A. Diddams, C. W. Oates, G. Wilpers, J. C. Bergquist, W. H. Oskay, and L. Hollberg, “Femtosecond-laser-based synthesis of ultrastable microwave signals from optical frequency references,” Opt. Lett. 30, 667–669 (2005).
[Crossref] [PubMed]

L.-S. Ma, Z. Bi, A. Bartels, L. Robertsson, M. Zucco, R. S. Windeler, G. Wilpers, C. Oates, L. Hollberg, and S. A. Diddams, “Optical frequency synthesis and comparison with uncertainty at the 10-19 level,” Science 303, 1843–1845 (2004).
[Crossref] [PubMed]

F.-L. Hong, S. A. Diddams, R. Guo, Z.-Y. Bi, A. Onae, H. Inaba, J. Ishikawa, K. Okumura, D. Katsuragi, J. Hirata, T. Shimizu, T. Kurosu, Y. Koga, and H. Matsumoto, “Frequency measurements and hyperfine structure of the R(85)33-0 transition of molecular iodine with a femtosecond optical comb,” J. Opt. Soc. Am. B 21, 88–95 (2004).
[Crossref]

J. Ye, J.-L. Peng, R. J. Jones, K. W. Holman, J. L. Hall, D. J. Jones, S. A. Diddams, J. Kitching, S. Bize, J. C. Bergquist, L. W. Hollberg, L. Robertsson, and L.-S. Ma, “Delivery of high-stability optical and microwave frequency standards over an optical fiber network,” J. Opt. Soc. Am. B 20, 1459–1467 (2003).
[Crossref]

S. A. Diddams, Th. Udem, J. C. Bergquist, E. A. Curtis, R. E. Drullinger, L. Hollberg, W. M. Itano, W. D. Lee, C. W. Oates, K. R. Vogel, and D. J. Wineland, “An optical clock based on a single trapped 199Hg+ ion,” Science 293, 825–828 (2001).
[Crossref] [PubMed]

S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, Th. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84, 5102–5105 (2000).
[Crossref] [PubMed]

D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, and S. T. Cundiff, “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,” Science 288, 635–639 (2000).
[Crossref] [PubMed]

Drullinger, R. E.

S. A. Diddams, Th. Udem, J. C. Bergquist, E. A. Curtis, R. E. Drullinger, L. Hollberg, W. M. Itano, W. D. Lee, C. W. Oates, K. R. Vogel, and D. J. Wineland, “An optical clock based on a single trapped 199Hg+ ion,” Science 293, 825–828 (2001).
[Crossref] [PubMed]

Ertmer, W.

F. Ruschewitz, J. L. Peng, H. Hinderthur, N. Schaffrath, K. Sengstock, and W. Ertmer, “Sub-kilohertz optical spectroscopy with a time domain atom interferometer,” Phys. Rev. Lett. 80, 3173–3176 (1998).
[Crossref]

Gill, P.

H. S. Margolis, G. P. Barwood, G. Huang, H. A. Klein, S. N. Lea, K. Szymaniec, and P. Gill, “Hertz-level measurement of the optical clock frequency in a single ion” Science 306, 1355–1358 (2004).
[Crossref] [PubMed]

Guo, R.

Hall, J. L.

J. Ye, J.-L. Peng, R. J. Jones, K. W. Holman, J. L. Hall, D. J. Jones, S. A. Diddams, J. Kitching, S. Bize, J. C. Bergquist, L. W. Hollberg, L. Robertsson, and L.-S. Ma, “Delivery of high-stability optical and microwave frequency standards over an optical fiber network,” J. Opt. Soc. Am. B 20, 1459–1467 (2003).
[Crossref]

S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, Th. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84, 5102–5105 (2000).
[Crossref] [PubMed]

D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, and S. T. Cundiff, “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,” Science 288, 635–639 (2000).
[Crossref] [PubMed]

F. -L. Hong, J. Ishikawa, J. Yoda, J. Ye, L.-S. Ma, and J. L. Hall, “Frequency comparison of 127I2-stabilized Nd:YAG lasers,” IEEE Trans. Instrum. Meas. 48, 532–536 (1999).
[Crossref]

Hänsch, T. W.

S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, Th. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84, 5102–5105 (2000).
[Crossref] [PubMed]

Th. Udem, J. Reichert, R. Holzwarth, and T. W. Hänsch, “Absolute optical frequency measurement of the cesium D1 line with a mode-locked laser,” Phys. Rev. Lett. 82, 3568–3571 (1999).
[Crossref]

Helmcke, J.

