Hidekazu Hachisu

Direct Comparison of Distant Optical Lattice Clocks at the 10-16 Uncertainty

Fiber-based remote comparison of 87Sr lattice clocks in 24 km distant laboratories is demonstrated. The instability of the comparison reaches 5×10-16 over an averaging time of 1000 s, which is two orders of magnitude shorter than that of conventional satellite links and is limited by the instabilities of the optical clocks. By correcting the systematic shifts that are predominated by the differential gravitational redshift, the residual fractional difference is found to be (1.0±7.3)×10-16, confirming the coincidence between the two clocks. The accurate and speedy comparison of distant optical clocks paves the way for a future optical redefinition of the second.

All-optical link for direct comparison of distant optical clocks

We developed an all-optical link system for making remote comparisons of two distant ultra-stable optical clocks. An optical carrier transfer system based on a fiber interferometer was employed to compensate the phase noise accumulated during the propagation through a fiber link. Transfer stabilities of 2 × 10(-15) at 1 second and 4 × 10(-18) at 1000 seconds were achieved in a 90-km link. An active polarization control system was additionally introduced to maintain the transmitted light in an adequate polarization, and consequently, a stable and reliable comparison was accomplished. The instabilities of the all-optical link system, including those of the erbium doped fiber amplifiers (EDFAs) which are free from phase-noise compensation, were below 2 × 10(-15) at 1 second and 7 × 10(-17) at 1000 seconds. The system was available for the direct comparison of two distant (87)Sr lattice clocks via an urban fiber link of 60 km. This technique will be essential for the measuring the reproducibility of optical frequency standards.

Direct comparison of a Ca+ single-ion clock against a Sr lattice clock to verify the absolute frequency measurement.

Optical frequency comparison of the (40)Ca(+) clock transition ν(Ca)((2)S(1/2-)(2D(5/2), 729 nm) against the (87)Sr optical lattice clock transition ν(Sr) ((1)S(0)-(3)P(0), 698 nm) has resulted in a frequency ratio ν(Ca) / ν(Sr) = 0.957 631 202 358 049 9(2 3). The rapid nature of optical comparison allowed the statistical uncertainty of frequency ratio ν(Ca) / ν(Sr) to reach 1 × 10(-15) in 1000s and yielded a value consistent with that calculated from separate absolute frequency measurements of ν(Ca) using the International Atomic Time (TAI) link. The total uncertainty of the frequency ratio using optical comparison (free from microwave link uncertainties) is smaller than that obtained using absolute frequency measurement, demonstrating the advantage of optical frequency evaluation. We note that the absolute frequency of (40)Ca(+) we measure deviates from other published values by more than three times our measurement uncertainty.Optical frequency comparison of the 40Ca+ clock transition \nu_{Ca} (2S1/2-2D5/2, 729nm) against the 87Sr optical lattice clock transition \nu_{Sr}(1S0-3P0, 698nm) has resulted in a frequency ratio \nu_{Ca} / \nu_{Sr} = 0.957 631 202 358 049 9(2 3). The rapid nature of optical comparison allowed the statistical uncertainty of frequency ratio \nu_{Ca} / \nu_{Sr} to reach 1x10-15 in only 1000s and yielded a value consistent with that calculated from separate absolute frequency measurements of \nu_{Ca} using the International Atomic Time (TAI) link. The total uncertainty of the frequency ratio using optical comparison (free from microwave link uncertainties) is smaller than that obtained using absolute frequency measurement, demonstrating the advantage of optical frequency evaluation. We report the absolute frequency of ^{40}Ca+ with a systematic uncertainty 14 times smaller than our previous measurement [1].

Stability Transfer between Two Clock Lasers Operating at Different Wavelengths for Absolute Frequency Measurement of Clock Transition in 87Sr

We demonstrated transferring the stability of one highly stable clock laser operating at 729 nm to another less stable laser operating at 698 nm. The two different wavelengths were bridged using an optical frequency comb. The improved stability of the clock laser at 698 nm enabled us to evaluate the systematic frequency shifts of the Sr optical lattice clock with a shorter averaging time. We determined the absolute frequency of the clock transition 1S0–3P0 in 87Sr to be 429 228 004 229 873.9 (1.4) Hz referenced to the SI second on the geoid via International Atomic Time (TAI).

