Coherent Control and Large-Scale Entanglement in Trapped Ion Crystals

Dr Michael J. Biercuk

The Quantum Control Laboratory, The University of Sydney

3:30 pm Friday, 23 April 2010,
EN413 (EN Building), Hawthorn.

We describe experimental efforts aimed at the realization of nonlinear multipartite interactions using planar ion crystals in a Penning trap. This system benefits from the ability to confine large ion arrays with regular and stable crystalline order without the need for significant trap engineering efforts. Of particular interest is the triangular lattice which arises naturally in a two-dimensional ion crystal, and which is known to show frustration in the presence of spin-spin interactions.

Coherent control over the spin state of the valence electron in ⁹Be⁺ is demonstrated and characterized through randomized benchmarking, yielding a single-bit operational fidelity of 99.92%. Coherence limits in this system are explored through use of dynamical decoupling protocols, demonstrating an ability to dramatically suppress decoherence-induced phase errors through sequence optimization. A global entangling interaction is engineered using state-dependent optical dipole forces, resulting in a simple distance-independent Ising interaction similar to single-axis-twisting spin squeezing. We present direct observations of optical-dipole-force excitation of the centre-of-mass (COM) mode for a planar crystal using phase-coherent Doppler velocimetry. By combining state-dependent excitation of the COM mode with microwave-mediated global spin control in arrays of up to ~150 ions, we observe hallmark signatures of spin-motional, and spin-spin entanglement. Experimental challenges such as the influence of spontaneous emission are discussed, and a new theoretical framework to fully account for dephasing induced by elastic Rayleigh scattering is presented.

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