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Space-Time Standards laboratory in National institute of Information and Communications Technology (NICT), Japan, has various activities of time and frequency metrology. NICT generates and disseminates Japan Standard Time (JST. In addition to the headquarter in Koganei, Tokyo, we built a substation of JST in Kobe, which is in western part of Japan. Every place in Japan has a potential to be attacked by a large earthquake. The combination of two stations provides a resilience. Two optical frequency standards are being developed. One is a strontium lattice clock and the other is a single In+ trap. The lattice clock is recognized as a secondary represenataion of the Second. Various technique of the time and frequency transfer is under developing, including GNSS, TWSTFT, VLBI and fiber link. We developed the two-way carrier phase technique and developed a modem for it, which is now commercially available in Japan. VLBI does not need satellites. We can compare the clocks without depending on GNSS providers or communication satellites. These various activity will be briefly introduced.

We report on the first, to the best of our knowledge, frequency ratio measurement of an

We report on the first, to the best of our knowledge, frequency ratio measurement of an single-ion clock and a optical lattice clock. A hydrogen maser serves as a flywheel oscillator to measure the ratio by independent optical combs. From 89,000 s of measurement time, the frequency ratio is determined to be 2.952 748 749 874 863 3(23) with relative uncertainty. The measurement creates a new connection in the network of frequency ratios of optical clocks.

We report on the progress of an optical clock based on an indium ion (115In+) sympathetically cooled with a calcium ion (40Ca+) in a linear trap. In our previous work, we have measured the clock transition frequency with an uncertainty of 5× 10− 15, prompting an update of the Comite International des Poids et Mesures (CIPM) recommended value. The uncertainty was mainly limited by the evaluation of the Zeeman shift which was complicated by unresolved sublevel components. In contrast to the previous measurement, Zeeman sublevels are now separated by application of a magnetic field. Combined with optical pumping to specific Zeeman substates, the magnetic field application method successfully reduces the observed linewidth of the clock transition spectrum down to about 80 Hz full width at half maximum (FWHM). Frequency locking of the clock laser to the transition is demonstrated for the first time. The clock reaches a relative instability of 1.5× 10− 15 for an integration time of 4000 seconds.

The indium ion (115In+) is one of the candidates for the single-ion optical clock with fractional frequency uncertainties in the 10−18 level proposed by Hans Dehmelt in 1980s. One of the first attempts for the realization has been started by the research group by Herbert Walther in Garching already in the late of 1980s. The first frequency measurement was published in 2001 [1], followed by two reports in 2007 [2, 3]. Unfortunately, there have been no updates since then, and the discrepancy of about 1 kHz in the reported frequencies is left unresolved. In the meanwhile the implementation with 27Al+ demonstrated the predicted fractional uncertainties in the 10−18 level. Recent theoretical calculations suggest that similar frequency uncertainties should be achieved with In+ and that enhanced frequency stability could be implemented by use of multiple In+ ions [4]. As the first step towards these goals we report on a new frequency measurement of the clock transition of In+ [5].

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.

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.

NICT carried out the first calibration of the GPS link between Koganei and Kobe by using GPS, TWSTFT and Clock Transport (CT). We presented the result of the calibration in EFTF 2015. The differential correction of GPS link by GPS, TWSTFT and CT were 102.5, 102.1 and 104.9 ns respectively. The CT result showed a discrepancy of 2 ns. By applying the relativistic effect correction for CT, we confirmed a good agreement of the results obtained by their three techniques.

We report results of frequency stability measurements of an extended cavity diode laser (ECDL) whose frequency is stabilized by a non-evacuated scanning transfer cavity. The transfer cavity is locked to a commercial frequency stabilized helium–neon laser. Frequency stability is measured by use of an optical frequency comb. The environmental perturbations (variations of temperature, air pressure, and humidity) are also simultaneously measured. The observed frequency drift of the ECDL is well explained by environmental perturbations. An atmospheric pressure variation, which is difficult to control with a non-evacuated cavity, is mainly affected to the frequency stability. Thus we put the cavity into a simple O-ring sealed (non-evacuated) tube. With this simple O-ring sealed tube, the frequency drift is reduced by a factor of 3, and the Allan variance reaches a value of 2.4×10−10, corresponds to the frequency stability of 83 kHz, at the average time of 3000 s. Since the actual frequency drift is well estimated by simultaneous measurement of the ambient temperature, pressure, and humidity, a feed-forward compensation of frequency drifts is also feasible in order to achieve a higher frequency stability with a simple non-evacuated transfer cavity.

