Recent theoretical investigations have indicated that rapid optical cycling should be feasible in complex polyatomic molecules with diverse constituents, geometries and symmetries. However, as a composite molecular mass grows, so does the required number of photon scattering events necessary to decelerate and confine molecular beams using laser light. Utilizing coherent momentum exchange between light fields and molecules can suppress spontaneous emission and significantly reduce experimental complexity for slowing and trapping. Working with BaH as a test species, we have identified a robust, experimentally viable configuration to achieve large molasses-like cooling forces for molecules using polychromatic optical fields addressing both $X-A$ and $X-B$ electronic transitions, simultaneously. Using numerical solutions of the time-dependent density matrix as well as Monte Carlo simulations, we demonstrate that creation of Suppressed Emission Rate (SupER) molasses with large capture velocities ($\sim 40$ m/s) is generically feasible for polyatomic molecules of increasing complexity that have an optical cycling center. Proposed SupER molasses are anticipated to not only extend quantum control to novel molecular species with abundant vibrational decay channels, but also significantly increase trapped densities for previously laser-cooled diatomic and triatomic species.
We present a 3D theoretical comparison between the radiation-pressure forces exerted on an atom in an isotropic light cooling scheme and in a six-beam molasses. We demonstrate that, in the case of a background vapor where all the space directions of the atomic motion have to be considered, the mean cooling rate is equal in both configurations. Nevertheless, we also point out what mainly differentiates the two cooling techniques: the force component orthogonal to the atomic motion. If this transverse force is always null in the isotropic light case, it can exceed the radiation-pressure-force lo…
We present an efficient scheme to implement a gray optical molasses for sub-Doppler cooling of $^{6}$Li atoms with minimum experimental overhead. To integrate the $D_1$ light for the gray molasses (GM) cooling into the same optical setup that is used for the $D_2$ light for a standard magneto-optical trap (MOT), we rapidly switch the injection seeding of a slave laser between the $D_2$ and $D_1$ light sources. Switching times as short as $30\,μ\textrm{s}$ can be achieved, inferred from monitor optical beat signals. The resulting low-intensity molasses cools a sample of $N=9\times10^8$ atoms to…
We report on a scheme to cool and compress trapped clouds of highly magnetic 52Cr atoms. This scheme combines sequences of gray molasses, which freeze the velocity distribution, and free evolutions in the (close to) harmonic trap, which periodically exchange the spatial and velocity degrees of freedom. Taken together, the successive gray molasses pulses cool all external degrees of freedom, which leads to an increase of the phase-space density (PSD) by a factor of about 250, allowing to reach a high final PSD of about 1.7*10^-3. These experiments are performed within an optical dipole trap, in…
We systematically investigate the dependence of the temperature of cold cesium atoms of polarization gradient cooling (PGC) in optical molasses on experimental parameters, which contain changing modes of cooling laser, PGC interaction time, cooling laser frequency and its intensity. The SR mode of cooling laser, that means the cooling laser frequency is changed with step mode and cooling laser intensity is changed with ramp mode, is found to be the best for PGC comparing with other SS, RS, and RR modes. We introduce a statistical explanation and an exponential decay function to explain the var…
We report on the efficient gray molasses cooling of sodium atoms using the $D_{2}$ optical transition at 589.1 nm. Thanks to the hyperfine split about 6$Γ$ between the $|F'=2\rangle$ and $|F'=3\rangle$ in the excited state 3$^{2}P_{3/2}$, this atomic transition is effective for the gray molasses cooling mechanism. Using this cooling technique, the atomic sample in $F = 2$ ground manifold is cooled from 700 $\upmu$K to 56 $\upmu$K in 3.5 ms. We observe that the loading efficiency into magnetic trap is increased due to the lower temperature and high phase space density of atomic cloud after gray…
We demonstrate sub-Doppler cooling of 40K on the D_1 atomic transition. Using a gray molasses scheme, we efficiently cool a compressed cloud of 6.5x10^8 atoms from ~ 4\mK to 20uK in 8 ms. After transfer in a quadrupole magnetic trap, we measure a phase space density of ~10^-5. This technique offers a promising route for fast evaporation of fermionic 40K.
