Ultracold Molecules Research Articles

Realizing multiband states with ultracold dipolar quantum simulators

The manipulation of dipolar interactions within ultracold molecular ensembles represents a pivotal advancement in experimental physics, aiming at the emulation of quantum phenomena unattainable through mere contact interactions. Our study uncovers regimes of multiband occupation which allow to probe more realistic, complex long-range interacting lattice models with ultracold dipolar simulators. By mapping out experimentally relevant ranges of potential depths, interaction strengths, particle fillings, and geometric configurations, we calculate the agreement between the state prepared in the quantum simulator and a target lattice state. We do so by separately calculating numerically exact many-body wave functions in the continuous space and single- or multiband lattice representations, and building their many-body state overlaps. Our findings reveal that for shallow lattices and stronger interactions above half filling, multiband population increases, resulting in fundamentally different ground states than the ones observed in simple lowest-band descriptions, e.g., striped vs checkerboard states. A wide range of probed parameter regimes in its turn provides a systematic and quantitative blueprint for realizing multiband states with two-dimensional quantum simulators employing ultracold dipolar molecules. Published by the American Physical Society 2024

SU(N) magnetism with ultracold molecules

Abstract Quantum systems with SU(N) symmetry are paradigmatic settings for quantum many-body physics. They have been studied for the insights they provide into complex materials and their ability to stabilize exotic ground states. Ultracold alkaline-earth atoms were predicted to exhibit SU(N) symmetry for N = 2I +1 = 1, 2, . . . , 10, where I is the nuclear spin. Subsequent experiments have revealed rich many-body physics. However, alkaline-earth atoms realize this symmetry only for fermions with repulsive interactions. In this paper, we predict that ultracold molecules shielded from destructive collisions with static electric fields or microwaves exhibit SU(N) symmetry, which holds because deviations of the s-wave scattering length from the spin-free values are only about 3% for CaF with static-field shielding and are estimated to be even smaller for bialkali molecules. They open the door to N as large as 32 for bosons and 36 for fermions. They offer important features unachievable with atoms, including bosonic systems and attractive interactions.

Three-Dimensional Magneto-Optical Trapping of Barium Monofluoride.

As a heavy molecule, barium monofluoride (BaF) presents itself as a promising candidate for measuring permanent electric dipole moment. Here we report the realization of three-dimensional magneto-optical trapping (MOT) of BaF molecules. Through the repumping of all the vibrational states up to v=3, and rotational states up to N=3, we effectively close the transition to a leakage level lower than 10^{-5}. This approach enables molecules to scatter a sufficient number of photons required for laser cooling and trapping. By employing a technique that involves chirping the slowing laser frequency, BaF molecules are decelerated to near-zero velocity, resulting in the capture of approximately 3×10^{3} molecules in a MOT. Our findings represent a significant step towards the realization of ultracold BaF molecules and the conduct of precision measurements with cold molecules.

Magnetically Tunable Electric Dipolar Interactions of Ultracold Polar Molecules in the Quantum Ergodic Regime.

By leveraging the hyperfine interaction between the rotational and nuclear spin degrees of freedom, we demonstrate extensive magnetic control over the electric dipole moments, electric dipolar interactions, and ac Stark shifts of ground-state alkali-dimer molecules such as KRb(X^{1}Σ^{+}). The control is enabled by narrow avoided crossings and the highly ergodic character of molecular eigenstates at low magnetic fields, offering a general and robust way of continuously tuning the intermolecular electric dipolar interaction for applications in quantum simulation, quantum sensing, and dipolar spinor physics.

