We designed the centerpiece of the new setup, a novel monolithic 3D linear Paul trap (Fig. 1), and had it fabricated using subtractive 3d printing and subsequent metallization for defining the trap electrodes. The trap geometry was designed to (1) provide good optical access to the ions, (2) feature low distortions of the trapping potential by terms that might couple energy into the ion motion, (3) enable efficient laser cooling, and (4) orient the electric field lines in a way that minimizes the influence of driven ion motion on quantum operations.
For housing the trap, we developed and built a new compact vacuum chamber (Fig. 2a) with excellent optical access and ultra-high vacuum. All laser sources for manipulating the ions were integrated into optical setups (Fig.2b) for addressing the ions by laser pulses. For coherent excitation, the laser pulse properties are controlled by a novel versatile radiofrequency source integrated into the experiment control program.
In an existing apparatus, random measurements were used to measure purities and entanglement properties of crystals with up to 20 ions (Science 364, 260 (2019)), demonstrating a cross-platform verification protocol (PRL 124, 010504 (2020)) and measuring quantum information scrambling (PRL 124, 240505 (2020)). Secondly, we demonstrated variational simulation of spin models by a feedback loop between a classical computer and a quantum co-processor, (Nature 569, 335 (2019)).
In the new setup, we created 2d-crystals with more than 100 ions. Structural configuration changes to metastable configurations are suppressed by fine-tuning the trap anisotropy. Preparation of the ion crystals at sub-mK temperatures is enabled by polarization-gradient cooling (NJP 22, 103013 (2020)) and EIT ground state cooling of the transverse motional modes (PRX Quantum 4, 040346 (2023)). For quantifying mode temperatures, a novel thermometry technique (PRX Quantum 4, 040346 (2023)) revealed motional heating rates of 0.5 phonons/s.
Qubits are encoded in the two Zeeman ground states of 40Ca+ ions. High-fidelity state measurements are achieved by electron shelving. Qubit manipulation is accomplished by a motion-sensitive Raman light field off-resonantly coupling the qubit states to P-states. Qubit coherence times >100 ms were found. An addressing unit has been developed for single-qubit control by differential light shifts. Raman beam are aligned by correlation spectroscopy (arXiv: 2203.12656). Long-range spin-spin interactions are realized by off-resonantly coupling all ions to all transverse modes using a bichromatic Raman interaction. In this way, we realized long-range Ising models for validating the performance of the simulator by comparing observed spin-spin correlations to exact simulations. Furthermore, we investigated non-equilibrium quantum dynamics by long-range XY models that generated spin squeezed states. Using the spin squeezing parameter as a benchmark, we demonstrated entanglement creation in 2d-crystals with >100 ions for which exact simulations are infeasible. We used a quantum-classical feedback loop for variationally optimizing the spin squeezing, obtaining up to 9.5 dB of noise reduction. Project results have been disseminated by scientific publications, talks at conferences and winter schools, and lab tours.
