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arXiv:1609.02254 · Physical Review Letters

Sisyphus Laser Cooling of a Polyatomic Molecule

Ivan Kozyryev, Louis Baum, Kyle Matsuda, Benjamin L. Augenbraun, Loïc Anderegg, Alexander P. Sedlack, John M. Doyle

Published Apr 24, 2017• 259 citations
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Abstract

We perform magnetically assisted Sisyphus laser cooling of the triatomic free radical strontium monohydroxide (SrOH). This is achieved with principal optical cycling in the rotationally closed P(N^{''}=1) branch of either the X[over ˜]^{2}Σ^{+}(000)↔A[over ˜]^{2}Π_{1/2}(000) or the X[over ˜]^{2}Σ^{+}(000)↔B[over ˜]^{2}Σ^{+}(000) vibronic transitions. Molecules lost into the excited vibrational states during the cooling process are repumped back through the B[over ˜](000) state for both the (100) level of the Sr-O stretching mode and the (02^{0}0) level of the bending mode. The transverse temperature of a SrOH molecular beam is reduced in one dimension by 2 orders of magnitude to ∼750 μK. This approach opens a path towards creating a variety of ultracold polyatomic molecules by means of direct laser cooling.

DOI
10.1103/physrevlett.118.173201

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Molecules containing three atoms have been laser-cooled to ultracold temperatures for the first time. John Doyle and colleagues at Harvard University in the US used a technique called Sisyphus cooling to chill an ensemble of about a million strontium-monohydroxide molecules to 750 μK.

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We perform magnetically assisted Sisyphus laser cooling of the triatomic free radical strontium monohydroxide (SrOH). This is achieved with principal optical cycling in the rotationally closed P(N^{''}=1) branch of either the X[over ˜]^{2}Σ^{+}(000)↔A[over ˜]^{2}Π_{1/2}(000) or the X[over ˜]^{2}Σ^{+}(000)↔B[over ˜]^{2}Σ^{+}(000) vibronic transitions. Molecules lost into the excited vibrational states during the cooling process are repumped back through the Bover ˜ state for both the (100) level of the Sr-O stretching mode and the (02^{0}0) level of the bending mode. The transverse temperature of a SrOH molecular beam is reduced in one dimension by 2 orders of magnitude to ∼750 μK. This approach opens a path towards creating a variety of ultracold polyatomic molecules by means of direct laser cooling.
We perform magnetically assisted Sisyphus laser cooling of the triatomic free radical strontium monohydroxide (SrOH).This is achieved with principal optical cycling in the rotationally closed PðN 00 ¼ 1Þ branch of either the X2 Σ þ ð000Þ ↔ Ã2 Π 1=2 ð000Þ or the X2 Σ þ ð000Þ ↔ B2 Σ þ ð000Þ vibronic transitions.Molecules lost into the excited vibrational states during the cooling process are repumped back through the Bð000Þ state for both the (100) level of the Sr-O stretching mode and the ð02 0 0Þ level of the bending mode.The transverse temperature of a SrOH molecular beam is reduced in one dimension by 2 orders of magnitude to ∼750 μK.This approach opens a path towards creating a variety of ultracold polyatomic molecules by means of direct laser cooling.
We demonstrate a two-dimensional Sisyphus laser cooling method that increases the number of strontium monohydroxide (SrOH) molecules loaded into a magneto-optical trap by a factor of 12. Subsequent loading into an optical dipole trap (ODT) achieves 2.2(3)×1042.2 (3)\times10^42.2(3)×104 ultracold SrOH molecules with a peak density of ∼2(1)×1010 cm−3\sim2(1)\times10^{10}~\mathrm{cm^{-3}}∼2(1)×1010 cm−3. The lifetime of molecules in the ODT is limited by two-body collisions characterized by a measured collision rate constant β∼4×10−10 cm3/sβ\sim 4\times10^{-10}~\mathrm{cm^3/s}β∼4×10−10 cm3/s. The cooling method developed here is generally applicable to all known cases of direct molecular laser cooling, including symmetric and asymmetric top molecules. Increases in trapped molecule number will directly improve the search for ultralight dark matter, position polyatomic molecules as a platform for probing CP-violating new particles with masses $\gg$10 TeV, and facilitate a broad range of further research in quantum science.
Abstract We demonstrate multiple photon cycling and radiative force deflection on the triatomic free radical strontium monohydroxide (SrOH). Optical cycling is achieved on SrOH in a cryogenic buffer-gas beam by employing the rotationally closed P ( N ″ = 1 ) branch of the vibronic transition X ˜ 2 Σ + ( 000 ) ↔ A ˜ 2 Π 1 / 2 ( 000 ) . A single repumping laser excites the Sr–O stretching vibrational mode, and photon cycling of the molecule deflects the SrOH beam by an angle of 0.2 ° via scattering of ∼100 photons per molecule. This approach can be used for direct laser cooling of SrOH and more complex, isoelectronic species.
Polyatomic molecules, while ubiquitous in nature, were generally perceived as too complicated and "unruly" for the majority of quantum physics experiments. However, recent theoretical analyses have pinpointed the multiple advantages of using complex molecules for diverse applications such as the creation of a universal quantum computer, quantum simulation of non-trivial many-body states, and tests of fundamental physical laws. Throughout my dissertation research I focused on extending experimental techniques from atomic physics to complex molecules. The goal of my work was to not only demonstrate new ways to control molecules, but also develop a novel experimental platform for quantum science - complex polyatomic molecules - and provide tools for physicists to conquer new frontiers in physics and chemistry.\nThe main theme of this thesis is the use of laser radiation to control internal and external degrees of freedom for the triatomic radical strontium monohydroxide (SrOH). The research achievements presented here can be primarily divided into the following areas:\ni) out-of-equilibrium studies of vibrationally inelastic collisions at cryogenic temperatures,\nii) demonstration of the radiation pressure force and the direct laser cooling of polyatomic molecules, and\niii) the development of concepts for laser cooling of molecules with six and more atoms.\nFor the collisional studies, off-diagonal optical pumping was used to populate excited Sr-O stretching state and the ratio of elastic to vibrational quenching collisions was determined to be ~700. In the laser cooling work, using only laser light, the transverse temperature of the cryogenic buffer-gas beam of SrOH was reduced from 50 mK to below 1 mK, leading to an order of magnitude increase in phase space density of the molecular beam. Additionally, the prospects of using a coherent bichromatic force for optical deceleration of polyatomics were analyzed and experimentally characterized with the transverse deflection of the SrOH beam. Our experimental results with SrOH and theoretical analysis of more complex strontium alkoxides not only open a wide range of future directions for laser manipulation and trapping of polyatomic molecules but also help overturn long held assumptions about the difficulties of performing "traditional" atomic physics experiments with complex molecules.
Abstract Recently, a few diatomic and polyatomic molecules have been identified as prospective systems for Doppler/Sisyphus cooling. Doppler/Sisyphus cooling allows us to decrease the kinetic energy of molecules down to μ K temperatures with high efficiency and allows their capture in molecular traps, including magneto-optical traps. Trapped molecules can be used for creating molecular fountains and/or performing controlled chemical reactions, high-precision spectra measurements, and a wealth of other applications. Polyatomic molecules with heavy nuclei are of considerable interest for the ‘new physics’ search outside of the Standard Model and other applications including cold chemistry and photochemistry, quantum informatics, etc. Here, we focus on the radium monohydroxide molecule (RaOH), which is on the one hand amenable to laser cooling and on the other hand opens excellent possibilities for  -odd and  ,  -odd effects research. At present, RaOH is the heaviest polyatomic molecule proposed for direct cooling with lasers.