We present new techniques in cooling 39K atoms using laser light close to the D1 transition. First, a new compressed-MOT configuration is taking advantage of gray molasses type cooling induced by blue-detuned D1 light. It yields an optimized density of atoms. Then, we use pure D1 gray molasses to further cool the atoms to an ultra-low temperature of 6\,$μ$K. The resulting phase-space density is $2 \times 10^{-4}$ and will ease future experiments with ultracold potassium. As an example, we use it to directly load up to $3\times 10^7$ atoms in a far detuned optical trap, a result that opens the …
Laser cooling of translational motion of small molecules is performed by addressing transitions that ensure spontaneous emission cannot cause net rotational excitation. This will not be possible once the rotational splitting becomes comparable to the operational excitation linewidth, as will occur for large molecules or wide bandwidth lasers. We show theoretically that in this regime, angular momentum transfer from red-detuned Doppler cooling light can also exert a damping torque on linear molecules, cooling rotation to the same Doppler limit (typically $\approx$ 500 $μ$K for molecules with $\…
We report on the realization of sub-Doppler laser cooling of sodium atoms in gray molasses using the D1 optical transition ($3s\, ^2S_{1/2} \rightarrow 3p\, ^2P_{1/2}$) at 589.8 nm. The technique is applied to samples containing $3\times10^9$ atoms, previously cooled to 350 $μ$K in a magneto-optical trap, and it leads to temperatures as low as 9 $μ$K and phase-space densities in the range of $10^{-4}$. The capture efficiency of the gray molasses is larger than 2/3, and we observe no density-dependent heating for densities up to $10^{11}$ cm$^{-3}$.
We report enhanced sub-Doppler cooling of the bosonic atoms of $^{39}$K facilitated by formation of dark states tuned for the Raman resonance in the $Λ-$configuration near the D1 transition. Temperature of about 12 $μ$K is achieved in the two stage D2-D1 molasses and spans a very large parameter region where quantum interference persists robustly. We also present results on enhanced radiation heating with sub-natural linewidth (0.07$Γ$) and signature Fano like profile of a coherently driven 3-level atomic system. The Optical Bloch Equations relevant for the three-level atom in bichromatic ligh…
We present a systematic study on the Raman gray molasses cooling (RGMC) of cesium atoms on the $D_2$ transition. Due to the large splitting in the excited hyperfine transitions of cesium $D_2$ line, it is relatively simple to implement the RGMC with suitable frequency control on the trapping and repumping lasers of the magneto-optical trap (MOT). We have achieved an atom temperature of 1.7$\pm$0.2 $μ$K after the RGMC with $3.2\times 10^8$ atoms. The phase space density is $1.43\times 10^{-4}$. Compared to the condition with a bare MOT or the MOT with a standard polarization gradient cooling, t…
Gray molasses is a powerful tool for sub-Doppler laser cooling of atoms to low temperatures. For alkaline atoms, this technique is commonly implemented with cooling lasers which are blue-detuned from either the D1 or D2 line. Here we show that efficient gray molasses can be implemented on the D2 line of 40K with red-detuned lasers. We obtained temperatures of 48(2) microKelvin, which enables direct loading of 9.2(3)*10^6 atoms from a magneto-optical trap into an optical dipole trap. We support our findings by a one-dimensional model and three-dimensional numerical simulations of the optical Bl…
Robust cooling and nondestructive imaging are prerequisites for many emerging applications of neutral atoms trapped in optical tweezers, such as their use in quantum information science and analog quantum simulation. The tasks of cooling and imaging can be challenged, however, by the presence of large trap-induced shifts of their respective optical transitions. Here, we explore a system of $^{39}$K atoms trapped in a near-detuned ($780$ nm) optical tweezer, which leads to relatively minor differential (ground vs. excited state) Stark shifts. We demonstrate that simple and robust loading, cooli…
We report observation of sub-Doppler cooling of lithium using an irregular-tetrahedral laser beam arrangement, which is produced by a nanofabricated diffraction grating. We are able to capture 11(2) % of the lithium atoms from a grating magneto-optical trap into $Λ$-enhanced $D_1$ gray molasses. The molasses cools the captured atoms to a radial temperature of 60(9) $μ$K and an axial temperature of 23(3) $μ$K. In contrast to results from conventional counterpropagating beam configurations, we do not observe cooling when our optical fields are detuned from Raman resonance. An optical Bloch equat…