G. Wilpers, T. Binnewies, C. Degenhardt, U. Sterr, J. Helmcke, and F. Riehle, “Optical clock with ultracold neutral atoms,” Phys. Rev. Lett. 89, 230801 (2002).
[Crossref] [PubMed]

Higashi, R.

M. Takamoto, F.-L. Hong, R. Higashi, and H. Katori, “An optical lattice clock,” Nature 435, 321–324 (2005).
[Crossref] [PubMed]

Hinderthur, H.

F. Ruschewitz, J. L. Peng, H. Hinderthur, N. Schaffrath, K. Sengstock, and W. Ertmer, “Sub-kilohertz optical spectroscopy with a time domain atom interferometer,” Phys. Rev. Lett. 80, 3173–3176 (1998).
[Crossref]

Hirata, J.

Hollberg, L.

A. Bartels, S. A. Diddams, C. W. Oates, G. Wilpers, J. C. Bergquist, W. H. Oskay, and L. Hollberg, “Femtosecond-laser-based synthesis of ultrastable microwave signals from optical frequency references,” Opt. Lett. 30, 667–669 (2005).
[Crossref] [PubMed]

L.-S. Ma, Z. Bi, A. Bartels, L. Robertsson, M. Zucco, R. S. Windeler, G. Wilpers, C. Oates, L. Hollberg, and S. A. Diddams, “Optical frequency synthesis and comparison with uncertainty at the 10-19 level,” Science 303, 1843–1845 (2004).
[Crossref] [PubMed]

S. A. Diddams, Th. Udem, J. C. Bergquist, E. A. Curtis, R. E. Drullinger, L. Hollberg, W. M. Itano, W. D. Lee, C. W. Oates, K. R. Vogel, and D. J. Wineland, “An optical clock based on a single trapped 199Hg+ ion,” Science 293, 825–828 (2001).
[Crossref] [PubMed]

C. W. Oates, E. A. Curtis, and L. Hollberg, “Improved short-term stability of optical frequency standards: approaching 1Hz in 1s with the Ca standard at 657 nm,” Opt. Lett. 25, 1603–1605 (2000).
[Crossref]

Hollberg, L. W.

Holman, K. W.

Holzwarth, R.

S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, Th. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84, 5102–5105 (2000).
[Crossref] [PubMed]

Th. Udem, J. Reichert, R. Holzwarth, and T. W. Hänsch, “Absolute optical frequency measurement of the cesium D1 line with a mode-locked laser,” Phys. Rev. Lett. 82, 3568–3571 (1999).
[Crossref]

Hong, F. -L.

F. -L. Hong, A. Onae, J. Jiang, R. Guo, H. Inaba, K. Minoshima, T. R. Schibli, H. Matsumoto, and K. Nakagawa, “Absolute frequency measurement of an acetylene-stabilized laser at 1542 nm,” Opt. Lett. 28, 2324–2326 (2003).
[Crossref] [PubMed]

F. -L. Hong, J. Ishikawa, J. Yoda, J. Ye, L.-S. Ma, and J. L. Hall, “Frequency comparison of 127I2-stabilized Nd:YAG lasers,” IEEE Trans. Instrum. Meas. 48, 532–536 (1999).
[Crossref]

Hong, F.-L.

M. Takamoto, F.-L. Hong, R. Higashi, and H. Katori, “An optical lattice clock,” Nature 435, 321–324 (2005).
[Crossref] [PubMed]

F.-L. Hong, J. Ishikawa, Y. Zhang, R. Guo, A. Onae, and H. Matsumoto, “Frequency reproducibility of an iodine-stabilized Nd:YAG laser at 532 nm,” Opt. Commun. 235, 377–385 (2004).
[Crossref]

F.-L. Hong, S. A. Diddams, R. Guo, Z.-Y. Bi, A. Onae, H. Inaba, J. Ishikawa, K. Okumura, D. Katsuragi, J. Hirata, T. Shimizu, T. Kurosu, Y. Koga, and H. Matsumoto, “Frequency measurements and hyperfine structure of the R(85)33-0 transition of molecular iodine with a femtosecond optical comb,” J. Opt. Soc. Am. B 21, 88–95 (2004).
[Crossref]

Huang, G.

H. S. Margolis, G. P. Barwood, G. Huang, H. A. Klein, S. N. Lea, K. Szymaniec, and P. Gill, “Hertz-level measurement of the optical clock frequency in a single ion” Science 306, 1355–1358 (2004).
[Crossref] [PubMed]

Ido, T.

