Coherent transfer of photoassociated molecules into the rovibrational ground state

Observation of Photoassociation Resonances in Ultracold Atom-Molecule Collisions

We report on the observation of photoassociation resonances in ultracold collisions between ^{23}Na^{40}K molecules and ^{40}K atoms. We perform photoassociation in a long-wavelength optical dipole trap to form deeply bound triatomic molecules in electronically excited states. The atom-molecule Feshbach resonance is used to enhance the free-bound Franck-Condon overlap. The photoassociation into well-defined quantum states of excited triatomic molecules is identified by observing resonantly enhanced loss features. These loss features depend on the polarization of the photoassociation lasers, allowing us to assign rotational quantum numbers. The observation of ultracold atom-molecule photoassociation resonances paves the way toward preparing ground-state triatomic molecules, provides a new high-resolution spectroscopy technique for polyatomic molecules, and is also important to atom-molecule Feshbach resonances.

Highly efficient creation and detection of deeply bound molecules via invariant-based inverse engineering with feasible modified drivings

Stimulated Raman Adiabatic Passage (STIRAP) and its variants, such as M-type chainwise-STIRAP, allow for efficiently transferring the populations in a multilevel system and have widely been used to prepare molecules in their rovibrational ground state. However, their transfer efficiencies are generally imperfect. The main obstacle is the presence of losses and the requirement to make the dynamics adiabatic. To this end, in the present paper, a new theoretical method is proposed for the efficient and robust creation and detection of deeply bound molecules in three-level Λ-type and five-level M-type systems via "Invariant-based shortcut-to-adiabaticity." In the regime of large detunings, we first reduce the dynamics of three- and five-level molecular systems to those of effective two- and three-level counterparts. By doing so, the major molecular losses from the excited states can be well suppressed. Consequently, the effective two-level counterpart can be directly compatible with two different "Invariant-based Inverse Engineering" protocols; the results show that both protocols give a comparable performance and have a good experimental feasibility. For the effective three-level counterpart, by considering a relation among the four incident pulses, we show that this model can be further generalized to an effective Λ-type one with the simplest resonant coupling. This generalized model permits us to borrow the "Invariant-based Inverse Engineering" protocol from a standard three-level Λ-type system to a five-level M-type system. Numerical calculations show that the weakly bound molecules can be efficiently transferred to their deeply bound states without strong laser pulses, and the stability against parameter variations is well preserved. Finally, the detection of ultracold deeply bound molecules is discussed.

Quantum control of field-free molecular orientation

Generating field-free (non-stationary) orientation of molecules in space has been a longstanding goal in the field of quantum control of molecular rotation, which has significant applications in physical chemistry, chemical physics, strong-field physics, and quantum information science. In this Perspective, we review and examine several representative control schemes developed in recent years and implemented in theoretical and experimental areas for generating field-free orientation of molecules. By conducting numerical simulations of different control schemes on the same molecular system, we demonstrate that quantum coherent control, specifically targeting a limited number of the lowest-lying rotational levels to achieve an optimal superposition, can result in a high degree of orientation. To this end, we provide an overview of our latest developed analytical method, which enables the precise design of terahertz field parameters through resonant excitation. This design approach facilitates the attainment of desired field-free orientations by optimizing the amplitudes and phases of rotational wave functions for the selected rotational levels. Finally, we outlook the significance of such progress in multiple frontier research fields, highlighting its potential applications in ultracold physics, quantum computation, quantum simulation, and quantum metrology.

Electroassociation of Ultracold Dipolar Molecules into Tetramer Field-Linked States

The presence of electric or microwave fields can modify the long-range forces between ultracold dipolar molecules in such a way as to engineer weakly bound states of molecule pairs. These so-called field-linked states [A. V. Avdeenkov and J. L. Bohn, Phys. Rev. Lett. 90, 043006 (2003).PRLTAO0031-900710.1103/PhysRevLett.90.043006; L. Lassablière and G. Quéméner, Phys. Rev. Lett. 121, 163402 (2018).PRLTAO0031-900710.1103/PhysRevLett.121.163402], in which the separation between the two bound molecules can be orders of magnitude larger than the molecules themselves, have been observed as resonances in scattering experiments [X.-Y. Chen et al., Nature (London) 614, 59 (2023).NATUAS0028-083610.1038/s41586-022-05651-8]. Here, we propose to use them as tools for the assembly of weakly bound tetramer molecules, by means of ramping an electric field, the electric-field analog of magnetoassociation in atoms. This ability would present new possibilities for constructing ultracold polyatomic molecules.

