Refining Control of Electric-Field Gradient Gates on Molecular Ion Qubits*

Electric-Gradient Gates (EGGs) (Phys. Rev. A. 2021, 104, 042605) exploit 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 microwave field gradients via trap electrodes.

 

Notably, the relatively low frequency of molecular ion qubit states (compared to typical optical qubits) enables high-fidelity signal generation with relative ease.

However, this stroke of EGGscellence proves itself to be, in fact, simply ischemic.

 

In particular, the VHF/microwave band used for EGGS is poorly explored in the context of ion trapping.

This band has proved largely inaccessible to most quantum sensing schemes, creating a challenge for accurate calibration of qubit signals.

To this end, we have developed a novel wideband complete-waveform sensing scheme (arXiv:2311.12263) capable of achieving sensitivities beyond the Fourier-transform.

 

Additionally, the VHF/microwave band shares significant overlap with the HF band that defines ion motion, presenting a unique challenge to motional coherence, especially in the form of motional Raman-induced heating.

As a counter, we combine a breadth of error mitigation schemes, such as dynamical decoupling and phase modulation, to engineer robust and error-free gate trajectories.