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 leveling with few-centimeter height resolution emerge. We are developing a transportable optical clock based on a single trapped aluminum 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 ⁴⁰Ca⁺. Important features are a segmented multilayer 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 ⁴⁰Ca⁺ repump laser system. We present preliminary estimates of the trap-induced frequency shifts on ²⁷Al⁺, derived from measurements with a single calcium ion. The micromotion-induced second-order Doppler shift for ²⁷Al⁺ has been determined to be δν_EMMν=-0.4_-0.3 ^+0.4×10^-18 and the black-body radiation shift is δν_BBR/ν = (-4.0 ± 0.4) × 10^-18. Moreover, heating rates of 30 (7) quanta per second at trap frequencies of ω_rad,Ca+ ≈ 2π × 2.5 MHz (ω_ax,Ca+ ≈ 2π × 1.5 MHz) in radial (axial) direction have been measured, enabling interrogation times of a few hundreds of milliseconds.