Indirect cooling of trapped ions through phonon rapid adiabatic passage

In recent demonstrations of the quantum charge-coupled device (QCCD) computer architecture, circuit time is dominated by cooling operations, which can limit operational fidelity via memory error. Some motional modes of a multi-ion crystal cool inefficiently due to laser geometry and ion mode participation, resulting in order-of-magnitude differences in cooling times. Previous work has shown that motional quanta can be transferred between modes by modulating local electric fields, opening the possibility of transferring phonons out of problematic modes. However, these techniques are hampered by the voltage filters needed to suppress high frequency noise, and are relatively sensitive to drifts in mode frequency and drive amplitude. Here we demonstrate a technique that overcomes these limitations, which we call phonon rapid adiabatic passage (phRAP). Analogous to adiabatic rapid passage, we couple harder-to-cool modes quasi-statically with easier ones using a DC electric field. Nearly-complete phonon population exchange results when the crystal is adiabatically driven through the resulting avoided crossing. Using this method, we demonstrate indirect ground-state cooling of all radial modes of a two-ion crystal, achieving an order of magnitude speedup in cooling times as compared with traditional sideband cooling.