T. Ido and H. Katori, “Recoil-free spectroscopy of neutral Sr atoms in the Lamb-Dicke regime,” Phys. Rev. Lett. 91, 053001 (2003).
[Crossref] [PubMed]

H. Katori, T. Ido, and M. Kuwata-Gonokami, “Optimal design of dipole potentials for efficient loading of Sr atoms,” J. Phys. Soc. Jpn. 68, 2479–2482 (1999).
[Crossref]

Inaba, H.

Ishikawa, J.

F.-L. Hong, S. A. Diddams, R. Guo, Z.-Y. Bi, A. Onae, H. Inaba, J. Ishikawa, K. Okumura, D. Katsuragi, J. Hirata, T. Shimizu, T. Kurosu, Y. Koga, and H. Matsumoto, “Frequency measurements and hyperfine structure of the R(85)33-0 transition of molecular iodine with a femtosecond optical comb,” J. Opt. Soc. Am. B 21, 88–95 (2004).
[Crossref]

F.-L. Hong, J. Ishikawa, Y. Zhang, R. Guo, A. Onae, and H. Matsumoto, “Frequency reproducibility of an iodine-stabilized Nd:YAG laser at 532 nm,” Opt. Commun. 235, 377–385 (2004).
[Crossref]

F. -L. Hong, J. Ishikawa, J. Yoda, J. Ye, L.-S. Ma, and J. L. Hall, “Frequency comparison of 127I2-stabilized Nd:YAG lasers,” IEEE Trans. Instrum. Meas. 48, 532–536 (1999).
[Crossref]

Itano, W. M.

S. A. Diddams, Th. Udem, J. C. Bergquist, E. A. Curtis, R. E. Drullinger, L. Hollberg, W. M. Itano, W. D. Lee, C. W. Oates, K. R. Vogel, and D. J. Wineland, “An optical clock based on a single trapped 199Hg+ ion,” Science 293, 825–828 (2001).
[Crossref] [PubMed]

Jiang, J.

Jones, D. J.

J. Ye, J.-L. Peng, R. J. Jones, K. W. Holman, J. L. Hall, D. J. Jones, S. A. Diddams, J. Kitching, S. Bize, J. C. Bergquist, L. W. Hollberg, L. Robertsson, and L.-S. Ma, “Delivery of high-stability optical and microwave frequency standards over an optical fiber network,” J. Opt. Soc. Am. B 20, 1459–1467 (2003).
[Crossref]

S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, Th. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84, 5102–5105 (2000).
[Crossref] [PubMed]

D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, and S. T. Cundiff, “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,” Science 288, 635–639 (2000).
[Crossref] [PubMed]

Jones, R. J.

Karshenboim, S. G.

E. Peik, B. Lipphardt, H. Schnatz, T. Schneider, Chr. Tamm, and S. G. Karshenboim, “Limit on the present temporal variation of the fine structure constant,” Phys. Rev. Lett. 93, 170801 (2004).
[Crossref] [PubMed]

Katori, H.

M. Takamoto, F.-L. Hong, R. Higashi, and H. Katori, “An optical lattice clock,” Nature 435, 321–324 (2005).
[Crossref] [PubMed]

H. Katori, M. Takamoto, V. G. Pal’chikov, and V. D. Ovsiannikov, “Ultrastable optical clock with neutral atoms in an engineered light shift trap,” Phys. Rev. Lett. 91, 173005 (2003).
[Crossref] [PubMed]

T. Ido and H. Katori, “Recoil-free spectroscopy of neutral Sr atoms in the Lamb-Dicke regime,” Phys. Rev. Lett. 91, 053001 (2003).
[Crossref] [PubMed]

M. Takamoto and H. Katori, “Spectroscopy of the 1S0 - 3P0 clock transition of 87Sr in an optical lattice,” Phys. Rev. Lett. 91, 223001 (2003).
[Crossref] [PubMed]

H. Katori, T. Ido, and M. Kuwata-Gonokami, “Optimal design of dipole potentials for efficient loading of Sr atoms,” J. Phys. Soc. Jpn. 68, 2479–2482 (1999).
[Crossref]

H. Katori, “Spectroscopy of strontium atoms in the Lamb-Dicke confinement,” in Proceedings of the 6th Symposium on Frequency Standards and Metrology, P. Gill, ed. (World Scientific, Singapore, 2002), pp. 323–330.
[Crossref]

Katsuragi, D.

