Title: Robust two-qubit gates in a linear ion crystal using a frequency-modulated driving force,
Abstract: In an ion trap quantum computer, collective motional modes are used to
entangle two or more qubits in order to execute multi-qubit logical gates. Any
residual entanglement between the internal and motional states of the ions
results in loss of fidelity, especially when there are many spectator ions in
the crystal. We propose using a frequency-modulated (FM) driving force to
minimize such errors. In simulation, we obtained an optimized FM two-qubit gate
that can suppress errors to less than 0.01\% and is robust against frequency
drifts over $\pm$1 kHz. Experimentally, we have obtained a two-qubit gate
fidelity of $98.3(4)\%$, a state-of-the-art result for two-qubit gates with 5
ions.