Time Scales Steered by Optical Clocks

Although optical frequency standards made rapid progress these days, microwave standards are still employed as source oscillators of time scales because an oscillator free from phase jumps is a prerequisite. Currently, for high-end users such as national metrology laboratories, hydrogen masers (H-masers) have adequate balance of the stability and reliability. Thus, optical clocks may play the role of the standards to which the time scales refer to in order to adjust their scale intervals. The benefit of using optical frequency standards, in this case, would be the capability to evaluate the scale interval of the H-maser more quickly. To investigate such possibilities of “optical” steering, we evaluated the behavior of an H-maser over a few months with reference to a 87Sr lattice clock. The evaluations clearly demonstrated a stable linear drift of the H-maser frequency, indicating the capability of compensating the drift for a stable time scale. This prospect was also supported by a numerical simulation based on the record of H-maser-UTC(NICT)-UTC link.

Optical direct comparison of 87Sr optical lattice clocks using a >50 km telecommunication fiber link

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.

Months-long real-time generation of a time scale based on an optical clock

Time scales consistently provide precise time stamps and time intervals by combining atomic frequency standards with a reliable local oscillator. Optical frequency standards, however, have not been applied to the generation of time scales, although they provide superb accuracy and stability these days. Here, by steering an oscillator frequency based on the intermittent operation of a 87Sr optical lattice clock, we realized an “optically steered” time scale TA(Sr) that was continuously generated for half a year. The resultant time scale was as stable as International Atomic Time (TAI) with its accuracy at the 10−16 level. We also compared the time scale with TT(BIPM16). TT(BIPM) is computed in deferred time each January based on a weighted average of the evaluations of the frequency of TAI using primary and secondary frequency standards. The variation of the time difference TA(Sr) – TT(BIPM16) was 0.79 ns after 5 months, suggesting the compatibility of using optical clocks for time scale generation. The steady signal also demonstrated the capability to evaluate one-month mean scale intervals of TAI over all six months with comparable uncertainties to those of primary frequency standards (PFSs).

Activities of Time and Frequency Metrology at NICT: Optical and Microwave Frequency Standards and Their Remote Comparisons

The National Institute of Information and Communications Technology (NICT) provides the national frequency standard of Japan as well as the Japan Standard Time (JST). Besides that, atomic frequency standards, including cesium fountains, a calcium ion clock, a strontium (Sr) lattice clock, and an indium ion clock, are also studied. This contribution briefly summarizes the current status of these frequency standards. We also study the technology to make remote comparison of these standards against others developed in external institutes. Sr lattice clock offers a convenient platform for this purpose because nearly ten Sr-based clocks are available worldwide including the one in NICT with the systematic uncertainty of 8.6 × 10−17. In 2011, a fiber link to the University of Tokyo was established with the baseline of 24 km, whereas that with a long baseline to PTB (Physikalisch-Technische Bundesanstalt, Germany) was realized in 2013 by a two-way satellite link using the carrier phase. The former revealed the gravitational redshift caused by the elevation difference of 56 m with uncertainty of 7 m. On the other hand, the latter for the first time realized direct comparison of two optical clocks in intercontinental scale.

Frequency Measurement System of Optical Clocks Without a Flywheel Oscillator

We developed a system for the remote frequency comparison of optical clocks. The system does not require a flywheel oscillator at the remote end, making it possible to evaluate optical frequencies even in laboratories, where no stable microwave reference, such as an Rb clock, a Cs clock, or a hydrogen maser exists. The system is established by the integration of several systems: a portable carrier-phase two-way satellite frequency transfer station and a microwave signal generation system by an optical frequency comb from an optical clock. The measurement was as quick as a conventional method that employs a local microwave reference. We confirmed the system uncertainty and instability to be at the low

Developments of optical frequency standards at NICT and all-optical comparison against a remote clock in University of Tokyo using 60km fiber link

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