In this paper we experimentally and theoretically investigate laser cooling of Strontium 88 atoms in one dimensional optical molasses. In our case, since the optical cooling dipole transition involves a $J_g=0$ groundstate, no Sisyphus-type mechanisms can occur. We are thus able to test quantitatively the predictions of the Doppler-cooling theory. We have found, in agreement with other similar experiments, that the measured temperatures are systematically larger than the theoretical predictions. We quantitatively interpret this discrepancy by taking into consideration the extra-heating mechani…
Following the bichromatic sub-Doppler cooling scheme on the D1 line of 40K recently demonstrated in (Fernandes et al. 2012), we introduce a similar technique for 7Li atoms and obtain temperatures of 60 uK while capturing all of the 5x10^8 atoms present from the previous stage. We investigate the influence of the detuning between the the two cooling frequencies and observe a threefold decrease of the temperature when the Raman condition is fulfilled. We interpret this effect as arising from extra cooling due to long-lived coherences between hyperfine states. Solving the optical Bloch equations …
Laser cooling based on dark states, i.e. states decoupled from light, has proven to be effective to increase the phase-space density of cold trapped atoms. Dark-states cooling requires open atomic transitions, in contrast to the ordinary laser cooling used for example in magneto-optical traps (MOTs), which operate on closed atomic transitions. For alkali atoms, dark-states cooling is therefore commonly operated on the $D_1$ transition $n S_{1/2}\rightarrow n P_{1/2}$. We show that, for $^{87}\text{Rb}$, thanks to the large hyperfine structure separations the use of this transition is not stric…
We demonstrate the production of large $^7$Li Bose-Einstein condensates in an optical dipole trap using $D_1$ gray molasses. The sub-Doppler cooling technique reduces the temperature of $4\times10^9$ atoms to $25~μ$K in 3~ms. After microwave evaporation cooling in a magnetic quadrupole trap, we transfer the atoms to a crossed optical dipole trap, where we employ a magnetic Feshbach resonance on the $|F=1,m_F=1\rangle$ state. Fast evaporation cooling is achieved by tilting the optical potential using a magnetic field gradient on the top of the Feshbach field. Our setup produces pure condensates…
We use D1 gray molasses to achieve Bose-Einstein condensation of a large number of $^{41}$K atoms in an optical dipole trap. By combining a new configuration of compressed-MOT with D1 gray molasses, we obtain a cold sample of $2.4\times10^9$ atoms with a temperature as low as 42 $μ$K. After magnetically transferring the atoms into the final glass cell, we perform a two-stage evaporative cooling. A condensate with up to $1.2\times10^6$ atoms in the lowest Zeeman state $|F=1,m_F=1\rangle$ is achieved in the optical dipole trap. Furthermore, we observe two narrow Feshbach resonances in the lowest…
In ultracold-atom and ion experiments, flexible control of the direction and amplitude of a uniform magnetic field is necessary. It is achieved almost exclusively by controlling the current flowing through coils surrounding the experimental chamber. Here, we present the design and characterization of a modular, analog electronic circuit that enables three-dimensional control of a magnetic field via the amplitude and direction of a current flowing through three perpendicular pairs of coils. Each pair is controlled by one module, and we are able to continuously change the current flowing thoroug…
Many predictions of Doppler cooling theory of two-level atoms have never been verified in a three-dimensional geometry, including the celebrated minimum achievable temperature $\hbar Γ/2 k_B$, where $Γ$ is the transition linewidth. Here, we show that, despite their degenerate level structure, we can use Helium-4 atoms to achieve a situation in which these predictions can be verified. We make measurements of atomic temperatures, magneto-optical trap sizes, and the sensitivity of optical molasses to a power imbalance in the laser beams, finding excellent agreement with the Doppler theory. We sho…
