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A clock at a higher altitude ticks faster than one at a lower altitude, in accordance with Einstein’s theory of general relativity. The outstanding stability and accuracy of optical clocks, at 10−18 levels1–5, allows height differences6 of a centimetre to be measured. However, such state-of-the-art clocks have been demonstrated only in well-conditioned laboratories. Here, we demonstrate an 18-digit-precision frequency comparison in a broadcasting tower, Tokyo Skytree, by developing transportable optical lattice clocks. The tower provides the clocks with adverse conditions to test the robustness and a 450 m height difference to test the gravitational redshift at (1.4 ± 9.1) × 10−5. The result improves ground-based clock comparisons7–9 by an order of magnitude and is comparable with space experiments10,11. Our demonstration shows that optical clocks resolving centimetres are technically ready for field applications, such as monitoring spatiotemporal changes of geopotentials caused by active volcanoes or crustal deformation12 and for defining the geoid13,14, which will have an immense impact on future society. A pair of transportable optical lattice clocks with 10−18 uncertainty is developed. The relativistic redshift predicted by the theory of general relativity has been tested at the 10–5 level by the two optical clocks with a height difference of 450 m on the ground.

We report three-stage laser cooling of Sr atoms using the $5{s}^{2}\phantom{\rule{0.16em}{0ex}}^{1}S_{0}$ ground state and the $5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}$ metastable state. $^{87}\mathrm{Sr}$ atoms are precooled in a magneto-optical trap simultaneously operated on the $5{s}^{2}\phantom{\rule{0.16em}{0ex}}^{1}S_{0}$--$5s5p\phantom{\rule{0.16em}{0ex}}^{1}P_{1}$ transition at 461 nm and on the $5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}$--$5s5d\phantom{\rule{0.16em}{0ex}}^{3}D_{3}$ transition at 496 nm. Atoms optically pumped to the $5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}(F=13/2)$ state are magneto-optically trapped and cooled down to 2.5(2) $\ensuremath{\mu}\mathrm{K}$ on the $5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}$--$5s4d\phantom{\rule{0.16em}{0ex}}^{3}D_{3}$ transition at $2.9\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Using this transition, we investigate one dimensional polarization gradient cooling of $^{88}\mathrm{Sr}$ atoms, which have simple energy levels without hyperfine structure, down to 0.22(18) $\ensuremath{\mu}\mathrm{K}$ or 17 times the recoil temperature. We discuss prospects for continuous generation of ultracold Sr atoms combined with a state transfer technique.

We report a cascaded optical fiber link which connects laboratories in RIKEN, the University of Tokyo, and NTT within a 100-km region using a transfer light at 1397 nm, a subharmonic of the Sr clock frequency. The multiple cascaded link employing several laser repeater stations benefits from a wide feedback bandwidth for fiber noise compensation, which allows constructing optical lattice clock networks based on the master-slave configuration. We developed the laser repeater stations based on planar lightwave circuits to significantly reduce the interferometer noise for improved link stability. We implemented a 240-km-long cascaded link in a UTokyo–NTT–UTokyo loop using light sent from RIKEN via a 30-km-long link. In environments with large fiber noise, the link instability is 3 × 10−16 at an averaging time of 1 s and reaches 1 × 10−18 at 2,600 s.

Modern ground-based gravitational wave (GW) detectors require a complex interferometer configuration with multiple coupled optical cavities. Since achieving the resonances of the arm cavities is the most challenging among the lock acquisition processes, the scheme called arm length stabilization (ALS) had been employed for lock acquisition of the arm cavities. We designed a new type of the ALS, which is compatible with the interferometers having long arms like the next generation GW detectors. The features of the new ALS are that the control configuration is simpler than those of previous ones and that it is not necessary to lay optical fibers for the ALS along the kilometer-long arms of the detector. Along with simulations of its noise performance, an experimental test of the new ALS was performed utilizing a single arm cavity of KAGRA. This paper presents the first results of the test where we demonstrated that lock acquisition of the arm cavity was achieved using the new ALS and residual noise was measured to be $8.2\,\mathrm{Hz}$ in units of frequency, which is smaller than the linewidth of the arm cavity and thus low enough to lock the full interferometer of KAGRA in a repeatable and reliable manner., Comment: 21 pages, 8figures

We developed a pair of transportable optical lattice clocks and conducted an 18-digit-precision frequency comparison in a broadcasting tower to give a height difference of 450 m, which tested the gravitational redshift at 10-5 level.

We present visible spectra of highly charged holmium ions, which is a potential candidate ion for constructing a precise atomic clock. A compact electron beam ion trap is used for producing and trapping holmium ions. By observing electron energy dependence of the spectra, the charge state is assigned for each emission line. We also present theoretical transition frequency and amplitude obtained with two different methods.

