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Interacting gases of ultracold polar molecules

Frauke Seeßelberg

Published Jan 1, 2019• 0 citations
Publisher

Abstract

Ultracold quantum gases are versatile model systems for exploring quantum physics or for the simulation of solid state materials. Meanwhile, they have been created from various atomic species - from alkali metals over alkaline earths to rare earth elements. The latest addition are quantum gases of different kinds of polar molecules. Expectations for quantum gases of heteronuclear molecules are high: Due to their large electric dipole moments, these molecules can interact with each other via long-range interactions - not just via contact interactions as is the case for most atoms. Additionally, they have vibrational and rotational degrees of freedom, with open up new possibilities for quantum simulation. But the same degrees of freedom also pose some challenges. For example, they make the preparation of the quantum gas more difficult, which is typically produced with a combination of laser cooling and evaporative cooling in the atomic case. Molecules mostly lack closed transitions in their spectra, which are required for laser cooling. Therefore, we create our molecular quantum gas from a mixture of two atomic quantum gases. In this work, such an experimental method was developed for fermionic NaK molecules, which is based on the two-photon process Stimulated Raman Adiabatic Passage (STIRAP). Within STIRAP, the hyperfine structure of the chosen intermediate state plays an important role. Experimentally, and with the help of a theoretical model describing the whole process, we find that we produce the most molecules when we use a large one-photon detuning, if the hyperfine structure of the intermediate state is unresolved. In another project, we explored the rotational level structure of the molecular ground state populated by STIRAP. Rotation is closely linked to the electric dipole moment. The superposition of the ground state with the first excited rotational state, for example, has a transition dipole moment of almost 60% of the permanent electric dipole moment. Unfortunately, coherence times of such superpositions are typically short, as the different polarizabilities of the rotational states lead to dephasing in optical traps. However, using a special polarization angle and a small, dc electric field, we can compensate these differences and realize a spin-decoupled magic trap. With this new technique we obtain record coherence times, at least for small molecular densities. For larger densities we observe first indications for dipolar interactions in a bulk gas of polar molecules, which we also model using the moving-average cluster expansion (MACE).

