Defect-Free Atom Arrays of ⁸⁸Sr in Optical Tweezers

Abstract

Neutral atoms trapped in optical microtraps, so called optical tweezers, have emerged as a platform for controlling large many-particle quantum systems, with applications in many-body physics, metrology, quantum information processing and cavity quantum electrodynamics. A key requirement for building a programmable quantum simulation platform is to gain control over the initial state of the system. It is essential to eliminate uncertainties in the initialization process, especially caused by variations in the number of stored atoms but also by uncertainties associated with the electronic and motional degrees of freedom of a trapped atom itself. <br /> In this work, we report on the design and characterization of a new optical tweezer setup, aimed at preparing, detecting, and manipulating single strontium atoms cooled to their motional ground state. We design a tweezer system around a high-NA microscope objective (NA=0.7) with different tweezer wavelengths to exploit their respective advantages. We load the tweezers from a MOT and by employing light-assisted collisions we create highly sub-Poissonian atom number distributions, where a trap is occupied by a single atom at most. In tweezers at 515nm, we cool atoms using resolved sideband cooling, where we observe a three-dimensional motional ground state fraction around 95% at a tweezer aspect ratio of 5.1(1), improving on existing experimental implementations. Further, we use a sisyphus cooling process to cool atoms in tweezers at 532nm and 813nm, where we also confirm temperatures close to the motional ground state. At 813nm, we confirm array homogeneities on the 1%-level, measured with up to 60 ultracold trapped atoms. The characterization of the sisyphus cooling process in tweezers at 532nm is, to our current knowledge, the first time that strontium atoms have been investigated in high NA tweezers at this wavelength. They confirm that a tweezer wavelength of 532nm is a viable candidate for future research, as commonly available lasers and optical elements can reduce complexity and cost of the experimental setup. <br /> As described above, we can reliably prepare single atoms in their electronic and motional ground state in a tweezer array. To address the remaining initial entropy of the atom array, namely the uncertainty associated with partially filled arrays, we rearrange atoms in the underlying array using a separate, spatially tunable optical tweezer. We develop a control-theory optimal trajectory model for moving an atom between sites, based on the kinematic parameters of the motion. We then present the assembly of defect-free arrays of 16 tweezers, where we determine a single move success probability ("move fidelity") of 97.8(22)%, on par with other state-of-the-art implementations. <br /> With this work we lay the foundation of a programmable quantum simulation platform, and in the future, the new experiment can be used to create large many-particle states, enabling exciting studies at the frontiers of experimental quantum simulation research.