Diffuse Basis Functions for Relativistic s and d Block Gaussian Basis Sets

Diffuse s, p, and d functions have been optimized for use with previously reported relativistic basis sets for the s and d blocks of the periodic table. The functions were optimized on the 4:1 weighted average of the s² and p² configurations of the anion, with the d shell in the dⁿ⁺¹ configuration for the d blocks. Exponents were extrapolated for groups 2 and 12, which have unstable or weakly bound anions. The diffuse basis sets have been tested by application to calculations of electron affinities of the group 11 elements (Cu, Ag, and Au), double electron affinities of the group 11 monocations, and potential energy curves of Mg₂ and Ca₂ van der Waals dimers, as well as some response properties of the group 1 anions (Rb^-, Cs^-, and Fr^-), the group 2 elements (Sr, Ba, and Ra), and RbLi, CsLi, and FrLi molecules.

Laser Cooling with an Intermediate State and Electronic Structure Studies of the Molecules CaCs and CaNa

The ground and excited electronic states of the diatomic molecules CaCs and CaNa have been investigated by implementing the ab initio CASSCF/(MRCI + Q) calculation. The potential energy curves of the doublet and quartet electronic low energy states in the representation ^2s+1Λ^(±) have been determined for the two considered molecules, in addition to the spectroscopic constants T _e, ω_e, B _e, R _e, and the values of the dipole moment μ_e and the dissociation energy D _e. The determination of vibrational constants E _v, B _v, D _v, and the turning points R _min and R _max up to the vibrational level v = 100 was possible with the use of the canonical functions schemes. Additionally, the transition and the static dipole moments curves, Einstein coefficients, the spontaneous radiative lifetime, the emission oscillator strength, and the Franck-Condon factors are computed. These calculations showed that the molecule CaCs is a good candidate for Doppler laser cooling with an intermediate state. A "four laser" cooling scheme is presented, along with the values of Doppler limit temperature T _D = 55.9 μK and the recoil temperature T _r = 132 nK. These results should provide a good reference for experimental spectroscopic and ultra-cold molecular physics studies.

Quantum control of reactions and collisions at ultralow temperatures

At temperatures close to absolute zero, the molecular reactions and collisions are dominantly governed by quantum mechanics. Remarkable quantum phenomena such as quantum tunneling, quantum threshold behavior, quantum resonances, quantum interference, and quantum statistics are expected to be the main features in ultracold reactions and collisions. Ultracold molecules offer great opportunities and challenges in the study of these intriguing quantum phenomena in molecular processes. In this article, we review the recent progress in the preparation of ultracold molecules and the study of ultracold reactions and collisions using ultracold molecules. We focus on the controlled ultracold chemistry and the scattering resonances at ultralow temperatures. The challenges in understanding the complex ultracold reactions and collisions are also discussed.

Nuclear spin conservation enables state-to-state control of ultracold molecular reactions

Quantum-state control of reactive systems has enabled microscopic probes of underlying interaction potentials and the alteration of reaction rates using quantum statistics. However, extending such control to the quantum states of reaction outcomes remains challenging. Here, we realize this goal by utilizing the conservation of nuclear spins throughout the reaction. Using resonance-enhanced multiphoton ionization spectroscopy to investigate the products formed in bimolecular reactions between ultracold KRb molecules we find that the system retains a near-perfect memory of the reactants' nuclear spins, manifested as a strong parity preference for the rotational states of the products. We leverage this effect to alter the occupation of these product states by changing the coherent superposition of initial nuclear spin states with an external magnetic field. In this way, we are able to control both the inputs and outputs of a reaction with quantum-state resolution. The techniques demonstrated here open up the possibilities to study quantum entanglement between reaction products and ultracold reaction dynamics at the state-to-state level.

