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Quantum degenerate mixtures of 23Na-39K and coherent transfer paths in NaK molecules

Torben Schulze

Published Jan 1, 2018• 5 citations
Publisher

Abstract

The discovery and development of methods to cool, trap and manipulate atomic ensembles have generated a revolution in atomic physics, culminating in the ever expanding research field of ultra-cold matter through the realization of Bose-Einstein condensates (BECs). Since the first realization of BEC more than two decades ago, the majority of studies were concerned with systems displaying short-range contact interaction. Following pioneering experiments in ultra-cold KRb, systems involving ultra-cold mixtures have rose to considerable interest through their capability to form diatomic polar molecules. The long-range character of dipolar particle interactions enables the study of a whole new spectrum of quantum many-body phenomena by giving access to the strongly correlated regime. In this context, NaK is a splendid candidate to investigate dipolar effects due to its large electric dipole moment of $2.72 \, $Debye, its chemical stability and the large history of spectroscopic studies devoted to its ground and excited state manifolds. This thesis reports on the first ever realization of a dual-species degenerate mixture of $^{23}\mathrm{Na}$ and $^{39}\mathrm{K}$. The experimental apparatus combines two pre-cooled atomic sources into a UHV collection region, where a two-color magneto-optical trap is operated. After transferring both species into an optically plugged quadrupole trap, the $^{23}\mathrm{Na}$ ensemble is cooled by selectively removing the hottest atoms from the trap through microwave transitions, while $^{39}\mathrm{K}$ is sympathetically cooled through its thermal contact to $^{23}\mathrm{Na}$. Following the transfer into an optical dipole trap, the mixture operation suffers from strong losses as the atomic clouds approach the high-density regime. Interspecies interactions are identified as the system parameter that drives the loss mechanism. In order to realize a quantum degenerate mixture, the dual-species collisional properties are investigated both theoretically and experimentally. By preparing both optically trapped ensembles in the spin state $|f=1,m_{f}=-1\rangle$, atom loss spectroscopy is performed in a magnetic field range from $0$ to $1000 \, \mathrm{G}$. The observed features include several s-wave poles and a zero crossing of the interspecies scattering length as well as inelastic two-body contributions in the $\mathcal{M} = m_{\mathrm{Na}}+m_{\mathrm{K}} = -2$ submanifold. Different magnetic field regions are identified for the purposes of sympathetic cooling of $^{39}\mathrm{K}$ and achieving a quantum degenerate mixture. Forced evaporation creates two Bose-Einstein condensates simultaneously at a magnetic field that provides sizable intra- and interspecies scattering rates needed for fast thermalization. The impact of the differential gravitational sag on the miscibility criterion for the mixture is discussed. The experimental setup and measurements are complemented by theoretical calculations that form a feasibility study of molecular NaK. Starting from ultra-cold Feshbach molecules, the study demonstrates possible pathways for the creation of ultra-cold polar NaK molecules in their absolute electronic and rovibrational ground state. In particular, a multi-channel analysis of the electronic ground and K(4p)+Na(3s) excited state manifold of NaK is presented. It analyzes the spin character of both the Feshbach molecular state and the electronically excited intermediate states and discusses possible coherent two-photon transfer paths from Feshbach molecules to rovibronic ground state molecules. %The theoretical study is complemented by the demonstration of STIRAP transfer from the X1Σ+(v=0) state to the a3Σ+ manifold on a molecular beam experiment. The experimental results serve as a promising starting point for the magnetoassociation into quantum degenerate $^{23}\mathrm{Na}^{39}\mathrm{K}$ Feshbach molecules. The theoretical analysis assures qualitative understanding as well as quantitative statements for the feasibility of the subsequent ground state conversion. In combination, they fill a critical gap towards the creation of chemically stable ultra-cold molecular Bose-Einstein condensates of NaK.

