アイテム詳細

要旨

Most ions lack the fast, cycling transitions that are necessary for direct
laser cooling. In most cases, they can still be cooled sympathetically through
their Coulomb interaction with a second, coolable ion species confined in the
same potential. If the charge-to-mass ratios of the two ion types are too
mismatched, the cooling of certain motional degrees of freedom becomes
difficult. This limits both the achievable fidelity of quantum gates and the
spectroscopic accuracy. Here we introduce a novel algorithmic cooling protocol
for transferring phonons from poorly- to efficiently-cooled modes. We
demonstrate it experimentally by simultaneously bringing two motional modes of
a Be$^{+}$-Ar$^{13+}$ mixed Coulomb crystal close to their zero-point energies,
despite the weak coupling between the ions. We reach the lowest temperature
reported for a highly charged ion, with a residual temperature of only
$T\lesssim200~\mathrm{\mu K}$ in each of the two modes, corresponding to a
residual mean motional phonon number of $\langle n \rangle \lesssim 0.4$.
Combined with the lowest observed electric field noise in a radiofrequency ion
trap, these values enable an optical clock based on a highly charged ion with
fractional systematic uncertainty below the $10^{-18}$ level. Our scheme is
also applicable to (anti-)protons, molecular ions, macroscopic charged
particles, and other highly charged ion species, enabling reliable preparation
of their motional quantum ground states in traps.