Microwave coherent control of ultracold ground-state molecules formed by short-range photoassociation

We report the observation of microwave coherent control of rotational states of ultracold ⁸⁵Rb¹³³Cs molecules formed in their vibronic ground state by short-range photoassociation. Molecules are formed in the single rotational state X(v = 0, J = 1) by exciting pairs of atoms to the short-range state (2)³Π_0^-(v = 11, J = 0), followed by spontaneous decay. We use depletion spectroscopy to record the dynamic evolution of the population distribution and observe clear Rabi oscillations while irradiating on a microwave transition between coupled neighbouring rotational levels. A density-matrix formalism that accounts for longitudinal and transverse decay times reproduces both the dynamic evolution during the coherent process and the equilibrium population. The coherent control reported here is valuable both for investigating coherent quantum effects and for applications of cold polar molecules produced by continuous short-range photoassociation.

A first principles study of the spin–orbit coupling effect in LiM (M = Na, K, Rb, Cs) molecules

Both fully relativistic and scalar-state based perturbation models provided the spin–orbit functions of the LiM (M = Na, K, Rb, Cs) molecules at almost experimental level of confidence.

Probing ultracold chemistry using ion spectrometry

Reactions between KRb molecules at sub-microkelvin temperatures were probed using ion spectrometry.

Production of ultracold 85Rb133Cs molecules in the lowest ground state via the B1Π1 short-range state

We investigate the production of cold ⁸⁵Rb¹³³Cs molecules in the lowest vibronic level of the ground electronic state via the B¹Π₁ short-range state. The photoassociation (PA) spectra of the B¹Π₁ state, including newly observed transition to 2 vibronic levels, are obtained by high sensitivity time-of-flight mass spectrometry. Based on these PA spectra, the harmonic and anharmonic constants of vibronic states are obtained, resulting in predicted vibronic energies with an uncertainty of 1-2 cm^-1. The B¹Π₁ (v = 3) state is found to have the maximum production rate for ground-state molecules with a value of 3(1) × 10⁴ s^-1, which is 3 times larger than the value via the previously studied 2³Π_0⁺ (v = 10, J = 0) state with two-photon cascade decay. The populations of J = 0, 1, and 2 rotational levels of X¹Σ⁺ (v = 0) state molecules formed via the B¹Π₁ (v = 3, J = 1) state are measured to be around 20%, 40%, and 20%. To quantify the coupling strength between the B¹Π₁ (v = 3) state and X¹Σ⁺ (v = 0) state, the transition dipole moment between them is measured to be 7.2(2) × 10^-3ea₀, which is also 3 times larger than the value between the 2³Π_0⁺ (v=10) state and X¹Σ⁺ (v = 0) state, meaning the B¹Π₁ (v = 3) state has a stronger coupling with the X¹Σ⁺ (v = 0) state. Our detailed measurements provide relevant parameters for investigation on direct stimulated Raman adiabatic passage transfer between the atomic scattering state and molecular bound state for ⁸⁵Rb¹³³Cs molecules.

Measurement of the variation of electron-to-proton mass ratio using ultracold molecules produced from laser-cooled atoms

Experimental techniques to manipulate cold molecules have seen great development in recent years. The precision measurements of cold molecules are expected to give insights into fundamental physics. Here we use a rovibrationally pure sample of ultracold KRb molecules to improve the measurement on the stability of electron-to-proton mass ratio \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {\mu = \frac{{m_{\mathrm{e}}}}{{M_{\mathrm{p}}}}} \right)$$\end{document}μ=meMp. The measurement is based upon a large sensitivity coefficient of the molecular spectroscopy, which utilizes a transition between a nearly degenerate pair of vibrational levels each associated with a different electronic potential. Observed limit on temporal variation of μ is \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\frac{1}{\mu }\frac{{d\mu }}{{dt}} = (0.30 \pm 1.0) \times 10^{ - 14} \, {\mathrm{year}}^{ - 1}$$\end{document}1μdμdt=(0.30±1.0)×10-14year-1, which is better by a factor of five compared with the most stringent laboratory molecular limits to date. Further improvements should be straightforward, because our measurement was only limited by statistical errors.

Ultracold molecules are suitable platforms for precision measurements due to their internal degrees of freedom. Here the authors derive a limit on the variation of the electron-to-proton mass ratio by using the spectroscopy of ultracold KRb molecules.