DOI
10.15488/3429

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The discovery and development of methods to cool, trap and manipulate atomic ensembles have generated a revolution in atomic physics, culminating in the ever expanding research field of ultra-cold matter through the realization of Bose-Einstein condensates (BECs). Since the first realization of BEC more than two decades ago, the majority of studies were concerned with systems displaying short-range contact interaction. Following pioneering experiments in ultra-cold KRb, systems involving ultra-cold mixtures have rose to considerable interest through their capability to form diatomic polar molecules. The long-range character of dipolar particle interactions enables the study of a whole new spectrum of quantum many-body phenomena by giving access to the strongly correlated regime. In this context, NaK is a splendid candidate to investigate dipolar effects due to its large electric dipole moment of 2.72 2.72 \, 2.72Debye, its chemical stability and the large history of spectroscopic studies devoted to its ground and excited state manifolds. This thesis reports on the first ever realization of a dual-species degenerate mixture of 23Na^{23}\mathrm{Na}23Na and 39K^{39}\mathrm{K}39K. The experimental apparatus combines two pre-cooled atomic sources into a UHV collection region, where a two-color magneto-optical trap is operated. After transferring both species into an optically plugged quadrupole trap, the 23Na^{23}\mathrm{Na}23Na ensemble is cooled by selectively removing the hottest atoms from the trap through microwave transitions, while 39K^{39}\mathrm{K}39K is sympathetically cooled through its thermal contact to 23Na^{23}\mathrm{Na}23Na. Following the transfer into an optical dipole trap, the mixture operation suffers from strong losses as the atomic clouds approach the high-density regime. Interspecies interactions are identified as the system parameter that drives the loss mechanism. In order to realize a quantum degenerate mixture, the dual-species collisional properties are investigated both theoretically and experimentally. By preparing both optically trapped ensembles in the spin state ∣f=1,mf=−1⟩|f=1,m_{f}=-1\rangle∣f=1,mf​=−1⟩, atom loss spectroscopy is performed in a magnetic field range from 000 to 1000 G1000 \, \mathrm{G}1000G. The observed features include several s-wave poles and a zero crossing of the interspecies scattering length as well as inelastic two-body contributions in the M=mNa+mK=−2\mathcal{M} = m_{\mathrm{Na}}+m_{\mathrm{K}} = -2M=mNa​+mK​=−2 submanifold. Different magnetic field regions are identified for the purposes of sympathetic cooling of 39K^{39}\mathrm{K}39K and achieving a quantum degenerate mixture. Forced evaporation creates two Bose-Einstein condensates simultaneously at a magnetic field that provides sizable intra- and interspecies scattering rates needed for fast thermalization. The impact of the differential gravitational sag on the miscibility criterion for the mixture is discussed. The experimental setup and measurements are complemented by theoretical calculations that form a feasibility study of molecular NaK. Starting from ultra-cold Feshbach molecules, the study demonstrates possible pathways for the creation of ultra-cold polar NaK molecules in their absolute electronic and rovibrational ground state. In particular, a multi-channel analysis of the electronic ground and K(4p)+Na(3s) excited state manifold of NaK is presented. It analyzes the spin character of both the Feshbach molecular state and the electronically excited intermediate states and discusses possible coherent two-photon transfer paths from Feshbach molecules to rovibronic ground state molecules. %The theoretical study is complemented by the demonstration of STIRAP transfer from the X1Σ+(v=0) state to the a3Σ+ manifold on a molecular beam experiment. The experimental results serve as a promising starting point for the magnetoassociation into quantum degenerate 23Na39K^{23}\mathrm{Na}^{39}\mathrm{K}23Na39K Feshbach molecules. The theoretical analysis assures qualitative understanding as well as quantitative statements for the feasibility of the subsequent ground state conversion. In combination, they fill a critical gap towards the creation of chemically stable ultra-cold molecular Bose-Einstein condensates of NaK.
The field of ultracold matter has developed rapidly over the course of the past twenty to thirty years. One outstanding achievement has been the realisation of Bose-Einstein condensation (BEC) only 23 years ago. Many other fascinating observations of quantum phenomena have been made at an incredibly fast pace using ultracold atomic samples with contact force interactions as a testbed. Due to the rapid successes of all these experiments on atomic ensembles, researchers began to aim for more elaborate inter-particle interactions. A possible additional one is the dipole-dipole interaction (DDI), the long-range character of which stands out in comparison to the contact interactions. Moreover, DDI is anisotropic and the possibility to tune the direction of the dipoles in the experiment as well as the interaction strength promises the possibility to study a plethora of new quantum phenomena. In recent years, several experiments using different combinations of alkali atoms to create ground-state hetero-nuclear molecules in the ultracold regime have been set up, following the pioneering work on KRb molecules. Of all possible combinations of two alkali atoms, NaK combines the advantages of a high induced electric dipole moment of 2.72 Debye, chemical stability of the ground-state molecules, the possibility to realise bosonic as well as fermionic molecules and a large knowledge base for cooling and trapping of the two atomic species. This thesis reports on the development and construction of an experiment apparatus for the production of ultracold bosonic dipolar ground state 23Na39K molecules. This includes the setup of an ultra-high vacuum (UHV) system, several laser light sources, microwave (MW) and radio frequency (RF) antennas and electronics to control the various components of the experiment. The system pre-cools the atomic samples of 23Na and 39K and loads them into a dual-species three-dimensional magneto-optical trap (3D MOT). After a transfer to a magnetic quadrupole trap, the mixture is cooled by forced MW evaporation until three-body losses limit dual-species operation at high densities and zero magnetic field. Using the pure optical trapping potential of a crossed optical dipole trap (cODT) and RF- state-transfer, the scattering properties of the bosonic mixture of 23Na+39K are investigated in extensive atom loss measurements for a magnetic field range from 0 to 750 G. Several Feshbach resonances, zero crossings of the interspecies scattering length and inelastic loss features in many different scattering channels are identified. The results of this study are used to refine the molecular potentials of NaK and to produce a dual-species quantum-degenerate mixture of two BECs in the state (f = 1, mf = −1) of 23Na + (f = 1, mf = −1) of 39K. The measurements and refined molecular potentials will considerably aid in the future realisation of 23Na39K molecules in their absolute ground state.