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.

We have laser-cooled all even isotopes of Zn+ ions confined in a linear radio-frequency ion trap, and measuredoptical isotope shifts in the 4s 2S1/2-4p 2P3/2 transition. Tunable continuous-wave coherent light near 202 nm was generated for this experiment by means of frequency conversion of light from diode and solid-state lasers. The measured isotope shifts are as follows: 66Zn+-64Zn+, 0.676(6) GHz; 68Zn+-66Zn+, 0.670(4) GHz; and 70Zn+-68Zn+, 0.568(10) GHz. In all cases, the transition line of the heavier isotope was observed at the higher frequency. The mass and the field shifts were estimated using a King plot. This is the first isotope-shift measurement in the transition involving the ground (4s 2S1/2) state of Zn+ ions.

We have developed an all-solid-state tunable continuous-wave coherent light source near 209 nm using two-stage frequency doubling of a Ti:sapphire laser. Two-stage frequency doubling was carried out with two enhancement cavities. A 209 nm output power of 100 µW was obtained with a single β-BaB2O4(BBO) crystal in the second-stage doubling. Compensation of the walk-off of the 209 nm radiation was also performed with two BBO crystals and an output power of 310 µW was obtained. The light source described here can be used satisfactorily for spectroscopy in the deep ultraviolet region.

and β-BaB2O4 crystals. The power of the generated 214.5-nm light amounts to more than 100 μW. This light source will be used for laser cooling of Cd+ ions.

This paper explains the tunable 214.5-nm continuous-wave coherent radiation generated by using an all-solidstatelight source system. The light source consists of the oscillator and amplifier system based on laser diodesand the two-stage frequency doubling system with KNbO3 and β-BaB2O4 crystals. The power of the generated 214.5-nm light amounts to more than 0.64 mW. This light source will be used for laser-cooling of Cd+ ions.

Direct comparison between a single 40Ca+ clock and an Sr lattice clock has resulted in a frequency ratio of 0.957 631 202 358 049 9(2 3). The rapid nature of optical comparison allowed the statistical uncertainty of the frequency ratio Ca+/Sr to be 1 × 10-15 in 1000 s and the value is consistent with that calculated from separate frequency measurements of Ca+ using the International Atomic Time (TAI) link. The total uncertainty of the measured ratio is smaller than that obtained at the absolute frequency measurement. The absolute frequency of Ca+ we measured deviates from other published values by more than three times our measurement uncertainty.

In a plane perpendicular to a magnetic field a collimated cesium beam was crossed at right angles by a pump laser beam. Cs2 molecules were excited to the D1Σu+ (v = 46, J = 54) level, which predissociated selectively to Cs(6s2S1/2) + Cs(6p2P3/2) atoms. The mj-resolved excitation spectra of the Cs(8s2S1/2,m′j ← 6p2P3/2,mj) transitions at a magnetic field of 1.91 kG were measured using a probe laser beam propagated either along the magnetic field or along the pump beam. The population of Cs(6p2P3/2,mj) atoms was found to be in the ratio 77 : 136 : 100 : 46 for mj = 3/2, 1/2, −1/2, and −3/2 based on an analysis of the line intensities. From the line shapes of the excitation spectra, Cs(6p2P3/2,±1/2) atoms were found to recoil predominantly parallel to the magnetic field H, and Cs(6p2P3/2,±3/2) atoms were found to recoil predominantly perpendicular to H.

High‐resolution spectra of the B1Π→X1Σ+ transition of 23Na85Rb molecule are measured by the technique of the Doppler‐free optical–optical double resonance polarization spectroscopy (OODRPS). The molecular constants of the X1Σ+(v″=5−30) levels are determined, and the potential energy curve is constructed up to v″=30 by the RKR method. The time‐resolved fluorescence intensity following the excitation to the B1Π(v′=5,J′= around 20) level is measured, and the lifetime of the B1Π(v′=5) level in collisionless limit is determined to be 17.8 ns. The absolute value of the electric dipole moment of the B1Π−X1Σ+ transition is determined to be 7.0 D in the region of 3.73 A