Interaction-Induced Fractionalization and Topological Superconductivity in the Polar Molecules Anisotropic t-J Model

We show that the interplay between antiferromagnetic interaction and hole motion gives rise to a topological superconducting phase. This is captured by the one dimensional anisotropic t-J model which can be experimentally achieved with ultracold polar molecules trapped onto an optical lattice. As a function of the anisotropy strength we find that different quantum phases appear, ranging from a gapless Luttinger liquid to spin gapped conducting and superconducting regimes. In the presence of appropriate z anisotropy, we also prove that a phase characterized by nontrivial topological order takes place. The latter is described uniquely by a finite nonlocal string parameter and presents robust edge spin fractionalization. These results allow us to explore quantum phases of matter where topological superconductivity is induced by the interaction.

Heuristic optimization of analytic laser pulses for vibrational stabilization of ultracold KRb

We proposed a methodology that allows to maximize the population transfer from a high vibrational state of the a ³Σ⁺ triplet state to the vibrational ground state of the X ¹Σ⁺ singlet state though the optimization of one pump and one dump laser pulses. The pump pulse is optimized using a fitness function, heuristically improved, that includes the effect of the spin-orbit coupling of the KRb [b-A]-scheme. The dump pulse is optimized to maximize the population transfer to the ground state. We performed a comparison with the case in which the pump and dump pulses are optimized to maximize the population transfer to the ground state employing a genetic algorithm with a single fitness function. The heuristic approach turned out to be 70% more efficient than a quantum optimal control optimization employing a single fitness function. The method proposed provides simple pulses that have an experimental realm.

Controlling the Scattering Length of Ultracold Dipolar Molecules

By applying a circularly polarized and slightly blue-detuned microwave field with respect to the first excited rotational state of a dipolar molecule, one can engineer a long-range, shallow potential well in the entrance channel of the two colliding partners. As the applied microwave ac field is increased, the long-range well becomes deeper and can support a certain number of bound states, which in turn bring the value of the molecule-molecule scattering length from a large negative value to a large positive one. We adopt an adimensional approach where the molecules are described by a rescaled rotational constant B[over ˜]=B/s_{E_{3}} where s_{E_{3}} is a characteristic dipolar energy. We found that molecules with B[over ˜]>10^{8} are immune to any quenching losses when a sufficient ac field is applied, the ratio elastic to quenching processes can reach values above 10^{3}, and that the value and sign of the scattering length can be tuned. The ability to control the molecular scattering length opens the door for a rich, strongly correlated, many-body physics for ultracold molecules, similar to that for ultracold atoms.

Magnetic Trapping of an Ultracold Gas of Polar Molecules

We demonstrate the efficient transfer of molecules from a magneto-optical trap into a conservative magnetic quadrupole trap. Our scheme begins with a blue-detuned optical molasses to cool SrF molecules to ≈50  μK. Next, we optically pump the molecules into a strongly trapped sublevel. This two-step process reliably transfers ≈40% of the molecules initially trapped in the magneto-optical trap into a single quantum state in the magnetic trap. Once loaded, the molecule cloud is compressed by increasing the magnetic field gradient. We observe a magnetic trap lifetime of over 1 s. This opens a promising new path to study ultracold molecular collisions, and potentially to produce quantum-degenerate molecular gases via sympathetic cooling with co-trapped atoms.

Observation of photoassociation of ultracold sodium and cesium at the asymptote Na (3S1/2) + Cs (6P1/2)

We report on the production of ultracold heteronuclear NaCs* molecules in a dual-species magneto-optical trap through photoassociation. The electronically excited molecules are formed below the Na (3S_1/2) + Cs (6P_1/2) dissociation limit. 12 resonance lines are detected using trap-loss spectroscopy based on a highly sensitive modulation technique. The highest observed rovibrational level exhibits clear hyperfine structure, which is detected for the first time. This structure is simulated within a simplified model consisting of 4 coupled levels belonging to the initially unperturbed Hund's case "a" electronic states, which have been explored in our previous work that dealt with the Na (3S_1/2) + Cs (6P_3/2) asymptote [W. Liu et al., Phys. Rev. A 94, 032518 (2016)].