Quantum sensing of ultralow temperature in biwire ultracold polar molecules

We present a systematic study of quantum sensing of ultralow temperature in biwire ultracold polar molecules of a quasi-one-dimensional (1D) trap by exploring the dynamics of two physically different qubit models. The two models consist of a trapped impurity atom that act as a temperature quantum sensor interacting with polar molecules reservoir, where dipole moments are aligned head-to-tail across the wires. Our model takes advantage of the adjustable interwire distance to accurately control the precision ultralow temperatures measurement. We show that the system undergoes a transition from Markovian to non-Markovian dynamics, which can be controlled by changing the interwire separation, the dipole-–dipole interaction (DDI), and the temperature. We characterize the thermometric performance using the quantum signal-to-noise ratio for both models and demonstrate that such a quantity exhibits a higher peak at ultralow temperature. We therefore emphasize that ultracold polar molecules are crucial for revolutionizing temperature sensing. Published by the American Physical Society 2024

Two-axis twisting using Floquet-engineered XYZ spin models with polar molecules.

Polar molecules confined in an optical lattice are a versatile platform to explore spin-motion dynamics based on strong, long-range dipolar interactions1,2. The precise tunability3 of Ising and spin-exchange interactions with both microwave and d.c. electric fields makes the molecular system particularly suitable for engineering complex many-body dynamics4-6. Here we used Floquet engineering7 to realize new quantum many-body systems of polar molecules. Using a spin encoded in the two lowest rotational states of ultracold 40K87Rb molecules, we mutually validated XXZ spin models tuned by a Floquet microwave pulse sequence against those tuned by a d.c. electric field through observations of Ramsey contrast dynamics. This validation sets the stage for the realization of Hamiltonians inaccessible with static fields. In particular, we observed two-axis twisting8 mean-field dynamics, generated by a Floquet-engineered XYZ model using itinerant molecules in two-dimensional layers. In the future, Floquet-engineered Hamiltonians could generate entangled states for molecule-based precision measurement9 or could take advantage of the rich molecular structure for quantum simulation of multi-level systems10,11.

Quantum state engineering in a five-state chainwise system by generalized coincident pulse technique.

In this paper, an exact analytical solution is presented for achieving coherent population transfer and creating arbitrary coherent superposition states in a five-state chainwise system by a train of coincident pulses. We show that the solution of a five-state chainwise system can be reduced to an equivalent three-state Λ-type one with the simplest resonant coupling under the assumption of adiabatic elimination together with a requirement of the relation among the four coincident pulses. In this method, the four coincident pulses at each step all have the same time dependence, but with specific magnitudes. The results show that, by using a train of appropriately coincident pulses, this technique not only enables complete population transfer, but also creates any desired coherent superposition between the initial and final states, while the population in all intermediate states is effectively suppressed. Furthermore, this technique can also exhibit a one-way population transfer behavior. The results are of potential interest in applications where high-fidelity multi-state quantum control is essential, e.g., quantum information, atom optics, formation of ultracold molecules, cavity QED, nuclear coherent population transfer, and light transfer in waveguide arrays.

Quantum gate control of polar molecules with machine learning.

We propose a scheme for achieving basic quantum gates using ultracold polar molecules in pendular states. The qubits are encoded in the YbF molecules trapped in an electric field with a certain gradient and coupled by the dipole-dipole interaction. The time-dependent control sequences consisting of multiple pulses are considered to interact with the pendular qubits. To achieve high-fidelity quantum gates, we map the control problem for the coupled molecular system into a Markov decision process and deal with it using the techniques of deep reinforcement learning (DRL). By training the agents over multiple episodes, the optimal control pulse sequences for the two-qubit gates of NOT, controlled NOT, and Hadamard are discovered with high fidelities. Moreover, the population dynamics of YbF molecules driven by the discovered gate sequences are analyzed in detail. Furthermore, by combining the optimal gate sequences, we successfully simulate the quantum circuit for entanglement. Our findings could offer new insights into efficiently controlling molecular systems for practical molecule-based quantum computing using DRL.