Kitching, J.

Klein, H. A.

H. S. Margolis, G. P. Barwood, G. Huang, H. A. Klein, S. N. Lea, K. Szymaniec, and P. Gill, “Hertz-level measurement of the optical clock frequency in a single ion” Science 306, 1355–1358 (2004).
[Crossref] [PubMed]

Klepczynski, W. J.

W. Lewandowski, J. Azoubib, and W. J. Klepczynski, “GPS: Primary tool for time transfer,” Proc. IEEE 87, 163–172 (1999).
[Crossref]

Knight, J. C.

J. C. Knight, “Photonic crystal fiber,” Nature 424, 847–851 (2003).
[Crossref] [PubMed]

Koga, Y.

Kolker, D.

I. Courtillot, a. Quessada, R. P. Kovacich, A. Brusch, D. Kolker, J.-J. Zondy, G. D. Rovera, and R. Lemonde, “Clock transition for a future optical frequency standard with trapped atoms” Phys. Rev. A 68, 030501 (2003).
[Crossref]

Kovacich, R. P.

I. Courtillot, a. Quessada, R. P. Kovacich, A. Brusch, D. Kolker, J.-J. Zondy, G. D. Rovera, and R. Lemonde, “Clock transition for a future optical frequency standard with trapped atoms” Phys. Rev. A 68, 030501 (2003).
[Crossref]

Kurosu, T.

Kuwata-Gonokami, M.

H. Katori, T. Ido, and M. Kuwata-Gonokami, “Optimal design of dipole potentials for efficient loading of Sr atoms,” J. Phys. Soc. Jpn. 68, 2479–2482 (1999).
[Crossref]

Larson, K. M.

K. M. Larson and J. Levine, “Carrier-phase time transfer,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr. 46, 1001–1012 (1999).
[Crossref]

Lea, S. N.

H. S. Margolis, G. P. Barwood, G. Huang, H. A. Klein, S. N. Lea, K. Szymaniec, and P. Gill, “Hertz-level measurement of the optical clock frequency in a single ion” Science 306, 1355–1358 (2004).
[Crossref] [PubMed]

Lee, W. D.

S. A. Diddams, Th. Udem, J. C. Bergquist, E. A. Curtis, R. E. Drullinger, L. Hollberg, W. M. Itano, W. D. Lee, C. W. Oates, K. R. Vogel, and D. J. Wineland, “An optical clock based on a single trapped 199Hg+ ion,” Science 293, 825–828 (2001).
[Crossref] [PubMed]

Lemonde, R.

I. Courtillot, a. Quessada, R. P. Kovacich, A. Brusch, D. Kolker, J.-J. Zondy, G. D. Rovera, and R. Lemonde, “Clock transition for a future optical frequency standard with trapped atoms” Phys. Rev. A 68, 030501 (2003).
[Crossref]

Levine, J.

K. M. Larson and J. Levine, “Carrier-phase time transfer,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr. 46, 1001–1012 (1999).
[Crossref]

Lewandowski, W.

W. Lewandowski, J. Azoubib, and W. J. Klepczynski, “GPS: Primary tool for time transfer,” Proc. IEEE 87, 163–172 (1999).
[Crossref]

Lipphardt, B.

E. Peik, B. Lipphardt, H. Schnatz, T. Schneider, Chr. Tamm, and S. G. Karshenboim, “Limit on the present temporal variation of the fine structure constant,” Phys. Rev. Lett. 93, 170801 (2004).
[Crossref] [PubMed]

Ma, L.-S.

L.-S. Ma, Z. Bi, A. Bartels, L. Robertsson, M. Zucco, R. S. Windeler, G. Wilpers, C. Oates, L. Hollberg, and S. A. Diddams, “Optical frequency synthesis and comparison with uncertainty at the 10-19 level,” Science 303, 1843–1845 (2004).
[Crossref] [PubMed]

J. Ye, J.-L. Peng, R. J. Jones, K. W. Holman, J. L. Hall, D. J. Jones, S. A. Diddams, J. Kitching, S. Bize, J. C. Bergquist, L. W. Hollberg, L. Robertsson, and L.-S. Ma, “Delivery of high-stability optical and microwave frequency standards over an optical fiber network,” J. Opt. Soc. Am. B 20, 1459–1467 (2003).
[Crossref]

F. -L. Hong, J. Ishikawa, J. Yoda, J. Ye, L.-S. Ma, and J. L. Hall, “Frequency comparison of 127I2-stabilized Nd:YAG lasers,” IEEE Trans. Instrum. Meas. 48, 532–536 (1999).
[Crossref]

Margolis, H. S.