A dynamical process of optically trapped singlet ground state 85Rb133Cs molecules produced via short-range photoassociation

We investigate the dynamical process of optically trapped X¹Σ⁺ (v'' = 0) state ⁸⁵Rb¹³³Cs molecules distributed in J'' = 1 and J'' = 3 rotational states. The considered molecules, formed from short-range photoassociation of mixed cold atoms, are subsequently confined in a crossed optical dipole trap. Based on a phenomenological rate equation, we provide a detailed study of the dynamics of ⁸⁵Rb¹³³Cs molecules during the loading and holding processes. The inelastic collisions of ⁸⁵Rb¹³³Cs molecules in the X¹Σ⁺ (v'' = 0, J'' = 1 and J'' = 3) states with ultracold ⁸⁵Rb (or ¹³³Cs) atoms are measured to be 1.0 (2) × 10^-10 cm³ s^-1 (1.2 (3) × 10^-10 cm³ s^-1). Our work provides a simple and generic procedure for studying the dynamical process of trapped cold molecules in the singlet ground states.

Photoassociation of ultracold NaLi

We perform photoassociation spectroscopy in an ultracold ²³Na-⁶Li mixture to study the c³Σ⁺ excited triplet molecular potential. We observe 50 vibrational states and their substructure to an accuracy of 20 MHz, and provide line strength data from photoassociation loss measurements. An analysis of the vibrational line positions using near-dissociation expansions and a full potential fit is presented. This is the first observation of the c³Σ⁺ potential, as well as photoassociation in the NaLi system.

Long-range behavior of the transition dipole moments of heteronuclear dimers XY (X, Y = Li, Na, K, Rb) based on ab initio calculations

The ab initio electronic transition dipole moments (ETDMs) of heteronuclear dimers XY (X, Y = Li, Na, K, Rb) were calculated between the ground and excited states converging to the lowest three dissociation limits. The spin-allowed ETDMs were evaluated in a wide range of interatomic distances, R, by means of the quasi-relativistic electronic wave functions obtained by the multi-reference configuration interaction method. The inner-shell electrons (2 electrons for Li and Na atoms, and 10 and 28 for K and Rb, respectively) were described using the non-empirical shape-consistent effective core potentials. The l-independent core polarization potentials of each atom were used to take core-polarization and core-valence correlation effects into account. The long-range behavior of both singlet-singlet X¹Σ⁺-(2,3)¹Σ⁺;(1,2)¹Π and triplet-triplet a³Σ⁺-(2,3)³Σ⁺;(1,2)³Π transition moments is perfectly fitted at large R-distance by the asymptotic formula of X. Chu and A. Dalgarno, Phys. Rev. A: At., Mol., Opt. Phys., 2002, 66, 024701: , where the coefficient β is equal to 2 and -1 for the Σ-Σ and Σ-Π transitions, respectively. The n²S-n²P transition moments, d^A, and dynamic polarizabilites, α^B, of the alkali atoms in the n²S state extracted from the present molecular calculations coincide with their empirical and ab initio counterparts to within a few percent.

Experimental studies of the NaCs 12(0+) [71Σ+] state: Spin-orbit and non-adiabatic interactions and quantum interference in the 12(0+) [71Σ+] and 11(0+) [53Π0] emission spectra

We present results from experimental studies of the 11(0⁺) and 12(0⁺) electronic states of the NaCs molecule. An optical-optical double resonance method is used to obtain Doppler-free excitation spectra. Selected data from the 11(0⁺) and 12(0⁺) high-lying electronic states are used to obtain Rydberg-Klein-Rees and Inverse Perturbation Approach potential energy curves. Interactions between these two electronic states are evident in the patterns observed in the bound-bound and bound-free fluorescence spectra. A model, based on two separate interaction mechanisms, is presented to describe how the wavefunctions of the two states mix. The electronic parts of the wavefunctions interact via spin-orbit coupling, while the individual rotation-vibration levels interact via a second mechanism, which is likely to be non-adiabatic coupling. A modified version of the BCONT program was used to simulate resolved fluorescence from both upper states. Parameters of the model that describe the two interaction mechanisms were varied until simulations were able to adequately reproduce experimental spectra.

