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.
Proc. SPIE. 8132, Time and Frequency Metrology III
KEYWORDS: Vacuum ultraviolet, High harmonic generation, Near infrared, Femtosecond phenomena, Prisms, Ferroelectric materials, Mirrors, Time metrology, Frequency metrology, Current controlled current source
We generate a VUV frequency comb centered at 159 nm as the 5th harmonic of Ti:S femto-second pulses by a passive enhancement using an external cavity. Average power up to 1.5 uW is measured by a solar-blind phototube. Stable generation about 10 minutes is obtained by active locking of the Ti:S frequencies to the enhancement cavity. Frequency tunability of our Ti:S comb teeth is also verified, which enables us to continuously sweep its frequency with more than twice a free spectral range. Those features reported here are indispensable to precision spectroscopy of unexplored atomic transitions in the VUV region.
We developed an optical frequency standard with the 4 <sup>2</sup><i>S</i><sub>1/2</sub>-3 <sup>2</sup>D<sub>5/2</sub> electric quadrupole transition of <sup>40</sup>Ca<sup>+</sup> 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.
Quantum-correlated twin beams were successfully generated from an optical parametric oscillator (OPO) pumped by a frequency-doubled diode laser. The 50-mW diode laser output at 1080 nm was first converted to 22.5-mW green light, and then was used to pump an OPO to yield signal and idler beams with a total power of 5 mW. Noise reduction of 4.3 dB below the shot-noise level was observed in the intensity-difference spectrum. The setup opens a simple way to a compact and low-cost source of diverse nonclassical states of light.