We present an experimental observation of the visible magnetic dipole transition between the fine structure levels in the ground term of ${\mathrm{Ba}}^{6+}$ ($5s{\phantom{\rule{0.16em}{0ex}}}^{2}5{p}^{2}$: $^{3}P_{0}\ensuremath{-}^{3}P_{1}$) with a compact electron beam ion trap. The wave number in vacuum of the transition is determined to be 15505.506(37) ${\mathrm{cm}}^{\ensuremath{-}1}$ by simultaneously observing the reference lines of neutral Ne. The present measurement reduces the uncertainty in the previously reported value obtained from observation of the electric dipole transitions to the ground terms in the extreme ultraviolet range [Phys. Scr. 46, 403 (1992)].

We report a frequency measurement on the $5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}\ensuremath{-}5s4d\phantom{\rule{0.16em}{0ex}}^{3}D_{3}$ transition of $^{88}\mathrm{Sr}$ atoms at $2.9\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$. The excitation spectra are obtained by measuring the photon momenta transferred to ultracold atoms. The transition frequency is determined to be 102 550 196 205(9) kHz using an erbium fiber-based optical frequency comb. We numerically investigate possible frequency corrections by taking an acceleration of atoms and attenuation of the excitation light into account. This measurement technique is particularly useful for transitions where laser-induced fluorescence is difficult to observe but the spatial imaging of atoms is possible.

Real-time geopotential measurements with two synchronously linked optical lattice clocks are demonstrated. A height difference between the two clocks separated by 15 km is determined, with an uncertainty of 5 cm, by means of a gravitational redshift. According to Einstein's theory of relativity, the passage of time changes in a gravitational field1,2. On Earth, raising a clock by 1 cm increases its apparent tick rate by 1.1 parts in 1018, allowing chronometric levelling3 through comparison of optical clocks1,4,5. Here, we demonstrate such geopotential measurements by determining the height difference of master and slave clocks separated by 15 km with an uncertainty of 5 cm. A subharmonic of the master clock laser is delivered through a telecom fibre6 to synchronously operate7 the distant clocks. Clocks operated under such phase coherence reject clock laser noise and facilitate proposals for linking clocks8,9 and interferometers10. Taken over half a year, 11 measurements determine the fractional frequency difference between the two clocks to be 1,652.9(5.9) × 10−18, consistent with an independent measurement by levelling and gravimetry11. Our system demonstrates a building block for an internet of clocks, which may constitute ‘quantum benchmarks’, serving as height references with dynamic responses.

UTokyo Research掲載「異なる原子の光格子時計の短時間精密比較に成功」 URI: http://www.u-tokyo.ac.jp/ja/utokyo-research/research-news/rapid-comparison-of-optical-lattice-clocks.html, UTokyo Research "Rapid comparison of optical lattice clocks" URI: http://www.u-tokyo.ac.jp/en/utokyo-research/research-news/rapid-comparison-of-optical-lattice-clocks.html

We performed the first direct measurement of the frequency ratio between a mercury (199Hg) and an ytterbium (171Yb) optical lattice clock to find νHg/νYb = 2.177 473 194 134 565 07(19) with the fractional uncertainty of 8.8 × 10−17. The ratio is in excellent agreement with expectations from the ratios νHg/νSr and νYb/νSr obtained previously in comparisons against a strontium (87Sr) optical lattice clock. The completed closure (νHg/νYb)(νYb/νSr)(νSr/νHg) − 1 = 0.4(1.3) × 10−16 tests the frequency reproducibility of the optical lattice clocks beyond what is achievable in comparison against the current realization of the second in the International System of Units (SI).

著者人数: 227名 (所属. 宇宙航空研究開発機構宇宙科学研究所(JAXA)(ISAS): 東谷, 千比呂), Number of authors: 227 (Affiliation. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency(JAXA)(ISAS): Tokoku, Chihiro), Accepted: 2017-11-21, 資料番号: SA1170370000

Continuous-wave output at 229 nm for the application of laser cooling of Cd atoms was generated by the 4th harmonic using two successive second harmonic generation stages. Employing a single-frequency optically pumped semiconductor laser as a fundamental source, 0.56 W of output at 229 nm was observed with a 10-mm long, Brewster-cut BBO crystal in an external cavity with 1.62 W of 458 nm input. Conversion efficiency from 458 nm to 229 nm was more than 34%. By applying a tapered amplifier as a fundamental source, we demonstrated magneto-optical trapping of all stable Cd isotopes including isotopes $^{111}$Cd and $^{113}$Cd, which are applicable to optical lattice clocks.