Improved Radio-Frequency Magneto-Optical Trap of SrF Molecules

We report the production of ultracold, trapped strontium monofluoride (SrF) molecules with number density and phase-space density significantly higher than previously achieved. These improvements are enabled by three distinct changes to our recently-demonstrated scheme for radio-frequency magneto-optical trapping of SrF: modification of the slowing laser beam geometry, addition of an optical pumping laser, and incorporation of a compression stage to the magneto-optical trap. With these improvements, we observe a trapped sample of SrF molecules at density 2.5×10⁵  cm^-3 and phase-space density 6×10^-14 , each a factor of 4 greater than in previous work. Under different experimental conditions, we observe trapping of up to 10⁴ molecules, a factor of 5 greater than in previous work. Finally, by reducing the intensity of the applied trapping light, we observe molecular temperatures as low as 250 μK.

Coherent phase control of population transfer through ladder system in two laser pulses with ω and nω

The ladder transitions controlled by two harmonic pulses are investigated theoretically using a time-dependent quantum wave packet method for the ground electronic state of HF molecule. By choosing [Formula: see text], [Formula: see text] and [Formula: see text] schemes, the population can be transferred to target states [Formula: see text]. The population distribution can be controlled by the average amplitude of total electric field which depends on the relative phase of two pulses. With the variation of the relative phase between [Formula: see text] and [Formula: see text] pulses, the variation of population has a period of [Formula: see text]. For [Formula: see text] and [Formula: see text] schemes, the population distributions show oscillation behavior with a period of [Formula: see text] by varying the relative phase. The two harmonic pulses can realize a nearly complete population transfer to the target state.

Submillikelvin Dipolar Molecules in a Radio-Frequency Magneto-Optical Trap

We demonstrate a scheme for magneto-optically trapping strontium monofluoride (SrF) molecules at temperatures one order of magnitude lower and phase space densities 3 orders of magnitude higher than obtained previously with laser-cooled molecules. In our trap, optical dark states are destabilized by rapidly and synchronously reversing the trapping laser polarizations and the applied magnetic field gradient. The number of molecules and trap lifetime are also significantly improved from previous work by loading the trap with high laser power and then reducing the power for long-term trapping. With this procedure, temperatures as low as 400  μK are achieved.

Long-range interactions between polar bialkali ground-state molecules in arbitrary vibrational levels

We have calculated the isotropic C6 coefficients characterizing the long-range van der Waals interaction between two identical heteronuclear alkali-metal diatomic molecules in the same arbitrary vibrational level of their ground electronic state X(1)Σ(+). We consider the ten species made up of (7)Li, (23)Na, (39)K, (87)Rb, and (133)Cs. Following our previous work [Lepers et al., Phys. Rev. A 88, 032709 (2013)], we use the sum-over-state formula inherent to the second-order perturbation theory, composed of the contributions from the transitions within the ground state levels, from the transition between ground-state and excited state levels, and from a crossed term. These calculations involve a combination of experimental and quantum-chemical data for potential energy curves and transition dipole moments. We also investigate the case where the two molecules are in different vibrational levels and we show that the Moelwyn-Hughes approximation is valid provided that it is applied for each of the three contributions to the sum-over-state formula. Our results are particularly relevant in the context of inelastic and reactive collisions between ultracold bialkali molecules in deeply bound or in Feshbach levels.

Perspective: Stimulated Raman adiabatic passage: The status after 25 years

The first presentation of the STIRAP (stimulated Raman adiabatic passage) technique with proper theoretical foundation and convincing experimental data appeared 25 years ago, in the May 1st, 1990 issue of The Journal of Chemical Physics. By now, the STIRAP concept has been successfully applied in many different fields of physics, chemistry, and beyond. In this article, we comment briefly on the initial motivation of the work, namely, the study of reaction dynamics of vibrationally excited small molecules, and how this initial idea led to the documented success. We proceed by providing a brief discussion of the physics of STIRAP and how the method was developed over the years, before discussing a few examples from the amazingly wide range of applications which STIRAP now enjoys, with the aim to stimulate further use of the concept. Finally, we mention some promising future directions.

Ultracold Dipolar Gas of Fermionic 23Na40 K Molecules in Their Absolute Ground State

We report on the creation of an ultracold dipolar gas of fermionic 23Na40 K molecules in their absolute rovibrational and hyperfine ground state. Starting from weakly bound Feshbach molecules, we demonstrate hyperfine resolved two-photon transfer into the singlet X 1Σ+|v=0,J=0⟩ ground state, coherently bridging a binding energy difference of 0.65 eV via stimulated rapid adiabatic passage. The spin-polarized, nearly quantum degenerate molecular gas displays a lifetime longer than 2.5 s, highlighting NaK's stability against two-body chemical reactions. A homogeneous electric field is applied to induce a dipole moment of up to 0.8 D. With these advances, the exploration of many-body physics with strongly dipolar Fermi gases of 23Na40K molecules is within experimental reach.