The geometric structures of 7-azaindole (7-AzI) and its dimer were investigated by laser-induced fluorescence (LIF). The rotationally resolved LIF spectrum (resolution: ∼0.00003 cm-1 of 7-AzI monomer shows that (i) rotational constants are A′′ 0.13082(5), B′′ = 0.05677(2), C′′ = 0.03970(1), A′ = 0.12468(5), B′ = 0.056752(2), and C′ = 0.03911(1), and (ii) the transition dipole moment direction is tilted at an angle of ± 21(5) degrees with respect to a axis. The LIF spectrum (resolution: ∼0.06 cm-1) of the nonreactive 7-AzI dimer suggests there are three possibilities for the geometric structure, having “single bonded”, “T-shaped”, and “non-planar” structures.The ionization energy of 7-AzI was determined to be 8.1171(5) eV by using pulsed-field ionization photoelectron spectroscopy (PFI-PES). Several vibronic bands of 7-AzI cation were observed and compared with those of neutral 7-AzI in the S0 and S1 states. The electron binding energies of 7-AzI cluster anions (2 ≤ n ≤ 5) were determined by photoelectron spectroscopy.

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.

Various activities of atomic frequency standards studied in National Institute of Information and Communications Technology (NICT) are briefly reviewed. After BIPM accepted the first cesium fountain clock in NICT as a reference to determine International Atomic Time (TAI), efforts to further reduce the uncertainty of collision shifts are ongoing. A second fountain clock using atomic molasses is being built to enable the operation with less atomic density. Single ion clock using calcium has been pursued for several years in NICT. The absolute frequency measured in 2008 has CIPM to adopt the Ca+ clock transition as a part of the list of radiation (LoR) to realize the meter. Sr lattice clock has started its operation last year. The absolute frequency agreed well with those obtained in other institutes. Study of stable cavities to stabilize clock lasers are also introduced.

We report on the progress toward 115In+ optical frequency standard by new approaches. The report is focused especially on the clock laser locked to a novel rectangular-shaped cavity.

We report improved frequency measurement of a single 40Ca+ optical clock towards the 10−15 uncertainty level. In contrast with our previous measurement, in which the accuracy was limited by the magnetic field fluctuation, the use of a magnetic shield significantly reduced the observed linewidth of its 2S 1/2 –2D 5/2 clock transition. From frequency comparison between the 40Ca+ clock and an 87Sr lattice clock, an Allan deviation of 7 × 10−16 at an averaging time of one thousand seconds was measured. We now measure the absolute frequency of the 40Ca+ clock transition with respect to the SI definition of the second.

At National Institute of Information and Communications Technology (NICT), we have lately developed a lattice clock using 87Sr atoms as well as a single ion clock based on 40Ca+. Besides the comparison of the two atomic standards inside the institute, all-optical fiber-link to transfer frequency standards to the University of Tokyo (UT) has been developed to evaluate the uncertainty and stability of our standards by referring the state-of-the-art lattice clock locating in UT. The initial evaluations have resulted in the relative stability of 10−16 level, clearly showing the differential frequency of 4–5Hz mainly caused by the 45m difference of the elevation.

The sub‐Doppler high resolution excitation spectrum of NaRb was measured by selectively monitoring the fluorescence intensity from perturbed levels to the (1)3Σ+ lower state. Many transitions were observed to the (2)3Σ+ state perturbed by B 1Π. Energy shifts of the perturbed levels were analyzed and the spin–orbit coupling constant between the (2)3Σ+ and B 1Π states was determined, together with molecular constants of each state. Fully resolved hyperfine splittings, induced by the coupling of electron spin with both 23Na and 85Rb nuclear spins, were observed in the transitions. By analysis of the observed splittings, the mechanism of perturbation was confirmed as due to Fermi contact interaction and hyperfine constants of the 23Na85Rb (2)3Σ+ state were determined. The ratio of electron spin densities at 23Na and 85Rb in the (2)3Σ+ state is estimated to be 0.84:0.16.

A 729 nm diode laser is used for the development of an extremely narrow line 729 nm clock laser for a quadrupole transition of a single 40Ca+ ion. The laser linewidth is decreased to 2 Hz, evaluated by a heterodyne beatnote between two narrow linewidth lasers that are individually stabilized to two independent ULE reference cavity systems. The ageing drift of the ULE cavity is observed over two years. It shows a decreasing with an exponential function. The long-term frequency drift is successfully minimized to 0.03 Hz/s by keeping the cavity at the zero expansion coefficient temperature. The Allan deviation is less than 5×10-15 at the averaging time of 1 s~10 s. To observe the clock transition, the clock laser light is transmitted to Ca+ ion vacuum chamber by a polarization-maintaining single-mode optical fiber of 40 m length. Fiber optical phase noise has been canceled. In order to compare with other optical frequency standards or rf frequency standards, the clock laser frequency is locked to the 2S1/2-2D5/2 electric quadrupole transition of the single trapped 40Ca+ ion. Stable locking has been observed over averaging times of 100 s. The Allan deviation approaches to 5×10-15 over averaging times of 1000 s. We are developing an ultrastable laser based on a much higher finesses and vibration insensitive cavity [8]. The ultimate aim is building a clock laser with a subherz linewidth and Allan deviation of

