Indirect Cooling of Weakly Coupled Trapped-Ion Mechanical Oscillators

TL;DR

This paper introduces a method for indirectly cooling weakly coupled motional modes in trapped-ion crystals by coupling them to strongly cooled modes via parametric modulation, enabling near-ground-state cooling in complex multi-ion systems.

Contribution

The authors demonstrate a novel indirect cooling technique for weakly coupled motional modes in multi-ion crystals using parametric modulation, applicable to various ion species and complex systems.

Findings

Achieved near-ground-state cooling of weakly coupled modes.

Successfully applied to multi-ion crystals with different species.

Method is broadly applicable to complex Coulomb crystals.

Abstract

Cooling the motion of trapped ions to near the quantum ground state is crucial for many applications in quantum information processing and quantum metrology. However, certain motional modes of trapped-ion crystals can be difficult to cool due to weak or zero interaction between the modes and the cooling radiation, typically laser beams. We overcome this challenge by coupling a mode with weak cooling radiation interaction to one with strong cooling radiation interaction using parametric modulation of the trapping potential, thereby enabling indirect cooling of the former. In this way, we demonstrate near-ground-state cooling of motional modes with weak or zero cooling radiation interaction in multi-ion crystals of the same and mixed ion species, specifically $^9$Be$^+$-$^9$Be$^+$, $^9$Be$^+$-$^{25}$Mg$^+$, and $^9$Be$^+$-$^{25}$Mg$^+$-$^9$Be$^+$ crystals. This approach can be generally applied to any Coulomb crystal where certain motional modes cannot be directly cooled efficiently, including crystals containing molecular ions, highly-charged ions, charged fundamental particles, or charged macroscopic objects.