Laser cooling to sub-Doppler temperatures by optical molasses is thought to be inhibited in atoms with unresolved, near-degenerate hyperfine structure in the excited state. We demonstrate that such cooling is possible in one to three dimensions, not only near the standard D2 line for laser cooling, but over a range extending to the D1 line. Via a combination of Sisyphus cooling followed by adiabatic expansion, we reach temperatures as low as 40 μK, which corresponds to atomic velocities a factor of 2.6 above the limit imposed by a single photon recoil. Our method requires modest laser power at…
We study theoretically the behavior of laser-cooled calcium monofluoride (CaF) molecules in an optical molasses and magneto-optical trap (MOT), and compare our results to recent experiments. We use multi-level optical Bloch equations to estimate the force and the diffusion constant, followed by a Fokker-Planck-Kramers equation to calculate the time-evolution of the velocity distribution. The calculations are done in three-dimensions, and we include all the relevant energy levels of the molecule and all the relevant frequency components of the light. Similar to simpler model systems, the veloci…
We show that with a purely blue-detuned cooling mechanism we can densely load single neutral atoms into large arrays of shallow optical tweezers. With this ability, more efficient assembly of larger ordered arrays will be possible - hence expanding the number of particles available for bottom-up quantum simulation and computation with atoms. Using Lambda-enhanced grey molasses on the D1 line of 87Rb, we achieve loading into a single 0.63 mK trap with 89% probability, and we further extend this loading to 100 atoms at 80% probability. The loading behavior agrees with a model of consecutive ligh…
We report magneto-optical trapping (MOT) of a polyatomic molecule, calcium monohydroxide (CaOH). The MOT contains $2.0(5)\times 10^4$ CaOH molecules at a peak density of $3.0(8)\times10^{6}$ cm$^{-3}$. CaOH molecules are further sub-Doppler laser cooled in an optical molasses, to a temperature of 110(4) $μ$K. The temperatures and densities achieved here make CaOH a viable candidate for a wide variety of quantum science applications, including the creation of optical tweezer arrays of CaOH molecules. This work also suggests that laser cooling and magneto-optical trapping of many other polyatomi…
We report on simultaneous sub-Doppler laser cooling of fermionic $^6$Li and $^{40}$K using the D$_1$ optical transitions. We compare experimental results to a numerical simulation of the cooling process applying a semi-classical Monte Carlo wavefunction method. The simulation takes into account the three dimensional optical molasses setup and the dipole interaction between atoms and the bichromatic light field driving the D$_1$ transitions. We discuss the physical mechanisms at play, we identify the important role of coherences between the ground state hyperfine levels and compare D$_1$ and D$…
The grating magneto-optical trap (GMOT) requires only one beam and three planar diffraction gratings to form a cloud of cold atoms above the plane of the diffractors. Despite the complicated polarization arrangement, we demonstrate sub-Doppler cooling of 87Rb atoms to a temperature of 7.6(0.6) uK through a multi-stage, far-detuned GMOT in conjunction with optical molasses. A decomposition of the electric field into polarization components for this geometry does not yield a mapping onto standard sub-Doppler laser-cooling configurations. With numerical simulations, we find that the polarization …
Complex molecular structure demands customized solutions to laser cooling by extending its general set of principles and practices. Yttrium monoxide (YO) has unique intramolecular interactions. The Fermi-contact interaction dominates over the spin-rotation coupling, resulting in two manifolds of closely spaced states, with one of them possessing a negligible Landé g-factor. This unique energy level structure favors dual-frequency DC magneto-optical trapping (MOT) and gray molasses cooling (GMC). We report exceptionally robust cooling of YO at 4 $μ$K over a wide range of laser intensity, detuni…
We demonstrate in situ fluorescence detection of $^7$Li atoms in a 1D optical lattice with single atom precision. Even though illuminated lithium atoms tend to boil out, when the lattice is deep, molasses beams without extra cooling retain the atoms while producing sufficient fluorescent photons for detection. When the depth of the potential well at an antinode is 2.4 mK, an atom remains trapped for 30 s while scattering probe photons at the rate of $1.7 \times 10^5$ s$^{-1}$. We propose a simple model that describes the dependence of the lifetime of an atom on well depth. When the number of t…
