Fast and Robust Two-Qubit Gates with Microwave-Driven Trapped Ions

We propose a pulsed dynamical decoupling protocol to be used as the generator of tunable, fast, and robust quantum phase gates between two microwave-driven hyperfine qubits in ion traps. The gate scheme consists of a suitably designed sequence of pi-pulses acting on the ions which seat along a magnetic field gradient. Unlike previous approaches, the proposed gate does not rely on the spectroscopic discrimination of a single vibrational mode to act as a quantum bus between the two trapped ions. Instead, in our design, the two vibrational modes in the axial direction of the ion chain are forced to cooperate under the influence of the externally applied microwave driving. In this manner, the gate speed can be significantly increased. Furthermore, our scheme is demonstrated to be robust against the dominant noise sources, which are errors on the magnetic field and microwave pulse intensities as well as motional heating, leading to fast and high-fidelity two-qubit gates. Our detailed numerical simulations, including realistic experimental considerations, show that with state-of-the-art trapped-ion technology, entangling gates with fidelities above 99.9% can be achieved in tens of microseconds.