Quantum control methods for robust entanglement of trapped ions

Abstract

A major obstacle in the way of practical quantum computing is achieving scalable and robust high-fidelity entangling gates. To this end, quantum control has become an essential tool, as it can make the entangling interaction resilient to sources of noise. Nevertheless, it may be difficult to identify an appropriate quantum control technique for a particular need given the breadth of work pertaining to robust entanglement. To this end, we attempt to consolidate the literature by providing a non-exhaustive summary and critical analysis. The quantum control methods are separated into two categories: schemes which extend the robustness to (i) spin or (ii) motional decoherence. We choose to focus on extensions of the σx ⊗ σx Mølmer-Sørensen interaction using microwaves and a static magnetic field gradient. Nevertheless, some of the techniques discussed here can be relevant to other trapped ion architectures or physical qubit implementations. Finally, we experimentally realize a proof-of-concept interaction with simultaneous robustness to spin and motional decoherence by combining several quantum control methods presented in this manuscript.

Batch Optimization of Frequency-Modulated Pulses for Robust Two-Qubit Gates in Ion Chains

“…To perform such precise control, various pulse-design methods have been proposed, such as using multichromatic beams with tunable amplitudes [10][11][12][13][14][15] and modulating amplitude [16][17][18][19][20][21], phase [22][23][24][25][26], and/or frequency [27][28][29][30] over many time segments. While these methods often guarantee high fidelity in the presence of small static offsets in experimental parameters [31], the parameters often fluctuate over time, which motivates pulse-design method that uses more precise information about the noise. Recently, characterization of noise in the motional modes [32,33], control signal [34], and ambient dephasing [35] has been experimentally demonstrated with trapped ions.…”