H. S. Margolis, G. P. Barwood, G. Huang, H. A. Klein, S. N. Lea, K. Szymaniec, and P. Gill, “Hertz-level measurement of the optical clock frequency in a single ion” Science 306, 1355–1358 (2004).
[Crossref] [PubMed]

Marion, H.

F. Pereira Dos Santos, H. Marion, S. Bize, Y. Sortais, A. Clairon, and C. Salomon, “Controlling the cold collision shift in high precision atomic interferometry,” Phys. Rev. Lett. 89, 233004 (2002).
[Crossref] [PubMed]

Matsumoto, H.

Minoshima, K.

Nakagawa, K.

Oates, C.

L.-S. Ma, Z. Bi, A. Bartels, L. Robertsson, M. Zucco, R. S. Windeler, G. Wilpers, C. Oates, L. Hollberg, and S. A. Diddams, “Optical frequency synthesis and comparison with uncertainty at the 10-19 level,” Science 303, 1843–1845 (2004).
[Crossref] [PubMed]

Oates, C. W.

Okumura, K.

Onae, A.

Oskay, W. H.

Ovsiannikov, V. D.

H. Katori, M. Takamoto, V. G. Pal’chikov, and V. D. Ovsiannikov, “Ultrastable optical clock with neutral atoms in an engineered light shift trap,” Phys. Rev. Lett. 91, 173005 (2003).
[Crossref] [PubMed]

Pal’chikov, V. G.

H. Katori, M. Takamoto, V. G. Pal’chikov, and V. D. Ovsiannikov, “Ultrastable optical clock with neutral atoms in an engineered light shift trap,” Phys. Rev. Lett. 91, 173005 (2003).
[Crossref] [PubMed]

Peik, E.

E. Peik, B. Lipphardt, H. Schnatz, T. Schneider, Chr. Tamm, and S. G. Karshenboim, “Limit on the present temporal variation of the fine structure constant,” Phys. Rev. Lett. 93, 170801 (2004).
[Crossref] [PubMed]

Peng, J. L.

F. Ruschewitz, J. L. Peng, H. Hinderthur, N. Schaffrath, K. Sengstock, and W. Ertmer, “Sub-kilohertz optical spectroscopy with a time domain atom interferometer,” Phys. Rev. Lett. 80, 3173–3176 (1998).
[Crossref]

Peng, J.-L.

Pereira Dos Santos, F.

F. Pereira Dos Santos, H. Marion, S. Bize, Y. Sortais, A. Clairon, and C. Salomon, “Controlling the cold collision shift in high precision atomic interferometry,” Phys. Rev. Lett. 89, 233004 (2002).
[Crossref] [PubMed]

Quessada, a.

I. Courtillot, a. Quessada, R. P. Kovacich, A. Brusch, D. Kolker, J.-J. Zondy, G. D. Rovera, and R. Lemonde, “Clock transition for a future optical frequency standard with trapped atoms” Phys. Rev. A 68, 030501 (2003).
[Crossref]

Quinn, T. J.

T. J. Quinn, “Practical realization of the definition of the metre, including recommended radiations of the optical frequency standards (2001),” Metrologia 40, 103–133 (2003).
[Crossref]

Ranka, J. K.

D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, and S. T. Cundiff, “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,” Science 288, 635–639 (2000).
[Crossref] [PubMed]

S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, Th. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84, 5102–5105 (2000).
[Crossref] [PubMed]

Reichert, J.

Th. Udem, J. Reichert, R. Holzwarth, and T. W. Hänsch, “Absolute optical frequency measurement of the cesium D1 line with a mode-locked laser,” Phys. Rev. Lett. 82, 3568–3571 (1999).
[Crossref]

Riehle, F.

G. Wilpers, T. Binnewies, C. Degenhardt, U. Sterr, J. Helmcke, and F. Riehle, “Optical clock with ultracold neutral atoms,” Phys. Rev. Lett. 89, 230801 (2002).
[Crossref] [PubMed]

Robertsson, L.

