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arXiv:2102.12427 · Physical Review X

Algorithmic Ground-State Cooling of Weakly Coupled Oscillators Using Quantum Logic

S. A. King, Lukas J. Spieß, P. Micke, Alexander Wilzewski, Tobias Leopold, J. R. Crespo López-Urrutia, Piet O. Schmidt

Published Dec 10, 2021• 25 citations
arXivHTMLPDF

Abstract

A new approach to laser cooling vastly extends this technique to many more species and even macroscopic objects, as demonstrated by cooling a highly charged ion to under 200 $\ensuremath{\mu}$K---close to the quantum-mechanical ground state.

DOI
10.1103/physrevx.11.041049

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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 9^99Be+^++-9^99Be+^++, 9^99Be+^++-25^{25}25Mg+^++, and 9^99Be+^++-25^{25}25Mg+^++-9^99Be+^++ 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.
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Abstract Efficient cooling of trapped charged particles is essential in many fundamental physics experiments, for high-precision metrology, and for quantum technology. Until now, ion-ion coupling for sympathetic cooling or quantum state control has been limited to ion species with accessible optical transitions or has required close-range Coulomb interactions. To overcome this limitation and further develop scalable quantum control techniques, there has been a sustained desire to extend laser-cooling techniques to particles in macroscopically separated traps, opening quantum control techniques to previously inaccessible particles such as highly charged ions, molecular ions, and antimatter particles. Here, we demonstrate sympathetic cooling of a single proton by laser cooled Be+ ions stored in a spatially separated Penning trap. The two traps are connected by a superconducting LC circuit that enables energy exchange over a distance of 9 cm. We simultaneously demonstrate the cooling of a resonant mode of a macroscopic LC circuit with laser-cooled ions and sympathetic cooling of an individually trapped proton, reaching temperatures far below the environment temperature. Importantly, as this technique does not rely on the direct Coulomb interaction but rather on image-current interactions, it can be easily applied to an experiment with antiprotons, facilitating improved precision in matter-antimatter comparisons and dark matter searches.