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arXiv:2107.11802 · Physical Review A

Optimized pulsed sideband cooling and enhanced thermometry of trapped ions

A. J. Rasmusson, Marissa D’Onofrio, Yuanheng Xie, Jiafeng Cui, Philip Richerme

Published Oct 21, 2021• 19 citations
arXivHTMLPDF

Abstract

Resolved-sideband cooling is a workhorse technique for a wide range of experiments with trapped ions, atoms, or micromechanical oscillators in the quantum regime. In this work, the authors provide both a novel approach to effectively optimize the cooling sequences using graph theory, as well as a technique to measure the ion temperature more accurately. The procedure should be applicable to any trapped-ion experiment.

DOI
10.1103/physreva.104.043108

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Resolved-sideband cooling is a workhorse technique for a wide range of experiments with trapped ions, atoms, or micromechanical oscillators in the quantum regime. In this work, the authors provide both a novel approach to effectively optimize the cooling sequences using graph theory, as well as a technique to measure the ion temperature more accurately. The procedure should be applicable to any trapped-ion experiment.
Resolved sideband cooling is a standard technique for cooling trapped ions below the Doppler limit to near their motional ground state. Yet, the most common methods for sideband cooling implicitly rely on low Doppler-cooled temperatures and tightly confined ions, and they cannot be optimized for different experimental conditions. Here, we introduce a framework which calculates the fastest possible sideband cooling sequence for any set of experimental parameters and verify its improvement compared to traditional methods using a trapped 171^{171}171Yb+^++ ion. After extensive cooling, we find that the ion motional distribution is distinctly non-thermal and thus not amenable to standard thermometry techniques. We therefore develop and experimentally validate an improved method to measure ion temperatures after sideband cooling. These techniques will enable more efficient cooling and thermometry within any trapped-ion system, especially those with high initial temperatures or spatially-extended ion wavepackets.
Sideband cooling enables preparation of trapped ion motion near the ground state and is essential for many scientific and technological applications of trapped ion devices. Here, we study the efficiency of continuous and pulsed sideband cooling using both first- and second-order sidebands applied to an ion where the motion starts outside the Lamb-Dicke regime. We find that after optimizing these distinct cooling methods, pulsed and continuous cooling achieve similar results based on simulations and experiments with a 40^{40}40Ca+^++ ion. We consider optimization of both average phonon number n‾\overline{n}n and population in the ground state. We also demonstrate the disparity between n‾\overline{n}n as measured by the sideband ratio method of trapped ion thermometry and the n‾\overline{n}n found by averaging over the ion's motional state distribution.
Sideband cooling enables preparation of trapped ion motion near the ground state and is essential for many scientific and technological applications of trapped ion devices. Here, we study the efficiency of continuous and pulsed sideband cooling using both first- and second-order sidebands applied to an ion where the motion starts outside the Lamb-Dicke regime. We find that after optimizing these distinct cooling methods, pulsed and continuous cooling achieve similar results based on simulations and experiments with a 40Ca+^{40}\mathrm{Ca}^{+}40Ca+ ion. We consider optimization of both average phonon number n‾\overline{n}n and population in the ground state. We also demonstrate the disparity between n‾\overline{n}n as measured by the sideband ratio method of trapped ion thermometry and the n‾\overline{n}n found by averaging over the ion's motional state distribution.
Coulomb crystals of cold trapped ions are a leading platform for the realization of quantum processors and quantum simulations and, in quantum metrology, for the construction of optical atomic clocks and for fundamental tests of the standard model. For these applications, it is not only essential to cool the ion crystal in all its degrees of freedom down to the quantum ground state but also to be able to determine its temperature with a high accuracy. However, when a large ground-state cooled crystal is interrogated for thermometry, complex many-body interactions take place, making it challenging to accurately estimate the temperature with established techniques. In this work, we present a new thermometry method tailored for ion crystals. The method is applicable to all normal modes of motion and does not suffer from a computational bottleneck when applied to large ion crystals. We test the temperature estimate with two experiments, namely with a one-dimensional linear chain of four ions and a two-dimensional crystal of 19 ions and verify the results, where possible, using other methods. The results show that the new method is an accurate and efficient tool for thermometry of ion crystals.
Coulomb crystals of cold trapped ions are a leading platform for the realization of quantum processors and quantum simulations and, in quantum metrology, for the construction of optical atomic clocks and for fundamental tests of the standard model.For these applications, it is not only essential to cool the ion crystal in all its degrees of freedom down to the quantum ground state but also to be able to determine its temperature with a high accuracy.However, when a large ground-state cooled crystal is interrogated for thermometry, complex many-body interactions take place, making it challenging to accurately estimate the temperature with established techniques.In this work, we present a new thermometry method tailored for ion crystals.The method is applicable to all normal modes of motion and does not suffer from a computational bottleneck when applied to large ion crystals.We test the temperature estimate with two experiments, namely with a one-dimensional linear chain of four ions and a two-dimensional crystal of 19 ions and verify the results, where possible, using other methods.The results show that the new method is an accurate and efficient tool for thermometry of ion crystals.
Coulomb crystals of cold trapped ions are a leading platform for the realisation of quantum processors and quantum simulations and, in quantum metrology, for the construction of optical atomic clocks and for fundamental tests of the Standard Model. For these applications, it is not only essential to cool the ion crystal in all its degrees of freedom down to the quantum ground state, but also to be able to determine its temperature with a high accuracy. However, when a large ground-state cooled crystal is interrogated for thermometry, complex many-body interactions take place, making it challenging to accurately estimate the temperature with established techniques. In this work we present a new thermometry method tailored for ion crystals. The method is applicable to all normal modes of motion and does not suffer from a computational bottleneck when applied to large ion crystals. We test the temperature estimate with two experiments, namely with a 1D linear chain of 4 ions and a 2D crystal of 19 ions and verify the results, where possible, using other methods. The results show that the new method is an accurate and efficient tool for thermometry of ion crystals.