Many-body localization in dipolar systems

Systems of strongly interacting dipoles offer an attractive platform to study many-body localized phases, owing to their long coherence times and strong interactions. We explore conditions under which such localized phases persist in the presence of power-law interactions and supplement our analytic treatment with numerical evidence of localized states in one dimension. We propose and analyze several experimental systems that can be used to observe and probe such states, including ultracold polar molecules and solid-state magnetic spin impurities.

Dipole polarizability of alkali-metal (Na, K, Rb)-alkaline-earth-metal (Ca, Sr) polar molecules: prospects for alignment

Electronic open-shell ground-state properties of selected alkali-metal-alkaline-earth-metal polar molecules are investigated. We determine potential energy curves of the (2)Σ(+) ground state at the coupled-cluster singles and doubles with partial triples (CCSD(T)) level of electron correlation. Calculated spectroscopic constants for the isotopes ((23)Na, (39)K, (85)Rb)-((40)Ca, (88)Sr) are compared with available theoretical and experimental results. The variation of the permanent dipole moment (PDM), average dipole polarizability, and polarizability anisotropy with internuclear distance is determined using finite-field perturbation theory at the CCSD(T) level. Owing to moderate PDM (KCa: 1.67 D, RbCa: 1.75 D, KSr: 1.27 D, RbSr: 1.41 D) and large polarizability anisotropy (KCa: 566 a.u., RbCa: 604 a.u., KSr: 574 a.u., RbSr: 615 a.u.), KCa, RbCa, KSr, and RbSr are potential candidates for alignment and orientation in combined intense laser and external static electric fields.

Spin waves and dielectric softening of polar molecule condensates

We consider an oblate Bose-Einstein condensate of heteronuclear polar molecules in a weak applied electric field. This system supports a rich quasiparticle spectrum that plays a critical role in determining its bulk dielectric properties. In particular, in sufficiently weak fields the system undergoes a polarization wave rotonization, leading to the development of textured electronic structure and a dielectric instability that is characteristic of the onset of a negative static dielectric function.

Ultracold hydrogen atoms: a versatile coolant to produce ultracold molecules

We show theoretically that ultracold hydrogen atoms have very favorable properties for sympathetic cooling of molecules to microkelvin temperatures. We calculate the potential energy surfaces for spin-polarized interactions of H atoms with the prototype molecules NH(3Σ-) and OH(2Π) and show that they are shallow (50 to 80  cm(-1)) and only weakly anisotropic. We carry out quantum collision calculations on H+NH and H+OH and show that the ratio of elastic to inelastic cross sections is high enough to allow sympathetic cooling from temperatures well over 1 K for NH and around 250 mK for OH.

Spectroscopy and applications of the 3 3Σ+ electronic state of 39K85Rb

We report new results on the spectroscopy of the 3 (3)Σ(+) electronic state of (39)K(85)Rb. The observations are based on resonance-enhanced multiphoton ionization of ultracold KRb molecules starting in vibrational levels v'' = 18-23 of the a (3)Σ(+) state and ionized via the intermediate 3 (3)Σ(+) state. The a-state ultracold molecules are formed by photoassociation of ultracold (39)K and (85)Rb atoms to the 3(0(+)) state of KRb followed by spontaneous emission. We discuss the potential applications of this state to future experiments, as a pathway for populating the lowest vibrational levels of the a state as well as the X state.

Realizing fractional Chern insulators in dipolar spin systems

Strongly correlated quantum systems can exhibit exotic behavior controlled by topology. We predict that the ν = 1/2 fractional Chern insulator arises naturally in a two-dimensional array of driven, dipolar-interacting spins. As a specific implementation, we analyze how to prepare and detect synthetic gauge potentials for the rotational excitations of ultracold polar molecules trapped in a deep optical lattice. With the motion of the molecules pinned, under certain conditions, these rotational excitations form a fractional Chern insulating state. We present a detailed experimental blueprint for its realization and demonstrate that the implementation is consistent with near-term capabilities. Prospects for the realization of such phases in solid-state dipolar systems are discussed as are their possible applications.

