Demonstration of Dipole-Phonon Quantum Logic with Optimized Sideband Cooling

Abstract

Using a single atom, we have observed the flip of a single molecule. The dipole-phonon interaction between the permanent dipole of a diatomic molecular ion and the secular oscillation of the ion chain manifests as a Jaynes-Cummings-type interaction. When combined with quantum logic using a co-trapped atomic ion, this interaction enables state preparation and measurement of quantum information encoded within a molecular ion. Here, we present the first experimental implementation of dipole- phonon quantum logic (DPQL) along with the preliminary research leading up to the demonstration. After an investigation of suitable molecular ions for experiments with DPQL, the ground rotational state of calcium oxide (CaO+) was identified as a prime candidate for the molecular ion qubit. We demonstrate sympathetic motional ground-state cooling with a co-trapped calcium ion (Ca+), and we showcase the ability to adiabatically ramp the trap secular frequency without perturbing the ions from the ground-state: two crucial steps toward implementing DPQL. However, due to the low population of the ground rotational state of CaO+ at room temperature, the molecular ion qubit space must be prepared via projective measurement. Because the rate of measurement is largely limited by ground-state cooling, we investigate via numerical optimization and experiments the comparative cooling performance of two commonly used methods of ground-state cooling, pulsed and continuous sideband cooling. Finally, we present the first observations of the dipole-phonon interaction via quantum logic readout, which have statistical significance as high as 7.4σ.