Implementing Robust Nondemolition Readout on Molecular Qubits via Electric-Field Gradient Gates

Due to their rich, radio-frequency addressable rovibrational structure, molecular ions have been proposed as a promising candidate on which to realize a scalable and high-fidelity trapped ion quantum computer (Phys. Rev. Lett. 2020, 125, 120501). One such architecture is Electric-Gradient Gates (EGGs) (Phys. Rev. A. 2021, 104, 042605), which encodes qubits on a molecular ion and uses a co-trapped atomic ion for sympathetic cooling and ancilla readout. With EGGS, a complete and laser-free set of quantum logic operations is achieved through the application of radio-frequency voltages on trap electrodes. Notably, a quantum nondemolition state detection scheme can be implemented by applying bichromatic microwave electric fields at the secular frequency sidebands of a molecular transition to sympathetically heat a co-trapped atomic ion.

To this end, we report sympathetic sideband cooling of an HCl+ - Ca+ ion chain to allow for high-fidelity readout resolution at the single-phonon level, as well as implementation of active feedback of our secular frequency to ensure robust detection of our qubit states.