Refining Motional Control towards Electric-Field Gradient Gates on Molecular Qubits*

Electric-Gradient Gates (EGGs) (Phys. Rev. A. 2021, 104, 042605) exploits the rich, microwave-addressable rovibrational structure of molecular ions to encode qubits. Together with a co-trapped atomic ion for sympathetic cooling and ancilla readout, EGGs achieves a complete and laser-free set of quantum logic operations by applying microwaves on trap electrodes.

However, motional control of ions remains a key obstacle towards near-term quantum computation. Particularly, state detection with EGGs involves the application of bichromatic microwave fields to generate state-dependent motional excitation.

To this end, we incorporate a body of quantum control techniques towards robust motional control with EGGs. Importantly, we implement a novel technique for in situ amplitude and phase calibration of qubit signals (arXiv:2311.12263). We integrate these improved control signals with continuous dynamical decoupling and waveform modulation to engineer robust sequences for nondemolition readout and quantum logic spectroscopy. Finally, we leverage parametric quantum amplification (Science. 2019, 364, 1163-1165) and optimal statistical design to ensure high-fidelity motional detection.