This thesis reports the formation of a dual-species Bose-Einstein condensate of 87^{87}87Rb and 133^{133}133Cs in the same trapping potential. Quantum degenerate mixtures exhibit rich physics inaccessible to single species experiments and provide an ideal starting point for the creation of ultracold dipolar molecules. These molecules offer a wealth of new research avenues including precision metrology, quantum simulation and computation. The experimental method exploits the efficient sympathetic cooling of 133^{133}133Cs via elastic collisions with 87^{87}87Rb, initially in a magnetic quadrupole trap and subsequently in a levitated optical trap. Evaporative cooling in the dipole trap must compete against a high interspecies three-body inelastic collision rate \mbox{∼10−25−10−26\sim10^{-25}-10^{-26}∼10−25−10−26~cm6/^{6}/6/s}. The two condensates each contain up to \mbox{2×1042\times10^{4}2×104} atoms and exhibit a striking phase separation, revealing the mixture to be immiscible due to strong repulsive interspecies interactions. Sacrificing all the 87^{87}87Rb during the cooling leads to the creation of single-species 133^{133}133Cs condensates of up to \mbox{6×1046\times10^{4}6×104} atoms. In addition this thesis reports the observation of an interspecies Feshbach resonance at 181.7(5)~G and the creation of a pure sample of Cs2_{2}2​ molecules via magneto-association on the 4(g)4 resonance at 19.8~G. These results represent important steps towards the creation of ultracold polar RbCs molecules.
This thesis reports on the construction of a novel experimental apparatus to create an ultracold sample of chemical stable 23Na40K^{23}\rm{Na}^{40}\rm{K}23Na40K polar molecules at their absolute ground states. The long range nature of the dipolar interactions between the ground state molecules, is expected to facilitate to observe new quantum many-body states and to simulate prototypical lattice spin models for exploring quantum magnetism and high TcT_cTc​ superconductivity. The experimental procedures are presented in this thesis including: trapping and cooling 23Na^{23}\rm{Na}23Na and $ ^{40}\rm{K}$ atoms to quantum degeneracy, associating weakly bound state molecules with Feshbach resonances, and transferring the Feshbach molecules to the rovibrational ground states via stimulated Raman adiabatic passage. As a result, successful transfer has been demonstrated in our experiment with a single-path transfer efficiency ≃60%\simeq 60\%≃60% via a hyperfine non-resolved intermediate state ∣d3Π,ν=5,J=1,Ω=1⟩\vert d^{3}\Pi, \nu=5, J=1, \Omega=1 \rangle∣d3Π,ν=5,J=1,Ω=1⟩. In addition, a perturbative theoretical study on the Fermi liquid properties of a single-component weakly-interacting two dimensional dipolar fermions with dipole moments ddd oriented perpendicularly to the plane of their translational motion is presented in the thesis. When the dipole moments are tilted with respect to the translational plane, the effect of anisotropic interaction appears, and collective excitations in the collisionless regime are studied in this configuration. Furthermore, stable dilute supersolid states with two-dimensional bosonic dipoles are predicted to exist with an additional three-body repulsive contact interaction. These novel quantum effects should be accessible experimentally once the NaK\rm{Na}\rm{K}NaK ground state molecules reach quantum degeneracy by further evaporative cooling.
