No Arabic abstract
The last few years have seen rapid progress in the application of laser cooling to molecules. In this review, we examine what kinds of molecules can be laser cooled, how to design a suitable cooling scheme, and how the cooling can be understood and modelled. We review recent work on laser slowing, magneto-optical trapping, sub-Doppler cooling, and the confinement of molecules in conservative traps, with a focus on the fundamental principles of each technique. Finally, we explore some of the exciting applications of laser-cooled molecules that should be accessible in the near term.
We demonstrate coherent microwave control of the rotational, hyperfine and Zeeman states of ultracold CaF molecules, and the magnetic trapping of these molecules in a single, selectable quantum state. We trap about $5times 10^{3}$ molecules for 2 s at a temperature of 65(11) $mu$K and a density of $1.2 times 10^{5}$ cm$^{-3}$. We measure the state-specific loss rate due to collisions with background helium.
We report laser cooling and trapping of yttrium monoxide (YO) molecules in an optical lattice. We show that gray molasses cooling remains exceptionally efficient for YO molecules inside the lattice with a molecule temperature as low as 6.1(6) $mu$K. This approach has produced a trapped sample of 1200 molecules, with a peak spatial density of $sim1.2times10^{10}$ cm$^{-3}$, and a peak phase-space density of $sim3.1times10^{-6}$. By adiabatically ramping down the lattice depth, we cool the molecules further to 1.0(2) $mu$K, twenty times colder than previously reported for laser-cooled molecules in a trap.
We present first indications of sympathetic cooling between two neutral, optically trapped atomic species. Lithium and cesium atoms are simultaneously stored in an optical dipole trap formed by the focus of a CO$_2$ laser, and allowed to interact for a given period of time. The temperature of the lithium gas is found to decrease when in thermal contact with cold cesium. The timescale of thermalization yields an estimate for the Li-Cs cross-section.
Recently, laser cooling methods have been extended from atoms to molecules. The complex rotational and vibrational energy level structure of molecules makes laser cooling difficult, but these difficulties have been overcome and molecules have now been cooled to a few microkelvin and trapped for several seconds. This opens many possibilities for applications in quantum science and technology, controlled chemistry, and tests of fundamental physics. This article explains how molecules can be decelerated, cooled and trapped using laser light, reviews the progress made in recent years, and outlines some future applications.
We demonstrate cryogenic buffer-gas cooling of gas-phase methyltrioxorhenium (MTO). This molecule is closely related to chiral organometallic molecules where the parity-violating energy differences between enantiomers may be measurable. The molecules are produced with a rotational temperature of approximately 6~K by laser ablation of an MTO pellet inside a cryogenic helium buffer gas cell. Facilitated by the low temperature, we demonstrate absorption spectroscopy of the 10.2~$mu$m antisymmetric Re=O stretching mode of MTO with a resolution of 8~MHz and a frequency accuracy of 30~MHz. We partially resolve the hyperfine structure and measure the nuclear quadrupole coupling of the excited vibrational state.