Speaker
Description
We aim at realization of an optical frequency standard with barium ion (Ba$^+$). The $^2$S$_{1/2}$ $(F = 2,m_F = 0)$ - $^2$D$_{3/2}$ $(F = 0,m_F = 0)$ clock transition in odd isotopes $^{135}$Ba$^+$ or $^{137}$Ba$^+$ is insensitive to quadrupole electric field[1]. Therefore, it is possible to improve the frequency stability by increase of the number of ions without degradation of uncertainty.
As a first step, we are developing an optical clock referenced to the $^2$S$_{1/2}$ - $^2$D$_{5/2}$ transition at 1.76 $\mu$m in $^{138}$Ba$^+$ ions[2-5]. So far, we conducted single-ion spectroscopy of the clock transition using a linenarrowed external-cavity laser diode (ECLD) and resolved motional sidebands [6]. To accelerate the detection of the spectra of the clock transition, we employed deexcitation of the $^2$D$_{3/2}$ state owing to its long lifetime of 31 s. We first drove the $^2$D$_{5/2}$ - $^2$P$_{3/2}$ transition at 614 nm by irradiating with radiation from an orange LED. The deexcitation rate was measured to be ≲ 10 s. Then, we irradiated with radiation around 614 nm from an optical frequency comb (OCF) based on a Yb:KYW laser[6] to further accelerate the deexcitation. We estimated the deexcitation time to be 200 ms, where the optical power in 30-nm bandwidth was 40 $\mu$W. This is a similar approach to use of an amplified spontaneous-emission in a Yb-doped fiber amplifier for deexcitation of the $^2$D$_{5/2}$ state in Sr$^+$ ions[8].
We also succeeded in laser cooling of single $^{137}$Ba$^+$ ions loaded through odd-isotope-selective photoionization[9]. We employ two-step photoionization of Ba atoms using the $^1$S$_{0}$ - $^1$P$_{1}$ transition at 553 nm as the first excitation. Ba atoms in the $^1$P$_{1}$ state is further excited using the second radiation above the ionization potential. Radiation at 553 nm is generated using a frequency-doubled ECLD and the second radiation is generated from a laser diode (LD) at 396 nm. Radiation for the first excitation is blue-detuned by 500 MHz from the absorption line of $^{138}$Ba$^+$ ions. We laser cooled $^{137}$Ba$^+$ ions by driving the $^2$S$_{1/2}$ - $^2$P$_{1/2}$ transition using two frequency-doubled ECLDs at 493 nm to avoid optical pumping in the hyperfine structures. We simultaneously drove the $^2$D$_{3/2}$ - $^2$P$_{1/2}$ transition using three ECLDs at 650 nm. We detected the fluorescence of a photon counting rate of 700 s$^{-1}$ for a $^{137}$Ba$^+$ ion.
[1] J. Sherman et al., 2005 Digest of the LEOS Summer Topical Meetings, p.99
[2] W. Nagourney et al., Opt. Commun., 79, 176 (1990)
[3] B. Appasamy et al., Appl. Phys. B, 60, 473 (1995)
[4] N. Kurz et al., Phys. Rev. A, 82, 030501(R) (2010)
[5] L. Slodička et al., Phys. Rev. A, 85, 043401 (2012)
[6] H. Fujisaki et al., 6th international conference on Trapped Charged Particles and Fundamental Physics (TCP2014), Japan, p.92
[7] M. Mitaki et al., Appl. Opt., 57, 5150 (2018)
[8] T. Fordell et al., Opt. Lett., 40 No.8, 1822 (2015)
[9] M. R. Dietrich et al., Phys. Rev. A, 81, 052328 (2010)