This thesis reports on experiments with quantum degenerate atomic mixtures and molecular gases at ultracold temperatures.The work describes the first realization of a quantum degenerate Fermi-Fermi mixture of two different species with unequal masses.In addition, the first quantum degenerate three-species mixture is realized.Furthermore, the first heteronuclear bosonic molecules with temperatures close to quantum degeneracy are created from a two-species mixture.Within this work a new and very versatile experimental platform to study quantum degenerate two-species Fermi mixtures is presented.The experimental concept relies on sympathetic cooling of two fermionic species ( 40 K and 6 Li) by a large bosonic gas ( 87 Rb).It is shown that large atom numbers and reliable operation are guaranteed by careful choice of the experimental components and parameters, which is essential to deal with the complexity of such an experimental platform with three atomic species.The first important milestone towards quantum degeneracy is realized by simultaneous trapping 87 Rb, 40 K and 6 Li in a magneto-optical trap.This marks the first realization of magneto-optical trapping of two fermionic species and also of three species.To achieve quantum degeneracy, different experimental challenges and difficulties of the three species with very different initial temperatures, scattering cross sections and masses were overcome.A combined compressed MOT and temporal dark MOT phase, a careful state cleaning process during the cooling process, species selective evaporative cooling of rubidium and removing of high energetic lithium atoms turn out to be essential for the successful realization of a quantum degenerate Fermi-Fermi-Bose mixture.Furthermore, at the end of the sympathetic cooling process, the cooling efficiency of lithium is improved by catalytic cooling through potassium.A quantum degenerate Fermi-Fermi mixture can be realized in the strongly interacting regime at a s-wave Feshbach resonance.By an adiabatic magnetic field sweep across the Feshbach resonance ultracold bosonic heteronuclear molecules are created with high efficiencies of up to 50 %.The associated number of molecules are up to 4 × 10 4 molecules, which is the largest number of heteronuclear molecules produced so far.Furthermore, close to resonance an increased molecular lifetime of more than 100 ms is observed.Therefore, this is the first system that may be used to explore many-body physics of a heteronuclear mixture in the strongly interacting regime across a Feshbach resonance.The heteronuclear Fermi mixture adds the mass ratio and the different internal structure as new parameters to quantum degenerate Fermi gases.The molecular cloud being close to quantum degeneracy marks the first starting point for the realization of a heteronuclear molecular BEC and to investigate the BEC-BCS crossover with unequal masses.Moreover, the molecules can be transferred into the rovibrational ground state to create a polar BEC with an anisotropic, long-range interaction.
We present measurements of interspecies Feshbach resonances and subsequent creation of dual-species Bose-Einstein condensates of 23Na^{23}\mathrm{Na}23Na and 39K^{39}\mathrm{K}39K. We prepare both optically trapped ensembles in the spin state $\left|f=1,{m}_{f}=\ensuremath{-}1\right\ensuremath{\rangle}$ and perform atom loss spectroscopy in a magnetic field range from 000 to 700G700\phantom{\rule{0.16em}{0ex}}\mathrm{G}700G. The observed features include several sss-wave poles and a zero crossing of the interspecies scattering length as well as inelastic two-body contributions in the $\mathcal{M}={m}_{\mathrm{Na}}+{m}_{\mathrm{K}}=\ensuremath{-}2$ submanifold. We identify and discuss the suitability of different magnetic field regions for the purposes of sympathetic cooling of 39K^{39}\mathrm{K}39K and achieving dual-species degeneracy. Two condensates are created simultaneously by evaporation at a magnetic field of about 150G150\phantom{\rule{0.16em}{0ex}}\mathrm{G}150G, which provides sizable intra- and interspecies scattering rates needed for fast thermalization. The impact of the differential gravitational sag on the miscibility criterion for the mixture is discussed. Our results serve as a promising starting point for the magnetoassociation into quantum degenerate 23Na39K^{23}\mathrm{Na}^{39}\mathrm{K}23Na39K Feshbach molecules.
Dual-species Bose-Einstein condensates of 23^{23}23Na and 41^{41}41K with tunable interactions
We report the creation of dual-species Bose-Einstein condensates (BECs) of 23^{23}23Na and 41^{41}41K. Favorable background scattering lengths enable efficient sympathetic cooling of 41^{41}41K via forced evaporative cooling of 23^{23}23Na in a plugged magnetic trap and an optical dipole trap. The 1/e1/e1/e lifetime of the thermal mixture in the stretched hyperfine state exceeds 5 s in the presence of background scattering. At the end of evaporation, we create dual BECs in the immiscible phase, with about 3×1053\times10^53×105 23^{23}23Na atoms surrounding 5×1045\times10^45×104 41^{41}41K atoms. To further enable the tuning of the interspecies interaction strength, we locate multiple Feshbach resonances at magnetic fields up to 100 G. The broadest sss-wave resonance located at 73.4(3) G features a favorable width of 1.8(2) G. This work sets the stage for the creation of ultracold gases of strongly dipolar bosonic 23^{23}23Na41^{41}41K molecules as well as the exploration of many-body physics in bosonic 23^{23}23Na-41^{41}41K mixtures.