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 developed an optical frequency standard with the 4 2S1/2-3 2D5/2 electric quadrupole transition of 40Ca+ 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.
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.