Figure 6

(a) Isotope purification and distillation of a single \({}^{134}\text {Ba}^+\) ion loaded via laser ablation from a natural abundance \(\text {BaCl}_2\) source. Stage I is an image from an intensified CCD camera of the ion cloud after laser ablation²¹ and contains naturally occurring isotopes of barium atomic ions and BaCl molecular ions. Stage II depicts evolution of the ion cloud after filtering out BaCl molecular ions by applying a dc-quadrupole to the ion trap electrodes. Stage III is the resulting linear ion chain after lowering the amplitude of trap RF voltage. Stage IV is a single \({}^{134}\text {Ba}^+\) ion distilled from the longer chain in stage III. (b) Atomic spectrum of the \(^{2}S_{1/2} \leftrightarrow ^{2}P_{1/2}\) transition in \({}^{134}\text {Ba}^+\) using a self-injection locked WGM laser near 493 nm. Using a fiber EOM, the frequency of the laser light near 493 nm (607 THz) is rapidly switched between laser cooling and probing of the atomic transition. Photons are counted using single photon counting photo multiplier tubes during the probe cycle resulting in the displayed spectrum. (c) Grotrian diagram of a barium atomic ion with nuclear spin \(I = 0\) (\(^{130,132,134,136,138}\text {Ba}^+\)). The figure shows the electric dipole transitions used for laser cooling and electron shelving and de-shelving^19,21. The narrow linewidth and high passive stability of the visible wavelength WGM laser could enable more robust operation of trapped ion qubits.