L.-S. Ma, Z. Bi, A. Bartels, L. Robertsson, M. Zucco, R. S. Windeler, G. Wilpers, C. Oates, L. Hollberg, and S. A. Diddams, “Optical frequency synthesis and comparison with uncertainty at the 10-19 level,” Science 303, 1843–1845 (2004).
[Crossref] [PubMed]

J. Ye, J.-L. Peng, R. J. Jones, K. W. Holman, J. L. Hall, D. J. Jones, S. A. Diddams, J. Kitching, S. Bize, J. C. Bergquist, L. W. Hollberg, L. Robertsson, and L.-S. Ma, “Delivery of high-stability optical and microwave frequency standards over an optical fiber network,” J. Opt. Soc. Am. B 20, 1459–1467 (2003).
[Crossref]

Rovera, G. D.

I. Courtillot, a. Quessada, R. P. Kovacich, A. Brusch, D. Kolker, J.-J. Zondy, G. D. Rovera, and R. Lemonde, “Clock transition for a future optical frequency standard with trapped atoms” Phys. Rev. A 68, 030501 (2003).
[Crossref]

Ruschewitz, F.

F. Ruschewitz, J. L. Peng, H. Hinderthur, N. Schaffrath, K. Sengstock, and W. Ertmer, “Sub-kilohertz optical spectroscopy with a time domain atom interferometer,” Phys. Rev. Lett. 80, 3173–3176 (1998).
[Crossref]

Salomon, C.

F. Pereira Dos Santos, H. Marion, S. Bize, Y. Sortais, A. Clairon, and C. Salomon, “Controlling the cold collision shift in high precision atomic interferometry,” Phys. Rev. Lett. 89, 233004 (2002).
[Crossref] [PubMed]

Schaffrath, N.

F. Ruschewitz, J. L. Peng, H. Hinderthur, N. Schaffrath, K. Sengstock, and W. Ertmer, “Sub-kilohertz optical spectroscopy with a time domain atom interferometer,” Phys. Rev. Lett. 80, 3173–3176 (1998).
[Crossref]

Schibli, T. R.

Schnatz, H.

E. Peik, B. Lipphardt, H. Schnatz, T. Schneider, Chr. Tamm, and S. G. Karshenboim, “Limit on the present temporal variation of the fine structure constant,” Phys. Rev. Lett. 93, 170801 (2004).
[Crossref] [PubMed]

Schneider, T.

E. Peik, B. Lipphardt, H. Schnatz, T. Schneider, Chr. Tamm, and S. G. Karshenboim, “Limit on the present temporal variation of the fine structure constant,” Phys. Rev. Lett. 93, 170801 (2004).
[Crossref] [PubMed]

Sengstock, K.

F. Ruschewitz, J. L. Peng, H. Hinderthur, N. Schaffrath, K. Sengstock, and W. Ertmer, “Sub-kilohertz optical spectroscopy with a time domain atom interferometer,” Phys. Rev. Lett. 80, 3173–3176 (1998).
[Crossref]

Shimizu, T.

Sortais, Y.

F. Pereira Dos Santos, H. Marion, S. Bize, Y. Sortais, A. Clairon, and C. Salomon, “Controlling the cold collision shift in high precision atomic interferometry,” Phys. Rev. Lett. 89, 233004 (2002).
[Crossref] [PubMed]

Stentz, A.

D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, and S. T. Cundiff, “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,” Science 288, 635–639 (2000).
[Crossref] [PubMed]

Sterr, U.

G. Wilpers, T. Binnewies, C. Degenhardt, U. Sterr, J. Helmcke, and F. Riehle, “Optical clock with ultracold neutral atoms,” Phys. Rev. Lett. 89, 230801 (2002).
[Crossref] [PubMed]

Szymaniec, K.

H. S. Margolis, G. P. Barwood, G. Huang, H. A. Klein, S. N. Lea, K. Szymaniec, and P. Gill, “Hertz-level measurement of the optical clock frequency in a single ion” Science 306, 1355–1358 (2004).
[Crossref] [PubMed]

Takamoto, M.

M. Takamoto, F.-L. Hong, R. Higashi, and H. Katori, “An optical lattice clock,” Nature 435, 321–324 (2005).
[Crossref] [PubMed]

H. Katori, M. Takamoto, V. G. Pal’chikov, and V. D. Ovsiannikov, “Ultrastable optical clock with neutral atoms in an engineered light shift trap,” Phys. Rev. Lett. 91, 173005 (2003).
[Crossref] [PubMed]

M. Takamoto and H. Katori, “Spectroscopy of the 1S0 - 3P0 clock transition of 87Sr in an optical lattice,” Phys. Rev. Lett. 91, 223001 (2003).
[Crossref] [PubMed]

Tamm, Chr.

