Towards a Transportable Aluminium Ion Quantum Logic Optical Clock

With the advent of optical clocks featuring fractional frequency uncertainties on the order of $10^{-17}$ and below, new applications such as chronometric levelling with few-cm height resolution emerge. We are developing a transportable optical clock based on a single trapped aluminium ion, which is interrogated via quantum logic spectroscopy. We employ singly-charged calcium as the logic ion for sympathetic cooling, state preparation and readout. Here we present a simple and compact physics and laser package for manipulation of $^{40}\mathrm{Ca}^+$. Important features are a segmented multi-layer trap with separate loading and probing zones, a compact titanium vacuum chamber, a near-diffraction-limited imaging system with high numerical aperture based on a single biaspheric lens, and an all-in-fiber \ca repump laser system. We present preliminary estimates of the trap-induced frequency shifts on $^{27}\mathrm{Al}^+$, derived from measurements with a single calcium ion. The micromotion-induced second-order Doppler shift for $^{27}\mathrm{Al}^+$ has been determined to be $\frac{\delta\nu_\mathrm{EMM}}{\nu}=\left({-0.4}^{+{0.4}}_{-{0.3}}\right)\times10^{-18}$ and the black-body radiation shift is $\delta\nu_\mathrm{BBR}/\nu=(-4.0\pm0.4)\times10^{-18}$. Moreover, heating rates of less than 10 quanta per second have been measured for all three motional modes at trap frequencies of $\omega_\mathrm{rad,Ca+} \approx2\pi\times2.5\,\mathrm{MHz}$ ($\omega_\mathrm{ax,Ca+} \approx2\pi\times1.5\,\mathrm{MHz}$) in radial (axial) direction, enabling interrogation times of 100s of milliseconds.