We show that parametric coupling techniques can be used to generate selective entangling interactions for multi-qubit processors. By inducing coherent population exchange between adjacent qubits under frequency modulation, we implement a universal ga
teset for a linear array of four superconducting qubits. An average process fidelity of $mathcal{F}=93%$ is estimated for three two-qubit gates via quantum process tomography. We establish the suitability of these techniques for computation by preparing a four-qubit maximally entangled state and comparing the estimated state fidelity against the expected performance of the individual entangling gates. In addition, we prepare an eight-qubit register in all possible bitstring permutations and monitor the fidelity of a two-qubit gate across one pair of these qubits. Across all such permutations, an average fidelity of $mathcal{F}=91.6pm2.6%$ is observed. These results thus offer a path to a scalable architecture with high selectivity and low crosstalk.
Anomalously large linear and quadratic Zeeman shifts are measured for weakly bound ultracold $^{88}$Sr$_2$ molecules near the intercombination-line asymptote. Nonadiabatic Coriolis coupling and the nature of long-range molecular potentials explain ho
w this effect arises and scales roughly cubically with the size of the molecule. The linear shifts yield nonadiabatic mixing angles of the molecular states. The quadratic shifts are sensitive to nearby opposite $f$-parity states and exhibit fourth-order corrections, providing a stringent test of a state-of-the-art textit{ab initio} model.
We have produced large samples of ultracold $^{88}$Sr$_2$ molecules in the electronic ground state in an optical lattice. The molecules are bound by 0.05 cm$^{-1}$ and are stable for several milliseconds. The fast, all-optical method of molecule crea
tion via intercombination line photoassociation relies on a near-unity Franck-Condon factor. The detection uses a weakly bound vibrational level corresponding to a very large dimer. This is the first of two steps needed to create Sr$_2$ in the absolute ground quantum state. Lattice-trapped Sr$_2$ is of interest to frequency metrology and ultracold chemistry.
We report on the implementation of a dynamically configurable, servomotor- controlled, permanent magnet Zeeman slower for quantum optics experiments with ultracold atoms and molecules. This atom slower allows for switching between magnetic field prof
iles that are designed for different atomic species. Additionally, through feedback on the atom trapping rate, we demonstrate that computer-controlled genetic optimization algorithms applied to the magnet positions can be used in situ to obtain field profiles that maximize the trapping rate for any given experimental conditions. The device is lightweight, remotely controlled, and consumes no power in steady state; it is a step toward automated control of quantum optics experiments.