E. Peik, B. Lipphardt, H. Schnatz, T. Schneider, Chr. Tamm, and S. G. Karshenboim, “Limit on the present temporal variation of the fine structure constant,” Phys. Rev. Lett. 93, 170801 (2004).
[Crossref] [PubMed]

Udem, Th.

S. A. Diddams, Th. Udem, J. C. Bergquist, E. A. Curtis, R. E. Drullinger, L. Hollberg, W. M. Itano, W. D. Lee, C. W. Oates, K. R. Vogel, and D. J. Wineland, “An optical clock based on a single trapped 199Hg+ ion,” Science 293, 825–828 (2001).
[Crossref] [PubMed]

S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, Th. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84, 5102–5105 (2000).
[Crossref] [PubMed]

Th. Udem, J. Reichert, R. Holzwarth, and T. W. Hänsch, “Absolute optical frequency measurement of the cesium D1 line with a mode-locked laser,” Phys. Rev. Lett. 82, 3568–3571 (1999).
[Crossref]

Vogel, K. R.

S. A. Diddams, Th. Udem, J. C. Bergquist, E. A. Curtis, R. E. Drullinger, L. Hollberg, W. M. Itano, W. D. Lee, C. W. Oates, K. R. Vogel, and D. J. Wineland, “An optical clock based on a single trapped 199Hg+ ion,” Science 293, 825–828 (2001).
[Crossref] [PubMed]

Wilpers, G.

A. Bartels, S. A. Diddams, C. W. Oates, G. Wilpers, J. C. Bergquist, W. H. Oskay, and L. Hollberg, “Femtosecond-laser-based synthesis of ultrastable microwave signals from optical frequency references,” Opt. Lett. 30, 667–669 (2005).
[Crossref] [PubMed]

L.-S. Ma, Z. Bi, A. Bartels, L. Robertsson, M. Zucco, R. S. Windeler, G. Wilpers, C. Oates, L. Hollberg, and S. A. Diddams, “Optical frequency synthesis and comparison with uncertainty at the 10-19 level,” Science 303, 1843–1845 (2004).
[Crossref] [PubMed]

G. Wilpers, T. Binnewies, C. Degenhardt, U. Sterr, J. Helmcke, and F. Riehle, “Optical clock with ultracold neutral atoms,” Phys. Rev. Lett. 89, 230801 (2002).
[Crossref] [PubMed]

Windeler, R. S.

L.-S. Ma, Z. Bi, A. Bartels, L. Robertsson, M. Zucco, R. S. Windeler, G. Wilpers, C. Oates, L. Hollberg, and S. A. Diddams, “Optical frequency synthesis and comparison with uncertainty at the 10-19 level,” Science 303, 1843–1845 (2004).
[Crossref] [PubMed]

S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, Th. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84, 5102–5105 (2000).
[Crossref] [PubMed]

D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, and S. T. Cundiff, “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,” Science 288, 635–639 (2000).
[Crossref] [PubMed]

Wineland, D. J.

S. A. Diddams, Th. Udem, J. C. Bergquist, E. A. Curtis, R. E. Drullinger, L. Hollberg, W. M. Itano, W. D. Lee, C. W. Oates, K. R. Vogel, and D. J. Wineland, “An optical clock based on a single trapped 199Hg+ ion,” Science 293, 825–828 (2001).
[Crossref] [PubMed]

Ye, J.

J. Ye, J.-L. Peng, R. J. Jones, K. W. Holman, J. L. Hall, D. J. Jones, S. A. Diddams, J. Kitching, S. Bize, J. C. Bergquist, L. W. Hollberg, L. Robertsson, and L.-S. Ma, “Delivery of high-stability optical and microwave frequency standards over an optical fiber network,” J. Opt. Soc. Am. B 20, 1459–1467 (2003).
[Crossref]

S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, Th. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84, 5102–5105 (2000).
[Crossref] [PubMed]

F. -L. Hong, J. Ishikawa, J. Yoda, J. Ye, L.-S. Ma, and J. L. Hall, “Frequency comparison of 127I2-stabilized Nd:YAG lasers,” IEEE Trans. Instrum. Meas. 48, 532–536 (1999).
[Crossref]

Yoda, J.

F. -L. Hong, J. Ishikawa, J. Yoda, J. Ye, L.-S. Ma, and J. L. Hall, “Frequency comparison of 127I2-stabilized Nd:YAG lasers,” IEEE Trans. Instrum. Meas. 48, 532–536 (1999).
[Crossref]

Zhang, Y.

