Kensuke Matsubara

Frequency Measurement of the Optical Clock Transition of 40Ca+ Ions with an Uncertainty of 10-14 Level

The absolute frequency of the 4 2S1/2–3 2D5/2 optical clock transition of 40Ca+ ions has been measured for the first time with respect to the Systeme International (SI) second. A single 40Ca+ ion is laser-cooled in a small ion trap and the transition frequency is measured as the average of two symmetrical Zeeman components. The frequency is determined to be 411 042 129 776 385 (±18) Hz from 48 measurements.

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].

Precise frequency-drift measurement of extended-cavity diode laser stabilized with scanning transfer cavity

A simple and effective method for stabilizing laser frequencies using scanning transfer cavities and a stabilized helium–neon laser was applied to extended-cavity diode lasers near 866 nm and 397 nm. The frequency drift of the stabilized 866 nm laser measured using an optical frequency synthesizer was smaller than 200 kHz for 1 h. The square root of the Allan variance was 1×10-10 at an averaging time of 103 s.

Stable Operation of Femtosecond Laser Frequency Combs with Uncertainty at the 10 17 Level toward Optical Frequency Standards

We have developed two femtosecond laser frequency combs (FLFCs) employing different designs and no external spectral broadening devices. The FLFCs referenced to a microwave standard were directly compared and confirmed to have a relative fractional uncertainty of 8.1 ×10-17 mainly due to imperfection in our comparison technique. They have also achieved uninterrupted daily operation typically more than 8 hours. They are found to be suitable for the optical standards from the absolute frequency measurement for a clock transition of 40Ca+ ions, that was recently performed in National Institute of Information and Communications Technology.

Narrow-Line and Frequency Tunable Diode Laser System for S–D Transition of Ca+ Ions

A new type of diode laser is used for the development of an extremely narrow linewidth and frequency-tunable clock laser system for a Ca+ ion optical frequency standard. The required reduction of linewidth was achieved by locking the laser to an ultrahigh-finesse ultralow-expansion glass (ULE) reference cavity. The long-term frequency drift is reduced by stabilizing a cavity temperature at the point that the thermal expansion coefficient of the ULE is zero. As a result, the laser linewidth is decreased to lower than 25 Hz and the long-term drift is less than 0.025 Hz/s. The other laser offset locked to the stabilized laser is continuously frequency tunable in the entire free spectral range of the reference cavity without degradation of stability and linewidth. This laser system is actually applied to optical frequency standard development and used for the observation of the 2S1/2–2D5/2 transition of Ca+ ions.

Preparing Single Ions in m=0 States in the Lamb-Dicke Regime

When a trapped ion is used for optical frequency standards, it is usually cooled by laser radiation and then pumped to the m=0 state. However, simple laser cooling cannot be applied to some ion species due to the formation of the dark state. This paper shows theoretically that such ions can also be cooled to the Doppler limit and can be prepared in the m=0 state by three laser beams with different polarizations and frequency detunings.

Development of femtosecond optical frequency combs for optical frequency standards in NICT

Optical frequency standards are being developed worldwide to lead a new definition of the unit of time. We are developing broadband optical frequency combs, which aims to count the laser frequency highly stabilized to the resonance of atomic reference and generate a rf frequency standard directly converted from the optical frequency. The developed frequency combs was successfully operated with a measurement accuracy of 3×10-14 at the averaging time of 1 s. From the primary demonstration, it was confirmed to be available for the frequency stability measurement of a clock laser used in an optical frequency standards. The frequency combs will be key components for the development of optical atomic clock.

Study for a 43Ca+ Optical Frequency Standard

The authors proceed with a research and development of an optical frequency standard based on the ²S_1/2 - ²D_5/2 transition of ⁴³Ca⁺ in a RF trap. For this purpose, motional sidebands of a single ⁴⁰Ca ⁺ ion were observed and a stable laser-diode system for spectroscopy of ⁴³Ca⁺ are being developed

Frequency measurement of the clock transition of an indium ion sympathetically-cooled in a linear trap

We report frequency measurement of the clock transition in an 115In+ ion sympathetically-cooled with Ca+ ions in a linear rf trap. The Ca+ ions are used as a probe of the external electromagnetic field and as the coolant for preparing the cold In+. The frequency is determined to be 1 267 402 452 901 049.9 (6.9) Hz by averaging 36 measurements using an optical frequency comb referenced to the frequency standards located in the same site.

Submilliwatt Continuous-Wave Coherent Light Generation near 214.5 nm by Two-Stage Frequency Doubling of a Diode Laser

We generated 0.64 mW of continuous-wave coherent light near 214.5 nm by using an all-solid-state light source. In this source, IR light from a master-laser and power-amplifier system based on diode lasers was frequency-quadrupled in two successive frequency-doubling stages. A continuous frequency-scanning range of more than 7 GHz was observed in the 214–215 nm wavelength region.