Searching for new fundamental interactions via isotopic shifts in molecular lattice clocks

Precision measurements with ultracold atoms and molecules are primed to probe beyond-the-standard model physics. Isotopologues of homonuclear molecules are a natural testbed for new Yukawa-type mass-dependent forces at nanometer scales, complementing existing mesoscopic-body and neutron scattering experiments. Here, we propose using isotopic shift measurements in molecular lattice clocks to constrain these new interactions from new massive scalar particles in the keV/c2 range: The new interaction would impart an extra isotopic shift to molecular levels on top of one predicted by the standard model. For the strontium dimer, a Hz-level agreement between experiment and theory could constrain the coupling of the new particles to hadrons by up to an order of magnitude over the most stringent existing experiments. Published by the American Physical Society 2024

Ultracold LiCr : A New Pathway to Quantum Gases of Paramagnetic Polar Molecules

Quantum gases of doubly polar molecules represent appealing frameworks for a variety of cross-disciplinary applications, encompassing quantum simulation and computation, controlled quantum chemistry, and precision measurements. Through a joint experimental and theoretical study, here we explore a novel class of ultracold paramagnetic polar molecules combining lithium alkali and chromium transition metal elements. Focusing on the specific bosonic isotopologue 6Li53Cr, leveraging on the Fermi statistics of the parent atomic mixture and on suitable Feshbach resonances recently discovered, we produce up to 50×103 ultracold LiCr molecules at peak phase-space densities exceeding 0.1, prepared within the least bound rotationless level of the LiCr electronic ground state X6Σ+. By also developing new probing methods, we thoroughly characterize the molecular gas, demonstrating the paramagnetic nature of LiCr dimers and the precise control of their quantum state. We investigate their stability against inelastic processes and identify a parameter region where pure LiCr samples exhibit lifetimes exceeding 0.2 s. Parallel to this, we employ state-of-the-art quantum chemical calculations to accurately predict the properties of LiCr ground and excited electronic states. This model, able to reproduce the experimental Li-Cr high-spin, scattering length, allows us to identify both efficient paths to coherently transfer weakly bound LiCr dimers to their absolute ground state, and suitable transitions for their subsequent optical manipulation. Our studies establish Li-Cr as a prime candidate to realize ultracold gases of doubly polar molecules with significant electric (3.3 D) and magnetic (5μB) dipole moments. Published by the American Physical Society 2024

Observation of Bose-Einstein condensation of dipolar molecules.

Ensembles of particles governed by quantum mechanical laws exhibit intriguing emergent behaviour. Atomic quantum gases1,2, liquid helium3,4 and electrons in quantum materials5-7 all exhibit distinct properties because of their composition and interactions. Quantum degenerate samples of ultracold dipolar molecules promise the realization of new phases of matter and new avenues for quantum simulation8 and quantum computation9. However, rapid losses10, even when reduced through collisional shielding techniques11-13, have so far prevented evaporative cooling to a Bose-Einstein condensate (BEC). Here we report on the realization of a BEC of dipolar molecules. By strongly suppressing two- and three-body losses viaenhanced collisional shielding, we evaporatively cool sodium-caesium molecules to quantum degeneracy and cross the phase transition to aBEC. The BEC reveals itself by a bimodal distribution when the phase-space density exceeds 1. BECs with a condensate fraction of 60(10)% and a temperature of 6(2) nK are created and found to be stable with a lifetime close to 2 s. This work opens the door to the exploration of dipolar quantum matter in regimes that have been inaccessible so far, promising the creation of exotic dipolar droplets14, self-organized crystal phases15 and dipolar spin liquids in optical lattices16.

Rotational magic conditions for ultracold molecules in the presence of Raman and Rayleigh scattering

Molecules have vibrational, rotational, spin-orbit and hyperfine degrees of freedom or quantum states, each of which responds in a unique fashion to external electromagnetic radiation. The control over superpositions of these quantum states is key to coherent manipulation of molecules. For example, the better the coherence time the longer quantum simulations can last. The important quantity for controlling an ultracold molecule with laser light is its complex-valued molecular dynamic polarizability. Its real part determines the tweezer or trapping potential as felt by the molecule, while its imaginary part limits the coherence time. Here, our study shows that efficient trapping of a molecule in its vibrational ground state can be achieved by selecting a laser frequency with a detuning on the order of tens of GHz relative to an electric-dipole-forbidden molecular transition. Close proximity to this nearly forbidden transition allows to create a sufficiently deep trapping potential for multiple rotational states without sacrificing coherence times among these states from Raman and Rayleigh scattering. In fact, we demonstrate that magic trapping conditions for multiple rotational states of the ultracold 23Na87Rb polar molecule can be created.

