Realization of the Fine-Structure Qubit in a Single Alkaline-Earth Atom: The Building Block for a 500-Qubit Neutral Atom Quantum Processor (QRydDemo)

We report on the realization of a novel neutral atom qubit encoded in the magnetically insensitive metastable fine-structure states ³P₀ and ³P₂ of single ⁸⁸Sr atoms trapped in an optical tweezer. Our encoding scheme enables fast single-qubit gates on the 100 ns timescale and single-photon excitation to a Rydberg state, wherein two-qubit gates on the same timescales can be achieved. As the first step towards this goal, we demonstrate preparation, read-out, and coherent control of this novel qubit. Coherent Rabi oscillations bridging an energy gap of more than 17 THz are demonstrated by using a pair of phase-locked lasers with ultra-narrow linewidths. We use Ramsey spectroscopy to extract the transverse qubit coherence time. By tuning the tensor polarizability of the ³P₂ state by using an external magnetic field, we achieve a magic trapping condition and measure an enhanced T₂ time of 1.2 ms. Using a microscopic quantum mechanical model, we simulate our experiments including dominant noise sources and identify the major constraints limiting our measured T₂ time and finally identify possible improvements to our system. Our novel encoding scheme enables key advantages to be brought together onto a single platform and thereby enables new synergies and possibilities in neutral atom based quantum computing.