Charge, parity, and time reversal (CPT) symmetry implies that a particle and its antiparticle have the same mass. The antiproton-to-electron mass ratio [Formula: see text] can be precisely determined from the single-photon transition frequencies of antiprotonic helium. We measured 13 such frequencies with laser spectroscopy to a fractional precision of 2.5 × 10 to 16 × 10 About 2 × 10 antiprotonic helium atoms were cooled to temperatures between 1.5 and 1.7 kelvin by using buffer-gas cooling in cryogenic low-pressure helium gas; the narrow thermal distribution led to the observation of sharp spectral lines of small thermal Doppler width. The deviation between the experimental frequencies and the results of three-body quantum electrodynamics calculations was reduced by a factor of 1.4 to 10 compared with previous single-photon experiments. From this, [Formula: see text] was determined as 1836.1526734(15), which agrees with a recent proton-to-electron experimental value within 8 × 10.

We have developed an optical frequency comb using a mode-locked fiber ring laser with an intra-cavity waveguide electro-optic modulator controlling the optical length in the laser cavity. The mode-locking is achieved with a simple ring configuration and a nonlinear polarization rotation mechanism. The beat note between the laser and a reference laser and the carrier envelope offset frequency of the comb were simultaneously phase locked with servo bandwidths of 1.3 MHz and 900 kHz, respectively. We observed an out-of-loop beat between two identical combs, and obtained a coherent δ-function peak with a signal to noise ratio of 70 dB/Hz.

We performed an absolute frequency measurement of the 1 S 0 -3 P 0 transition in 87 Sr with a fractional uncertainty of 1.2 × 10 −15 , which is less than one third that of our previous measurement. A caesium fountain atomic clock was used as a transfer oscillator to reduce the uncertainty of the link between a strontium optical lattice clock and the SI second. The absolute value of the transition frequency is 429 228 004 229 873.56(49) Hz.Recently, some optical clocks have reached the 10 −18 level 1, 2 in both uncertainty and stability, and these values surpass the caesium fountain microwave primary standards used to realise the SI unit of time. The high-performance of the optical clocks means that the scientific community is discussing a re-definition of the second. Therefore, there is a need for the metrology community to make a strenuous effort to determine the absolute frequencies of the optical frequency standards in relation to the current primary frequency standards, so that the length of one second remains unchanged after the re-definition.At the National Metrology Institute of Japan (NMIJ) we have developed atomic clocks based on optical transitions in an ensemble of neutral atoms trapped in Stark-shift-free optical lattices, 3-5 which are called optical lattice clocks. 6 In 2014, we measured the frequency of the 1 S 0 -3 P 0 clock transition in 87 Sr. 5 At that time the uncertainty of the absolute frequency (3.7 × 10 −15 ) was mainly limited by the uncertainty of a comparison with NMIJ coordinated ‡ These two authors contributed equally to this work.

Rare-earth chelate-doped graded index (GI) polymer optical fibers (POF) are proposed and fabricated. The attenuation loss was measured to be 0.4 dB/m at 650 nm for a GI POF doped with 1 wt % of europium (Eu) chelate. Lifetime shortening and spectral narrowing verified the occurrence of superfluorescence at 614 nm in the Eu chelate-doped GI POF pumped with xenon flashlamps. The demonstration of superfluorescence shows that rare-earth chelate-doped GI POFs are appealing as optical amplifiers and superfluorescent sources in a variety of communication and sensor applications.