Sideband cooling of a $$\hbox {Ca}^+$$ Ca + – $$\hbox {In}^+$$ In + ion chain toward the quantum logic spectroscopy of $$\hbox {In}^+$$ In +

Abstract

Indium ions ( $${}^{115}\hbox {In}^+$$ ) are a promising candidate for an optical clock with a frequency inaccuracy below $$10^{-17}$$ . Quantum logic spectroscopy (QLS) plays a fundamental role in realizing such a clock by supplying a high-fidelity fast quantum state detection method. Initializing a phonon mode of an ion chain consisting of $$\hbox {In}^+$$ and an ancillary ion to the ground state is a fundamental step toward the implementation of QLS. We report experimental results of the sideband cooling and heating rate measurements of a $${}^{40}\hbox {Ca}^+$$ – $${}^{115}\hbox {In}^+$$ chain using optical transitions in $$\hbox {Ca}^+$$ . The sideband cooling of the out-of-phase mode of the axial motion down to an estimated phonon number of 0.096 is demonstrated.