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High-Fidelity Quantum Control Using 9Be+ Ion Crystals in a Penning Trap



Michael J. Biercuk, Hermann Uys, Aaron Vandevender, Nobuyasu Shiga, Wayne M. Itano, John J. Bollinger


We provide an introduction to the use of ion crystals in a Penning trap for experiments in quantum information. Macroscopic Penning traps allow for the containment of a few to a few million atomic ions whose internal states may be used in quantum information experiments. Ions are laser Doppler cooled, and the mutual Coulomb repulsion of the ions leads to the formation of crystalline arrays. The structure and dimensionality of the resulting ion crystals may be tuned using a combination of control laser beams and external potentials. We discuss the use of two-dimensional 9Be+ ion crystals for experimental tests of quantum control techniques. Our primary qubit is the 124 GHz ground-state electron spin flip transition, which we drive using a home-built microwave system. An ion crystal represents a spatial ensemble of qubits, but as we demonstrate, the effects of inhomogeneities across a typical crystal are small, and as such we treat the ensemble as a single effective spin. We are able to strongly initialize the qubits in a simple state and perform a strong projective measurement on the system, unlike some other technologies using ensemble control and measurement techniques. In this manuscript we present a detailed accounting of our ability to fully control the qubit Bloch vector, performing arbitrary high-fidelity rotations. Randomized Benchmarking demonstrates an error per driven computational gate (a Pauli-randomized pi/2=2 and pi pulse pair) of 8 {+/-} 1 × 10-4. Ramsey interferometry and spin-locking measurements are used to elucidate the limits of qubit coherence in the system, yielding a typical free-induction decay coherence time of T_2 ~2 ms, and a limiting T_1rho ~688 ms. These experimental specifications make ion crystals in a Penning trap an ideal candidate for novel experiments in quantum control. As such, we briefly describe recent efforts aimed at studying the error-suppressing capabilities of dynamical decoupling pulse sequences, demonstrating an ability to dramatically extend qubit coherence and suppress phase errors. This article concludes with a discussion of future avenues for experimental exploration, including the use of additional nuclear-spin-flip transitions for effective multiqubit protocols, and the potential for Coulomb crystals to form a useful testbed for studies of large-scale entanglement.
Quantum Information & Computation


Trapped Ion, Quantum Information, Quantum Computing, Fidelity, Quantum Control, Dynamical Decoupling, Penning Trap, Decoherence, Dephasing


Biercuk, M. , Uys, H. , Vandevender, A. , Shiga, N. , Itano, W. and Bollinger, J. (2009), High-Fidelity Quantum Control Using 9Be+ Ion Crystals in a Penning Trap, Quantum Information & Computation, [online], (Accessed April 15, 2024)
Created August 19, 2009, Updated February 19, 2017