  In recent demonstrations of the quantum charge-coupled device (QCCD) computer
architecture, circuit times are dominated by cooling. Some motional modes of
multi-ion crystals take orders-of-magnitude longer to cool than others because
of low coolant ion participation. Here we demonstrate a new technique, which we
call phonon rapid adiabatic passage (phrap), that avoids this issue by
coherently exchanging the thermal populations of selected modes on timescales
short compared to direct cooling. Analogous to adiabatic rapid passage, we
quasi-statically couple these slow-cooling modes with fast-cooling ones using
DC electric fields. When the crystal is then adiabatically ramped through the
resultant avoided crossing, nearly-complete phonon population exchange results.
We demonstrate this on two-ion crystals, and show the indirect ground-state
cooling of all radial modes--achieving an order of magnitude speedup compared
to direct cooling. We also show the technique's insensitivity to trap potential
and control field fluctuations, and find that it still achieves sub-quanta
temperatures starting as high as n~200.