A new method for detecting wavefront aberrations using a Fabry–Perot cavity is described. A Fabry–Perot cavity transmits the cavity eigenmode which satisfies a resonance condition, while the other modes are reflected from the cavity. Utilizing this mode discrimination property, we can extract aberration components from incident aberrated light as reflected light from the cavity. Under the condition of a weak aberration, the amplitude profile of the reflected light can be approximated to the wavefront aberration. We can therefore determine the wavefront aberrations in a visualized form with the technique. We present the detection scheme and the first experimental verification of the technique. The detected aberration using this technique was in good agreement with the result on the basis of a conventional wavefront sensor.

In order to detect a gravitational wave, a kilometer-scale interferometer with a high-power, single-frequency laser with extremely low noises is required. In the Japanese gravitational wave detector named Large-scale Cryogenic Gravitational wave Telescope (LCGT), a single-frequency laser with an output power of more than 100 W will be used. The laser system used in LCGT consists of a master laser, an amplifier, a pre-modecleaner, a modulation system, and a modecleaner. The system also includes stabilization systems for laser intensity and frequency. We have already developed most of them. This article describes the characteristics of the laser system used in LCGT and the current status of our developments.

We report on a frequency ratio measurement of a (199)Hg-based optical lattice clock referencing a (87)Sr-based clock. Evaluations of lattice light shift, including atomic-motion-dependent shift, enable us to achieve a total systematic uncertainty of 7.2×10(-17) for the Hg clock. The frequency ratio is measured to be νHg/νSr=2.629 314 209 898 909 60(22) with a fractional uncertainty of 8.4×10(-17), which is smaller than the uncertainty of the realization of the International System of Units (SI) second, i.e., the SI limit.

This article describes the recent progress of optical lattice clocks with neutral strontium ($^{87}$Sr), ytterbium ($^{171}$Yb) and mercury ($^{199}$Hg) atoms. In particular, we present frequency comparison between the clocks locally via an optical frequency comb and between two Sr clocks at remote sites using a phase-stabilized fibre link. We first review cryogenic Sr optical lattice clocks that reduce the room-temperature blackbody radiation shift by two orders of magnitude and serve as a reference in the following clock comparisons. Similar physical properties of Sr and Yb atoms, such as transition wavelengths and vapour pressure, have allowed our development of a compatible clock for both species. A cryogenic Yb clock is evaluated by referencing a Sr clock. We also report on a Hg clock, which shows one order of magnitude less sensitivity to blackbody radiation, while its large nuclear charge makes the clock sensitive to the variation of fine-structure constant. Connecting all three types of clocks by an optical frequency comb, the ratios of the clock frequencies are determined with uncertainties smaller than possible through absolute frequency measurements. Finally, we describe a synchronous frequency comparison between two Sr-based remote clocks over a distance of 15 km between RIKEN and the University of Tokyo, as a step towards relativistic geodesy., 11 pages, 5 figures, invited review article in Comptes Rendus de Physique 2015

We report on a frequency ratio measurement of a (199)Hg-based optical lattice clock referencing a (87)Sr-based clock. Evaluations of lattice light shift, including atomic-motion-dependent shift, enable us to achieve a total systematic uncertainty of 7.2×10(-17) for the Hg clock. The frequency ratio is measured to be νHg/νSr=2.629 314 209 898 909 60(22) with a fractional uncertainty of 8.4×10(-17), which is smaller than the uncertainty of the realization of the International System of Units (SI) second, i.e., the SI limit.

All-polarization-maintaining, single-port Er:fiber combs offer long-term robust operation as well as high stability. We have built two such combs and evaluated the transfer noise for linking optical clocks. A uniformly broadened spectrum over 135-285 THz with a high signal-to-noise ratio enables the optical frequency measurement of the subharmonics of strontium, ytterbium, and mercury optical lattice clocks with the fractional frequency-noise power spectral density of $(1-2)\times 10^{-17}$ Hz$^{-1/2}$ at 1 Hz. By applying a synchronous clock comparison, the comb enables clock ratio measurements with $10^{-17}$ instability at 1 s, which is one order of magnitude smaller than the best instability of the frequency ratio of optical lattice clocks., Comment: 14 pages, 4 figures

We have developed an apparatus to test Lorentz invariance in the photon sector by measuring the resonant frequency difference between two counterpropagating directions of an asymmetric optical ring cavity using a double-pass configuration. No significant evidence for the violation was found at the level of $\delta c /c \lesssim 10^{-14}$. Details of our apparatus and recent results are presented., Comment: 4 pages, 1 figure. Presented at the Sixth Meeting on CPT and Lorentz Symmetry, Bloomington, Indiana, June 17-21, 2013