DOI
10.5282/edoc.24316

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Ultracold quantum gases are versatile model systems for exploring quantum physics or for the simulation of solid state materials. Meanwhile, they have been created from various atomic species - from alkali metals over alkaline earths to rare earth elements. The latest addition are quantum gases of different kinds of polar molecules. Expectations for quantum gases of heteronuclear molecules are high: Due to their large electric dipole moments, these molecules can interact with each other via long-range interactions - not just via contact interactions as is the case for most atoms. Additionally, they have vibrational and rotational degrees of freedom, with open up new possibilities for quantum simulation. But the same degrees of freedom also pose some challenges. For example, they make the preparation of the quantum gas more difficult, which is typically produced with a combination of laser cooling and evaporative cooling in the atomic case. Molecules mostly lack closed transitions in their spectra, which are required for laser cooling. Therefore, we create our molecular quantum gas from a mixture of two atomic quantum gases. In this work, such an experimental method was developed for fermionic NaK molecules, which is based on the two-photon process Stimulated Raman Adiabatic Passage (STIRAP). Within STIRAP, the hyperfine structure of the chosen intermediate state plays an important role. Experimentally, and with the help of a theoretical model describing the whole process, we find that we produce the most molecules when we use a large one-photon detuning, if the hyperfine structure of the intermediate state is unresolved. In another project, we explored the rotational level structure of the molecular ground state populated by STIRAP. Rotation is closely linked to the electric dipole moment. The superposition of the ground state with the first excited rotational state, for example, has a transition dipole moment of almost 60% of the permanent electric dipole moment. Unfortunately, coherence times of such superpositions are typically short, as the different polarizabilities of the rotational states lead to dephasing in optical traps. However, using a special polarization angle and a small, dc electric field, we can compensate these differences and realize a spin-decoupled magic trap. With this new technique we obtain record coherence times, at least for small molecular densities. For larger densities we observe first indications for dipolar interactions in a bulk gas of polar molecules, which we also model using the moving-average cluster expansion (MACE).
The realization of rovibrationally stable dense samples of ultracold diatomic molecules remains one of the main stepping stones to achieve the next slate of major goals in the field of atomic and molecular physics. Though obtaining diatomic alkali molecules was seen as a logical next step following the optical cooling of atoms, many of the possible applications currently under investigation extend beyond atomic and molecular physics. For example, spectroscopy of ultracold molecules can help in testing extensions of the Standard Model via the search for a permanent electric dipole moment of the electron (1; 2), or the energy difference between enantiomers of chiral molecules (3). Various molecular transitions can be utilized to track the time dependence of fundamental constants, including the fine structure constant and the proton to electron mass ratio (4). They also open the way for cold and ultracold chemistry, where the interacting species and products are in a coherent quantum superposition state (5) and reactions can happen via quantum tunneling. Dipolar ultracold quantum gases promise to show a plethora of new phenomena due to anisotropic long-range dipole-dipole interactions (6). Dipolar molecules in optical lattices can be employed as a quantum simulator of condensed matter systems, and they are predicted to demonstrate new quantum phases such as a dipolar crystal, supersolid, checkerboard and collapse phases (7; 8). Ultracold polar molecules also represent an attractive platform for quantum computation (9). They offer a variety of long-lived states for qubit encoding, including rotational, spin and hyperfine (if electronic and nuclear spins are non-zero), Λ and Ω-doublet states (10) and scalability to a large number of qubits. Polar molecules can be easily controlled by DC electric and magnetic fields, as well as bymicrowave and optical fields, allowing the design of various traps (11; 12). The main appeal of polar molecules for quantum information processing, however, comes from their permanent electric dipole moment, permitting them to interact via a long-range dipole-dipole interaction. The dipole-dipole interaction offers a tool to construct two-qubit gates, required for universal quantum computation (9; 13). Ultracold molecules in their ground vibrational state v = 0 (and even in specific rotational, hyperfine or Zeeman states) are required formany of these applications since they have a large permanent electric dipole moment and are stable with respect to collisions and spontaneous emission. Currently translationally ultracold (100 nK 1 mK) molecules are produced by magneto(14) and photo-association (15) techniques. In a typical photoassociation scheme, 3
Ultracold atomic gases have proven to be a versatile tool for the study of many-body quantum physics thanks to the high degree of control that they offer. However, interactions between neutral atoms are usually short-ranged and isotropic, which limits their applicability as a simulator of real condensed matter systems. One of the most common methods of introducing stronger interactions into an ultracold gas is associating pairs of atoms of two different elements into molecules. Such molecules possess a body-fixed electric dipole moment, which can be addressed by an external electric field or microwave radiation to engineer long-ranged, tunable dipolar interactions. In addition to constituting an analogue quantum simulator, ultracold dipolar molecules show potential as a platform for digital quantum computation, precision measurements, and the study of quantum chemistry. The main result presented in this thesis is the first successful synthesis of ultracold 39K133Cs molecules in their vibrational and rotational ground state. Using a novel simultaneous cooling scheme for potassium and caesium, our team has been able to produce mixtures that are cold enough for efficient magnetoassociation into weakly bound molecules to take place. By performing high-resolution laser spectroscopy of these molecules, I have identified a two-photon pathway to the ground state via an exceptionally narrow intermediate excited state. I was then able to perform Stimulated Raman Adiabatic Passage (STIRAP) along that pathway with one-way efficiencies up to 71%, producing trapped samples of up to 3500 ground-state molecules at temperatures of 1 µK. The lifetime of these samples is currently limited by a near-universal two-body loss process, a further study of which could shed light on similar loss mechanisms in other molecular species. I have also carried out preliminary rotational spectroscopy, which paves the way towards using KCs molecules as a controlled interacting spin system and a refinement of their hyperfine coupling constants. The thesis ends with a detailed outline of further steps that should be taken to improve the efficiency of STIRAP, as well as the population and coherence lifetimes in KCs samples. It is followed by a list of suggested experiments that could be carried out using our system after it undergoes the described technical upgrades. Some of these experiments are directly inspired by the results I have obtained during my six-month research stay in the KRb laboratory at JILA.
