Abstract
The majority of ions and other charged particles of spectroscopic interest 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 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 in each of the two modes, corresponding to a residual mean motional phonon number of . Combined with the lowest observed electric-field noise in a radio-frequency ion trap, these values enable an optical clock based on a highly charged ion with fractional systematic uncertainty below the 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.
- Received 2 March 2021
- Revised 31 August 2021
- Accepted 20 September 2021
DOI:https://doi.org/10.1103/PhysRevX.11.041049
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Laser cooling of trapped particles is a powerful technique to reduce their kinetic energy down to temperatures that are only a tiny fraction of a degree above absolute zero. Unfortunately, only a few atomic species can be laser cooled directly. If a laser-coolable “refrigerant” atom is stored side by side with the particle of interest in a trap, heat can be removed from both through the refrigerant atom. This works well for particles with identical charges and masses, but if those are very different, the cooling effect is greatly diminished. Here, we overcome this limitation via a sequence of laser pulses that transfers motional excitation from the atom of interest over to the refrigerant atom, from which it can be removed by laser cooling.
Our technique is based on “algorithmic cooling,” a well-established protocol for transferring heat from one part of a system to another, from which it can be removed by coupling to a bath. To demonstrate our method, we trap a two-ion crystal composed of a single beryllium ion (the refrigerant) and a highly charged argon ion (the target). Using tailored laser pulses, we cool the argon ion to less than —close to the motional ground state—reducing its temperature to one billionth of what it was when it was formed in a hot plasma of electrons and atoms, making it the coldest highly charged atom on Earth.
Since our technique is universal, applications extend across a multitude of fields, such as the development of ultraprecise clocks based on trapped highly charged atoms and molecules, localization and motional control over macroscopic particles such as nanodiamonds, precision measurements of the properties of antiprotons, and improving the fidelity of quantum computation.