A search for Lorentz violation in electrodynamics was performed by measuring the resonant frequency difference between two counterpropagating directions of an optical ring cavity. Our cavity contains a dielectric element, which makes our cavity sensitive to the violation. The laser frequency is stabilized to the counterclockwise resonance of the cavity, and the transmitted light is reflected back into the cavity for resonant frequency comparison with the clockwise resonance. This double-pass configuration enables a null experiment and gives high common mode rejection of environmental disturbances. We found no evidence for odd-parity anisotropy at the level of $\delta c /c \lesssim 10^{-14}$. Within the framework of the Standard Model Extension, our result put more than 5 times better limits on three odd-parity parameters $\tilde{\kappa}^{JK}_{o+}$ and a 12 times better limit on the scalar parameter $\tilde{\kappa}_{\tr}$ compared with the previous best limits., Comment: 4 pages, 3 figures

We are developing optical lattice clocks with a scope of attaining 10−18 fractional uncertainty. Cryogenic silicon cavity targeting 2×10−17 stability at 1s, will allow full utilization of the potential stability of optical lattice clocks. In order to reduce the blackbody radiation shift, which is the most serious source of uncertainties, Sr clocks in cryogenic environment and Hg clocks are underdevelopment. We discuss prospects for clock comparison, no dead time operation of the clocks and fiber link of the clocks between Riken and the University of Tokyo.

We investigated thermal properties depending on boundary conditions of wavelength conversion crystals. With a tight aperture, we demonstrated 19-W single-pass 532-nm SHG at a conversion efficiency of 26.5% in a 10-mm-long PPMgSLT crystal without saturation.

We demonstrated a 40-W, CW, cavity-enhanced second-harmonic generation (SHG) with kHz linewidth of an injection-locked Nd:YAG laser. The nonlinear characteristics of LiB 3 O 5 were measured with a single-pass configuration to design a SHG cavity.

We investigated the properties of single-pass second-harmonic generation of CW 532 nm radiation with a single frequency Nd:YAG laser by QPM in Mg:SLT. SHG upper limit can be increased by shifting the focus position.

Amplitude modulation of laser light is required for resonant sideband extraction employed in gravitational-wave detectors. Amplitude modulation with electro-optic phase modulators is realized by interferometric phase-to-amplitude conversion. Although two outputs modulated at opposite phases to each other are obtained, usually only one of them is utilized and the other is abandoned. The reuse of this abandoned light improves the power efficiency of the modulation. This can be realized by inverting the modulation phase of one output with a delay line and adding it to the other coherently. Moreover, this system selects a high-efficiency operating point and modulates the light in a linear range. We demonstrate that the modulation system can be operated with a power loss that is due only to the losses of the optical components.

Second-generation gravitational wave detectors require a highly stable laser with an output power greater than 100 W to attain their target sensitivity. We have developed a frequency stabilization system for a 100-W injection-locked Nd:YAG (yttrium aluminum garnet) laser. By placing an external wideband electro-optic modulator used as a fast-frequency actuator in the optical path of the slave output, we can circumvent a phase delay in the frequency control loop originating from the pole of an injection-locked slave cavity. Thus, we have developed an electro-optic modulator made of a MgO-doped stoichiometric LiNbO(3) crystal. Using this modulator, we achieve a frequency control bandwidth of 800 kHz and a control gain of 180 dB at 1 kHz. These values satisfy the requirement for a laser frequency control loop in second-generation gravitational wave detectors.

We report on the wideband frequency stabilization of a 100-W injection-locked Nd:YAG laser using an external electrooptic modulator made of MgO-doped stoichiometric LiNbO3 crystals. In order to circumvent a phase delay in the frequency control loop caused by an injection-locked slave cavity, the electrooptic modulator was placed at the optical path of the slave output. The realized control bandwidth was 85 kHz, which was four-times wider than that obtained without an external electrooptic modulator. We achieved the frequency noise level suppressed relative to a reference cavity of 6×10-2 Hz/√Hz at around 100 Hz.

We investigated thermal behaviors of single-pass second-harmonic generation of continuous wave green radiation with high efficiency by quasi-phase matching in periodically poled Mg-doped stoichiometric lithium tantalate (PPMgSLT). Heat generation turned out to be directly related to the green light absorption in the material. Strong relation between an upper limit of the second harmonic power and confocal parameter was found. Single-pass second-harmonic generation of 16.1 W green power was achieved with 17.6% efficiency in Mg:SLT at room temperature.

We have developed an electrooptic modulator (EOM) using stoichiometric MgO-doped LiNbO3 (MgO-doped SLN) crystals, and evaluated this EOM for the phase control of a high-power laser whose power is over 100 W. We succeeded in the phase modulation of a 100 W laser without a decrease in phase modulation index, due to a nonlinear optical effect, which can occur when a high-power laser beam enters the EOM. We also estimated wavefront distortions caused by passing through the EOM with a Shack–Hartmann wavefront sensor, and found that the additional wavefront distortion was negligible. Thus, we confirmed that MgO-doped SLN crystals are a suitable material for use in a high-power laser system.