Recently ground state polar molecules have been successfully realized using stimulated Raman adiabatic passage (STIRAP) technique, which has stimulated new experimental activities along this direction in many laboratories. It is very promising that a stable and quantum degenerate gas of polar molecules can be realized in the near future. Such polar molecules have a large permanent electric dipole moment and hence strong long-range dipolar interaction effects in the presence of an electric field, giving rise to new few- and many-body phenomena that are absent in the traditional atomic systems explored so far.
Ultracold bialkali polar molecules play a leading part at the frontline of quantum physics. They recently attract a lot of attention in the field of ultracold quantum chemistry, quantum many-body physics and quantum simulations. The key for their success is the rich internal level structure with rotational and vibrational degrees of freedom and their large electric dipole moments. Still, only a handful of molecular species are available at ultracold temperatures until now, although it is highly desirable to produce new molecular species to further expand the range of applications. Besides direct laser cooling methods for molecules, the assembly of heteronuclear ground-state molecules from ultracold atomic mixtures is the most promising approach for the creation of polar molecules. It includes the formation of weakly bound Feshbach molecules from the diatomic mixture and the subsequent two-photon stimulated Raman adiabatic passage (STIRAP) transfer to the rovibrational ground state. This creation strategy has been successfully demonstrated for the first time in the pioneering experiments at JILA with ultracold 40K87Rb molecules. Since then, only a few more molecular species from different alkali atoms have been created, namely 6Li23Na, 23Na40K, 23Na87Rb and 87Rb133Cs. In this thesis, I report the successful creation of a new species of ultracold polar ground-state molecules: 23Na39K. Starting from an ultracold mixture of bosonic 23Na and 39K atoms, weakly bound molecules are created. For this purpose, a Feshbach resonance in a high angular momentum scattering channel is chosen, experimentally identified and characterized. Close to this resonance the weakly bound Feshbach molecules are formed using resonant radio frequency radiation. For the two-photon ground-state transfer, a unique, highly specialized two-color laser system is designed and realized. It is used for one- and two-photon spectroscopy to identify the relevant transitions for the ground-state transfer. Based on the obtained data, a local model of the singlet-triplet mixed excited state manifolds is developed, with which the hyperfine structure and the magnetic field dependence is predicted with high accuracy. According to these findings, a suitable pathway to a single hyperfine ground state is chosen considering selection rules and experimental conditions such as laser polarization and beam alignment. To precisely determine the two-photon resonance condition for STIRAP, electromagnetically induced transparency measurements are performed. The ground-state transfer is then performed using STIRAP. The experimental findings regarding the STIRAP are successfully supported theoretically by a model based on a five-level master equation. The pure molecular gas shows evidence for two-body dominated loss mechanisms, such as sticky four-body collisions. The molecule-atom mixture of 23Na39K+39K reveals an unexpectedly low loss rate coefficient although sticky three-body collisions are assumed to occur. This behavior demands further investigations regarding different hyperfine collisional chan-nels and magnetic field regions.
A quantum gas of ultracold polar molecules, with long-range and anisotropic interactions, not only would enable explorations of a large class of many-body physics phenomena but also could be used for quantum information processing. We report on the creation of an ultracold dense gas of potassium-rubidium (40K87Rb) polar molecules. Using a single step of STIRAP (stimulated Raman adiabatic passage) with two-frequency laser irradiation, we coherently transfer extremely weakly bound KRb molecules to the rovibrational ground state of either the triplet or the singlet electronic ground molecular potential. The polar molecular gas has a peak density of 10(12) per cubic centimeter and an expansion-determined translational temperature of 350 nanokelvin. The polar molecules have a permanent electric dipole moment, which we measure with Stark spectroscopy to be 0.052(2) Debye (1 Debye = 3.336 x 10(-30) coulomb-meters) for the triplet rovibrational ground state and 0.566(17) Debye for the singlet rovibrational ground state.
Ultracold polar molecular gases promise new directions and exciting appli-cations in precision measurements, ultracold chemistry, electric-field controlled collisions, dipolar quantum gases, and quantum information sciences. This the-sis presents experimental realization of a near quantum degenerate gas of polar molecules, where the phase-space density of the gas achieved is more than 10 or-ders of magnitude higher than previous results. The near quantum degenerate gas of polar molecules is created using two coherent steps. First, atoms in an ul-tracold gas mixture are converted into extremely weakly bound molecules near a Fano-Feshbach resonance. Second, the weakly bound molecules are transferred to the ro-vibronic ground state using a coherent two-photon Raman technique. The fact that these ground-state molecules are polar is confirmed with a spectroscopic measurement of the permanent electric dipole moment. Finally, manipulation of the molecular hyperfine state is demonstrated; this allows molecules to be pop-ulated in a single quantum state, in particular, the lowest energy state. With an ultracold gas of molecules, full control of molecular internal state, and electric field as a new handle, ultracold molecular collisions, including ultracold chemical reactions and dipolar collisions, are studied.