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) $μ$K. This approach has produced a trapped sample of 1200 molecules, with a peak spatial density of $\sim1.2\times10^{10}$ cm$^{-3}$, and a peak phase-space density of $\sim3.1\times10^{-6}$. By adiabatically ramping down the lattice depth, we cool the molecules further to 1.0(2) $μ$K, twenty times colder than previously reported for laser-cooled molecule…
We have studied the effects of loading $^{87}$Rb into a far off resonant trap (FORT) in the presence of an ultracold cloud of $^{85}$Rb. The presence of the $^{85}$Rb resulted in a marked decrease of the $^{87}$Rb load rate. This decrease is consistent with a decrease in the laser cooling efficiency needed for effective loading. While many dynamics which disrupt loading efficency arise when cooling in a dense cloud of atoms (reabsorption, adverse optical pumping, etc.), the large detuning between the transitions of $^{85}$Rb and $^{87}$Rb should isolate the isotopes from these effects. For our…
We report the all-optical production of Bose Einstein condensates (BEC) of $^{39}$K atoms. We directly load $3 \times 10^{7}$ atoms in a large volume optical dipole trap from gray molasses on the D1 transition. We then apply a small magnetic quadrupole field to polarize the sample before transferring the atoms in a tightly confining optical trap. Evaporative cooling is finally performed close to a Feshbach resonance to enhance the scattering length. Our setup allows to cross the BEC threshold with $3 \times 10^5$ atoms every 7s. As an illustration of the interest of the tunability of the inter…
We report on an apparatus for cooling and trapping of neutral dysprosium. We characterize and optimize the performance of our Zeeman slower and 2D molasses cooling of the atomic beam by means of Doppler spectroscopy on a 136 kHz broad transition at 626 nm. Furthermore, we demonstrate the characterization and optimization procedure for the loading phase of a magneto-optical trap (MOT) by increasing the effective laser linewidth by sideband modulation. After optimization of the MOT compression phase, we cool and trap up to $10^9$ atoms within 3 seconds in the MOT at temperatures of 9 μK and phas…
The ability to cool atoms below the Doppler limit -- the minimum temperature reachable by Doppler cooling -- has been essential to most experiments with quantum degenerate gases, optical lattices and atomic fountains, among many other applications. A broad set of new applications await ultracold molecules, and the extension of laser cooling to molecules has begun. A molecular magneto-optical trap has been demonstrated, where molecules approached the Doppler limit. However, the sub-Doppler temperatures required for most applications have not yet been reached. Here we cool molecules to 50 uK, we…
Recent work with laser-cooled molecules in attractive optical traps has shown that the differential AC Stark shifts arising from the trap light itself can become problematic, limiting collisional shielding efficiencies, rotational coherence times, and laser-cooling temperatures. In this work, we explore trapping and laser-cooling of CaF molecules in a ring-shaped repulsive optical trap. The observed dependences of loss rates on temperature and barrier height show characteristic behavior of repulsive traps and indicate strongly suppressed average AC Stark shifts. Within the trap, we find that $…
We report on the transverse laser cooling of a magnetically guided beam of ultra cold chromium atoms. Radial compression by a tapering of the guide is employed to adiabatically heat the beam. Inside the tapered section heat is extracted from the atom beam by a two-dimensional optical molasses perpendicular to it, resulting in a significant increase of atomic phase space density. A magnetic offset field is applied to prevent optical pumping to untrapped states. Our results demonstrate that by a suitable choice of the magnetic offset field, the cooling beam intensity and detuning, atom losses an…
We report non-destructive imaging of optically trapped calcium monofluoride (CaF) molecules using in-situ $Λ$-enhanced gray molasses cooling. $200$ times more fluorescence is obtained compared to destructive on-resonance imaging, and the trapped molecules remain at a temperature of $20\,μ\text{K}$. The achieved number of scattered photons makes possible non-destructive single-shot detection of single molecules with high fidelity.
