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Physical Review Letters

Doppler Cooling Trapped Ions with a UV Frequency Comb

Josue Davila-Rodriguez, Akira Ozawa, Theodor W. Hänsch, Thomas Udem

Published Jan 26, 2016• 26 citations
Publisher

Abstract

We demonstrate Doppler cooling of trapped magnesium ions using a frequency comb at 280 nm obtained from a frequency tripled Ti:sapphire laser. A comb line cools on the 3s_{1/2}-3p_{3/2} transition, while the nearest blue-detuned comb line contributes negligible heating. We observe the cooling-heating transition and long-term cooling of ion chains with several sympathetically cooled ions. Spatial thermometry shows that the ion is cooled to near the Doppler limit. Doppler cooling with frequency combs has the potential to open many additional atomic species to laser cooling by reaching further into the vacuum and extreme ultraviolet via high-harmonic generation and by providing a broad bandwidth from which multiple excitation sidebands can be obtained.

DOI
10.1103/physrevlett.116.043002

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We demonstrate Doppler cooling of trapped magnesium ions using a frequency comb at 280 nm obtained from a frequency tripled Ti:sapphire laser. A comb line cools on the 3s_{1/2}-3p_{3/2} transition, while the nearest blue-detuned comb line contributes negligible heating. We observe the cooling-heating transition and long-term cooling of ion chains with several sympathetically cooled ions. Spatial thermometry shows that the ion is cooled to near the Doppler limit. Doppler cooling with frequency combs has the potential to open many additional atomic species to laser cooling by reaching further into the vacuum and extreme ultraviolet via high-harmonic generation and by providing a broad bandwidth from which multiple excitation sidebands can be obtained.
We demonstrate the use of a frequency-doubled optical frequency comb to load, cool, and crystallize trapped atomic ions as an alternative to ultraviolet (UV) or even deep UV continuous-wave lasers. We find that the Doppler shift from the atom's oscillation in the trap, driven by the blue-detuned comb teeth, introduces additional cooling and amplification which gives rise to steady-state phonon lasing of the ion's harmonic motion in the trap. The phonon laser's gain saturation keeps the optical frequency comb from continually adding energy without bound. This protection allows us to demonstrate loading and crystallization of hot ions directly with the comb, eliminating the need for a continuous-wave cooling laser, a technique that is extendable to the deep UV.
We report on laser cooling of neutral rubidium atoms by using a single mode of a frequency comb. Cooling is achieved on a dipole-allowed transition at 780 nm in a one-dimensional retro-reflected beam geometry. Temperatures are measured using standard time-of-flight imaging. We show the dependence of the temperature on the cooling time, intensity and detuning of the frequency comb. The lowest temperature achieved is approximately equal to the Doppler temperature and is limited by the intensity of the comb mode driving the cooling transition. Additionally, we verify the analogy between frequency comb and continuous-wave laser cooling. Our work is a step towards laser cooling of atoms with strong cycling transitions in the vacuum ultraviolet, such as hydrogen, deuterium and antihydrogen, where generation of continuous-wave laser light is limited by current laser technology. Achieving efficient cooling at these wavelengths would significantly improve the precision of optical frequency standards, enable measurements of fundamental constants with unprecedented accuracy, improve tests of charge, parity, and time reversal symmetry, and open the way to achieving quantum degeneracy width new atomic species.
An experimental study of sideband laser cooling of trapped ions in miniature radio-frequency traps is presented. In+{\mathrm{In}}^{+}In+ ions are laser cooled by exciting the narrow intercombination line ${5s}^{2}{}^{1}{S}_{0}\ensuremath{\rightarrow}5s5p{}^{3}{P}_{1}$, which shows resolved sidebands due to the vibrational motion of the ions in the trap. The influence of the micromotion at the frequency of the oscillating trapping field is investigated. Heating processes through coupling with the trapping field occur for certain regions of negative laser detuning. A method of bichromatic sideband cooling is introduced that combines efficient cooling from high temperatures with the possibility of reaching low mean vibrational quantum numbers 〈n〉<1〈n〉<1〈n〉<1 by using narrow optical transitions. This technique allows recording of high-resolution excitation spectra of the cooling transition in the Lamb-Dicke regime and thus determination of 〈n〉〈n〉〈n〉. Single ions as well as two-ion Coulomb crystals have been cooled to temperatures below $100 \ensuremath{\mu}\mathrm{K}$, and population of the ground state of motion to more than 50% over long time scales is demonstrated. These values are close to the lower limit for the detection of vibrational excitation with the present method.
Trapped atomic ions are a multifaceted platform that can serve as a quantum information processor, precision measurement tool and sensor. However, in order to perform these experiments, the trapped ions need to be cooled substantially below room temperature. Doppler cooling has been a tremendous work horse in the ion trapping community. Hydrogen-like ions are good candidates because they have typically have a simple closed cycling transition that requires only a few lasers to Doppler cool. The \\textsuperscript{2}S to \\textsuperscript{2}P transition for these ions however typically lies in the UV to deep UV regime which makes buying a standard CW laser difficult as optical power here is hard to come by. Rather using a CW laser, which requires frequency stabilization and produces low optical power, this thesis explores how a mode-locked laser in the comb regime Doppler cools Yb ions. This thesis explores how a mode-locked laser is able to laser cool trapped ions and the consequences of using a broad spectrum light source. I will first give an overview of the architecture of our oblate Paul trap. Then I will discuss how a 10 picosecond optical pulse with a repetition rate of 80 MHz interacts with a 20 MHz linewidth \\textsuperscript{2}S\\(_{1/2}\\) to \\textsuperscript{2}P\\(_{1/2}\\) transition. Next I will go over our experiment involving ultrafast Ramsey experiment between the \\textsuperscript{2}S\\(_{1/2}\\) and \\textsuperscript{2}P\\(_{1/2}\\) states. Finally, I will present the results of these experiments.
Optical frequency combs (FCs) have revolutionised ultra-high precision metrology by providing a link between optical and microwave frequencies. While FCs are traditionally used for calibrating a CW probe-laser, also direct frequency comb spectroscopy (DFCS) is possible. We have recently demonstrated the versatility of DFCS performed in the environment of an ion trap, combining the broad spectral range of a Ti: sapphire FC with the feasibility of trapping and (sympathetic) cooling of different ion species. In the presented contribution it is shown that by employing single-ion imaging, this technique can be extended even to dipole-forbidden transitions, despite the low power per unamplified comb mode of ~0.1 μW.
Cooling of atoms and ions using a red-detuned laser has had a profound impact on science and technology. In this work simultaneous laser cooling and blue-detuned laser pumping of a Mg+ ion in a Paul trap is studied. Blue-detuned pumping is conventionally referred to as the heating regime, and in early work, remarkably complex behaviors (bistability and limit cycles) have been associated with this regime. These behaviors have so far not been fully explained. Here, it is shown that blue-detuned pumping, as opposed to heating, causes stimulated emission of center-of-mass phonons, leading to coherent oscillatory motion of the ion in analogy with a laser. Mechanical amplification is calculated as well as the threshold pumping condition for oscillation. A single ion in a linear radio-frequency trap is studied to verify these predictions. Blue-detuned pumping of the magnesium D2 transition at 279.6 nm provides amplification along the long axis of the ion trap so as to excite only axial oscillations. A slightly off-axis, red-detuned beam cools the center-of-mass motion to approximately 1 mK.