F.-L. Hong, J. Ishikawa, Y. Zhang, R. Guo, A. Onae, and H. Matsumoto, “Frequency reproducibility of an iodine-stabilized Nd:YAG laser at 532 nm,” Opt. Commun. 235, 377–385 (2004).
[Crossref]

Zondy, J.-J.

I. Courtillot, a. Quessada, R. P. Kovacich, A. Brusch, D. Kolker, J.-J. Zondy, G. D. Rovera, and R. Lemonde, “Clock transition for a future optical frequency standard with trapped atoms” Phys. Rev. A 68, 030501 (2003).
[Crossref]

Zucco, M.

L.-S. Ma, Z. Bi, A. Bartels, L. Robertsson, M. Zucco, R. S. Windeler, G. Wilpers, C. Oates, L. Hollberg, and S. A. Diddams, “Optical frequency synthesis and comparison with uncertainty at the 10-19 level,” Science 303, 1843–1845 (2004).
[Crossref] [PubMed]

IEEE Trans. Instrum. Meas. (1)

F. -L. Hong, J. Ishikawa, J. Yoda, J. Ye, L.-S. Ma, and J. L. Hall, “Frequency comparison of 127I2-stabilized Nd:YAG lasers,” IEEE Trans. Instrum. Meas. 48, 532–536 (1999).
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H. Katori, T. Ido, and M. Kuwata-Gonokami, “Optimal design of dipole potentials for efficient loading of Sr atoms,” J. Phys. Soc. Jpn. 68, 2479–2482 (1999).
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T. J. Quinn, “Practical realization of the definition of the metre, including recommended radiations of the optical frequency standards (2001),” Metrologia 40, 103–133 (2003).
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J. C. Knight, “Photonic crystal fiber,” Nature 424, 847–851 (2003).
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F.-L. Hong, J. Ishikawa, Y. Zhang, R. Guo, A. Onae, and H. Matsumoto, “Frequency reproducibility of an iodine-stabilized Nd:YAG laser at 532 nm,” Opt. Commun. 235, 377–385 (2004).
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H. Katori, M. Takamoto, V. G. Pal’chikov, and V. D. Ovsiannikov, “Ultrastable optical clock with neutral atoms in an engineered light shift trap,” Phys. Rev. Lett. 91, 173005 (2003).
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Figures (7)

Fig. 1.
Fig. 1. Schematic diagram of the experimental setup. AOM, acousto-optic modulator; SYN, synthesizer; PLL, phase-lock loop; PCF, photonic crystal fiber; DM, dichroic mirror; D, detector; PBS, polarization beam splitter; AMP, amplifier; SMF, single-mode fiber; PZT, piezoelectric transducer; GPS, global positioning system.
Fig. 2.
Fig. 2. Allan standard deviation of the commercial Cs clock (black solid curve), an H-maser (dashed curve), the iodine-stabilized Nd:YAG laser (green solid curve), the Sr lattice clock (dotted line) and the measured beat frequency between the Nd:YAG laser and the clock laser (curve with filled circles).
Fig. 3.
Fig. 3. (a) Measured AOM offset frequency δ(t). (b) Measured clock-laser frequency f c(t) using the comb referenced to the commercially available Cs clock. (c) Absolute frequency of the clock transition ν 0(k) deduced from δ(t) and f c(t). (d) Histogram of ν 0(k) with a Gaussian fitting.
Fig. 4.
Fig. 4. Spectra of the original comb of the Ti:sapphire laser (dashed curve) and the broadened comb after the photonic crystal fiber (solid curve). For the frequency measurement, beat note frequencies were observed between the clock laser and the broadened comb at 698 nm, and between the Nd:YAG laser and the broadened comb at 1064 nm.
Fig. 5.
Fig. 5. (a) Measured AOM offset frequency δ(t). (b) Measured clock laser frequency f c(t) using the comb locked to the iodine-stabilized Nd:YAG laser. (c) Absolute frequency of the clock transition ν 0(k) deduced from δ(t) and f c(t). (d) Histogram of ν 0(k) with a Gaussian fitting.
Fig. 6.
Fig. 6. Absolute frequency measurement of the 1 S 0 - 3 P 0 transition of 87Sr atoms in an optical lattice. Data shown in this figure do not include systematic corrections.
Fig. 7.
Fig. 7. Time interval between the 1 p.p.s. signals from the Cs clock and the GPS-DO, started with the Cs clock signal and stopped with the GPS-DO signal. The slope α indicates the frequency difference between the two clocks.

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