Iku Hirano

Atomic fountain clock with very high frequency stability employing a pulse-tube-cryocooled sapphire oscillator

The frequency stability of an atomic fountain clock was significantly improved by employing an ultra-stable local oscillator and increasing the number of atoms detected after the Ramsey interrogation, resulting in a measured Allan deviation of 8.3 × 10-14τ-1/2. A cryogenic sapphire oscillator using an ultra-low-vibration pulse-tube cryocooler and cryostat, without the need for refilling with liquid helium, was applied as a local oscillator and a frequency reference. High atom number was achieved by the high power of the cooling laser beams and optical pumping to the Zeeman sublevel mF = 0 employed for a frequency measurement, although vapor-loaded optical molasses with the simple (001) configuration was used for the atomic fountain clock. The resulting stability is not limited by the Dick effect as it is when a BVA quartz oscillator is used as the local oscillator. The stability reached the quantum projection noise limit to within 11%. Using a combination of a cryocooled sapphire oscillator and techniques to enhance the atom number, the frequency stability of any atomic fountain clock, already established as primary frequency standard, may be improved without opening its vacuum chamber.

Preliminary Evaluation of the Cesium Fountain Primary Frequency Standard NMIJ-F2

We describe the preliminary evaluation of the frequency corrections and their uncertainty in the cesium fountain primary frequency standard (PFS) NMIJ-F2 under development at National Metrology Institute of Japan (NMIJ). In NMIJ-F2, cold atoms generated from a vapor-loaded optical molasses in the (001) configuration are optically pumped to the Zeeman sublevels of mF = 0 to increase the number of atoms involved in the Ramsey interrogation. Moreover, a cryocooled sapphire oscillator with ultralow phase noise is employed as the local oscillator to avoid degradation of the frequency stability due to the Dick effect. As a result, we have obtained a very high fractional frequency stability of 9.7 × 10-14 τ-1/2. As for systematic frequency shifts, the fractional correction for the second-order Zeeman shift is experimentally estimated to be (-165.5 ± 0.5) × 10-15 from the first-order Zeeman shift of atoms in mF = +1 launched to various heights. The fractional frequency correction for cold-atom collisions is estimated to be (+3.3 ± 0.4) × 10-15 by extrapolating the frequency to zero density from the frequencies measured for various nonzero atom numbers. We will soon be able to make a comparison with other atomic fountain PFSs at the 1 × 10-15 level.

Development of the cesium fountain frequency standard, NMIJ-F2

We have made much progress on NMIJ-F2, which is our second cesium fountain frequency standard aiming an uncertainty of <; 1×10^-15 as an immediate goal. The frequency stability is improved to 8.3×10^-14τ^-1/2 (τ: averaging time) by applying a cryogenic sapphire oscillator using a pulse-tube cryocooler as a local oscillator and optically pumping the atoms to the Zeeman sublevel mF = 0. Then, the homogeneous magnetic field in the interrogation region is obtained with magnetic shielding, a long solenoid coil, and two additional coils. The fractional frequency correction for the 2nd-order Zeeman shift is evaluated to be -165.5×10^-15. Moreover, the fractional frequency correction for the collisional shift (the frequency shift due to collisions between cold atoms) is measured to be (+3.3±0.5)×10^-15 by an extrapolation method.

Collisional Quenching Rates by He, N2 and CH4 for the 4D3/2 State in SrII

Sr+ ions were confined in a rf trap with buffer gases (He, N2 and CH4) at several different pressures, and the fluorescence from 5p2P1/2 to 5s2S1/2 was detected. Increasing in the fluorescence using a pump-back laser enabled the estimation of collisional quenching rates for the 4d2D3/2 state of Sr+. They are 0.27, 0.83 and 2.3 in units of 10-16 m3 s-1 for He, N2 and CH4, respectively. The experimental collisional quenching rates were compared with the Langevin reaction rates obtained by the classical ion-molecule collision theory.

DETERMINATION OF COLLISIONAL QUENCHING RATE FOR THE 4D3/2 STATE IN SRII

Sr+ ions were confined in a rf trap with He buffer gas. Weak fluorescence from Sr+ ions was detected using a frequency-doubled semiconductor laser, a CW fiber laser (pump-back light) and the photon counting method. Increase in fluorescence using the pump-back laser enabled the determination of collisional quenching rates for the 4d2D3/2 state. A collisonal quenching rate of 6400(s-1Pa-1) was obtained. This result reveals that collisional quenching rate for the metastable state can be determined by using CW lasers instead of pulse lasers.