Spectroscopy of the double minimum 3 (3)ΠΩ electronic state of 39K85Rb

We report the observation and analysis of the 3 (3)ΠΩ double-minimum electronic excited state of (39)K(85)Rb. The spin-orbit components (0(+), 0(-), 1, and 2) of this state are investigated based on potentials developed from the available ab initio potential curves. We have assigned the vibrational levels v' = 2-11 of the 3 (3)Π1,2 potentials and v' = 2-12 of the 3(3)Π0(+/-) potential. We compare our experimental observations of the 3 (3)ΠΩ state with predictions based on theoretical potentials. The observations are based on resonance enhanced multiphoton ionization of ultracold KRb in vibrational levels v" = 14-25 of the a (3)Σ(+) state. These a-state ultracold molecules are formed by photoassociation of ultracold (39)K and (85)Rb atoms to the 5(1) state of KRb followed by spontaneous emission to the a state.

Topological flat bands from dipolar spin systems

We propose and analyze a physical system that naturally admits two-dimensional topological nearly flat bands. Our approach utilizes an array of three-level dipoles (effective S=1 spins) driven by inhomogeneous electromagnetic fields. The dipolar interactions produce arbitrary uniform background gauge fields for an effective collection of conserved hard-core bosons, namely, the dressed spin flips. These gauge fields result in topological band structures, whose band gap can be larger than the corresponding bandwidth. Exact diagonalization of the full interacting Hamiltonian at half-filling reveals the existence of superfluid, crystalline, and supersolid phases. An experimental realization using either ultracold polar molecules or spins in the solid state is considered.

Rovibrational cooling of molecules by optical pumping

We demonstrate rotational and vibrational cooling of cesium dimers by optical pumping techniques. We use two laser sources exciting all the populated rovibrational states, except a target state that thus behaves like a dark state where molecules pile up thanks to absorption-spontaneous emission cycles. We are able to accumulate photoassociated cold Cs(2) molecules in their absolute ground state (v = 0, J = 0) with up to 40% efficiency. Given its simplicity, the method could be extended to other molecules and molecular beams. It also opens up general perspectives in laser cooling the external degrees of freedom of molecules.

Two-step condensation of the charged Bose gas

The condensation of the spinless ideal charged Bose gas in the presence of a magnetic field is revisited. The conventional approach is extended to include the macroscopic occupation of excited kinetic states lying in the lowest Landau level, which plays an essential role in the case of large magnetic fields. In that limit, signatures of two diffuse phase transitions (crossovers) appear in the specific heat. In particular, at temperatures lower than the cyclotron frequency, the system behaves as an effectively one-dimensional free boson system, with the specific heat equal to 1/2Nk(B) and a gradual condensation at lower temperatures.

Nonadiabatic preparation of spin crystals with ultracold polar molecules

We study the growth dynamics of ordered structures of strongly interacting polar molecules in optical lattices. Using a dipole blockade of microwave excitations, we map the system onto an interacting spin-1/2 model possessing ground states with crystalline order, and describe a way to prepare these states by nonadiabatically driving the transitions between molecular rotational levels. The proposed technique bypasses the need to cross a phase transition and allows for the creation of ordered domains of considerably larger size compared to approaches relying on adiabatic preparation.

Experimental studies of the NaCs 5(3)Π0 and 1(a)3Σ+ states

We report high resolution measurements of 372 NaCs 5(3)Π(0)(v, J) ro-vibrational level energies in the range 0 ≤ v ≤ 22. The data have been used to construct NaCs 5(3)Π(0) potential energy curves using the Rydberg-Klein-Rees and inverted perturbation approximation methods. Bound-free 5(3)Π(0)(v, J) → 1(a)(3)Σ(+) emission has also been measured, and is used to determine the repulsive wall of the 1(a)(3)Σ(+) state and the 5(3)Π(0) → 1(a)(3)Σ(+) relative transition dipole moment function. Hyperfine structure in the 5(3)Π(0) state has not been observed in this experiment. This null result is explained using a simple vector coupling model.