An electric-field-assisted magnetic trap for cold molecular evaporative cooling: Toward molecular Bose–Einstein condensation

Ultracold molecules provide fundamental new insights into molecular interaction dynamics in the quantum regime and represent a new platform for chemical physics where quantum behaviors play a dominant role in molecular interaction and dynamics. An electric-field-assisted magnetic trap for trapping and further evaporative cooling of cold molecules to ultracold regime is proposed utilizing the perturbation of the hyperfine levels in a mixed field, and the depth of the trap can be tuned by adjusting the assisted electric field rapidly. Thus, the evaporative cooling of [Formula: see text]I[Formula: see text]Br molecules in the [Formula: see text] state of the rovibronic ground state is simulated. It shows that IBr molecules would be cooled from 26[Formula: see text][Formula: see text]K to 556[Formula: see text]nK within 598.9[Formula: see text]ms and eventually we would obtain a number of [Formula: see text] molecules in a volume of [Formula: see text][Formula: see text]cm3 with the number density of [Formula: see text][Formula: see text]cm[Formula: see text]. The Bose–Einstein condensation of the alkali diatomic molecules would probably be realized in the proposed trap if the present experimentally available samples are loaded.

Fine Control of Ultracold Polar Molecules

Fine Control of Ultracold Polar Molecules

A resonance triggers chemical reactions between the coldest molecules

The rich energy structure of ultracold molecules (at -459 °F or -273 °C), gives rise to collisional dynamics where the state-of-the-art models are inadequate for describing collisional resonances. We have discovered a pronounced magnetically tuned resonance in collisions between two NaLi molecules, which enhances the chemical reaction rate by more than a factor of a hundred.

ABLRI: A program for calculating the long-range interaction energy between two monomers in their non-degenerate states.

An accurate description of the long-range (LR) interaction is essential for understanding the collision between cold or ultracold molecules. However, to our best knowledge, there lacks a general approach to construct the intermolecular potential energy surface (IPES) between two arbitrary molecules and/or atoms in the LR region. In this work, we derived analytical expressions of the LR interaction energy, using the multipole expansion of the electrostatic interaction Hamiltonian and the non-degenerate perturbation theory. To make these formulae practical, we also derived the independent Cartesian components of the electrostatic properties, including the multipole moments and polarizabilities, of the monomer for a given symmetry using the properties of these components and the group-theoretical methods. Based on these newly derived formulae, we developed a FORTRAN program, namely ABLRI, which is capable of calculating the interaction energy between two arbitrary monomers both in their non-degenerate electronic ground states at large separations. To test the reliability of this newly developed program, we constructed IPESs for the electronic ground state of H2O-H2 and O2-H systems in the LR region. The interaction energy computed by our program agreed well with the abinitio calculation, which shows the validity of this program.

Prospects for thermalization of microwave-shielded ultracold molecules

Toward more efficient schemes for achieving deeply degenerate molecular Fermi gases, we study anisotropic thermalization in dilute gases of microwave shielded polar molecular fermions. For collision energies above the threshold regime, we find that thermalization is suppressed due to a strong preference for forward scattering and a reduction in total cross section with energy, significantly reducing the efficiency of evaporative cooling. We perform close-coupling calculations on the effective potential energy surface derived by Deng [] to obtain accurate two-body elastic differential cross sections across a range of collision energies. We use Gaussian process regression to obtain a global representation of the differential cross section over a wide range of collision angles and energies. The route to equilibrium is then analyzed with cross-dimensional rethermalization experiments, quantified by a measure of collisional efficiency toward achieving thermalization. Published by the American Physical Society 2024

Quantum computation and quantum simulation with ultracold molecules

Quantum computation and quantum simulation with ultracold molecules

Cold and ultracold molecules

Cold and ultracold molecules

Quantum state manipulation and cooling of ultracold molecules

Quantum state manipulation and cooling of ultracold molecules