We developed an optical frequency standard with the 4 2 S 1/2 -3 2 D 5/2 electric quadrupole transition of 40 Ca + ions. Its absolute transition frequency is 411 042 129 776 390(±7) Hz. The accuracy is limited by the electric quadrupole shift and the ambient magnetic field fluctuation. To determine the absolute transition frequency with a better accuracy, we have observed two pairs of the symmetrically-splitting Zeeman components and measured the transition frequency corrected for the electric quadrupole shift. In addition, we are developing a magnetic-shielded ion-trap chamber to suppress the transition-line broadening caused by the magnetic field fluctuation.

We developed an optical frequency standard using 40Ca+ with the 4 2S 1/2 −3 2D5/2 electric quadrupole transition. Its absolute transition frequency is 411 042 129 776 390(7) Hz. The uncertainty is limited by the electric quadrupole shift and ambient magnetic field fluctuation. To determine the absolute transition frequency with a better uncertainty, we have observed two pairs of the symmetrically-splitting Zeeman components and measured the transition frequency which is corrected for the electric quadrupole shift. It will be used to determine the absolute transition frequency with respect to the SI base units of time and length. In addition, we are developing a magnetic-shielded ion-trap chamber to reduce the transition-line broadening caused by the magnetic field fluctuation.

A narrow linewidth diode laser is being developed. The laser linewidth is reduced to 66 Hz. The long term frequency drift is reduced 0.5 Hz per second, measured by Gigajet 20 W optical frequency comb.

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.

This paper discusses two open problems on the feasible frequency standard, namely, initial state preparation method and attainable frequency uncertainty.

We are proceeding with a development of an optical frequency standard using a 2Sfrac12-2D5/2 electric quadrupole transition of Ca+ ions in a small radio-frequency trap. We have theoretically confirmed that an uncertainty of < 1 times 10-15 is attainable in measurement of the 2Sfrac12 - 2D5/2 transition frequency. Experimentally, we have developed stable laser-diode systems and observed the quantum jump of single 40Ca+ ions to the 2D5/2 state

This paper shows two procedures of the clock laser system development: the first is reducing the linewidth of a diode laser (master laser); and the second is the tunable offset lock of the slave laser to the stabilized master laser. Linewidth and frequency stability of the laser system is also evaluated.

An optical frequency standard based on the $S\text{\ensuremath{-}}D$ transition of trapped $^{43}\mathrm{Ca}^{+}$ is possible by using just laser diodes for cooling and probing. We discuss two open problems related to the feasibility of this frequency standard, the attainable frequency uncertainty and the method of initial-state preparation. The uncertainty of the clock transition frequency can be so small as $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$. Using a laser system consisting of three lasers at different detunings and polarizations, the ion can be cooled down to the Lamb-Dicke regime and then prepared in the $m=0$ state within a period much shorter than the cooling period without being heated. The details of our numerical simulation are described.

A compact tunable ultraviolet coherent light source based on diode lasers would have wide application. However, there are only a few reports on that at wavelength below 250 nm. We have generated continuous-wave coherent light at 214.5 nm by frequency-quadrupling an output of a high-power diode laser system through two frequency doubling stages [1]. Our motivation to this subject is a development of an all-solid-state light source for laser cooling of Cd+ ions, which we propose as a candidate for a future frequency standard [2].

The authors proceed with a research and development of an optical frequency standard based on the 2S1/2 - 2 D5/2 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

An extended-cavity Littman-type diode laser, aiming the linewidth below 1 Hz, is in process to be developed. This laser is planned as the clock laser for the precise spectroscopy of the electric quadrupole 4s 2Sfrac12ap2D5/2 transition of a 43Ca+ ion

Deep-UV cw radiation around 202 nm was generated by frequency conversion of a LD-pumped solid-state laser and a LD-based master-oscillator power-amplifier system. The maximum power is 0.36 mW and the wavelength is tunable from 201.5 to 203.1 nm. We measured an optogalvanic spectrum with this light source.

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.

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.

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.