We report on the all-optical production of Bose-Einstein condensates in microgravity using a combination of grey molasses cooling, light-shift engineering and optical trapping in a painted potential. Forced evaporative cooling in a 3-m high Einstein elevator results in $4 \times 10^4$ condensed atoms every 13.5 s, with a temperature as low as 35 nK. In this system, the atomic cloud can expand in weightlessness for up to 400 ms, paving the way for atom interferometry experiments with extended interrogation times and studies of ultra-cold matter physics at low energies on ground or in Space.
We demonstrate sub-Doppler laser cooling and magneto-optical trapping of the rare earth element Holmium. Atoms are loaded from an atomic beam source and captured in six-beam $σ_+ - σ_-$ molasses using a strong $J=15/2 \leftrightarrow J=17/2$ cycling transition at $λ=410.5 \rm nm$. Due to the small difference in hyperfine splittings and Landé $g$-factors in the lower and upper levels of the cooling transition the MOT is self-repumped without additional repump light, and deep sub-Doppler cooling is achieved with the magnetic trap turned on. We measure the leakage out of the cycling transition to…
PyLCP: A python package for computing laser cooling physics
National Institute of Standards and Technology（美国国家标准与技术研究院）；Joint Quantum Institute, National Institute of Standards and Technology and University of Maryland（国家量子研究所（由美国国家标准与技术研究院和马里兰大学联合建立））
We demonstrate site-resolved imaging of individual bosonic $^{174}\mathrm{Yb}$ atoms in a Hubbard-regime two-dimensional optical lattice with a short lattice constant of 266 nm. To suppress the heating by probe light with the $^1S_0$-$^1P_1$ transition of the wavelength $λ$ = 399 nm for high-resolution imaging and preserve atoms at the same lattice sites during the fluorescence imaging, we simultaneously cool atoms by additionally applying narrow-line optical molasses with the $^1S_0$-$^3P_1$ transition of the wavelength $λ$ = 556 nm. We achieve a low temperature of $T = 7.4(1.3)\ μ\mathrm{K}$…
The problem of deep laser cooling of $^{24}$Mg atoms is theoretically studied. We propose two-stage sub-Doppler cooling strategy using electro-dipole transition $3^3P_2$$\to$$3^3D_3$ ($λ$=383.9 nm). The first stage implies exploiting magneto-optical trap with $σ^+$ and $σ^-$ light beams, while the second one uses a lin$\perp$lin molasses. We focus on achieving large number of ultracold atoms (T$_{eff}$ < 10 $μ$K) in a cold atomic cloud. The calculations have been done out of many widely used approximations and based on quantum treatment with taking full account of recoil effect. Steady-state a…
Simultaneous sub-Doppler laser cooling and optical trapping of bosonic $^{39}$K-$^{133}$Cs and $^{41}$K-$^{133}$Cs mixtures
Institute of Experimental Physics, University of Warsaw（华沙大学实验物理研究所）；Durham University（杜伦大学）；Stockholm University（斯德哥尔摩大学）；eleQtron GmbH（eleQtron有限公司）；Warsaw University of Technology（华沙理工大学）
