Synthetic quantum systems with interacting constituents play an important role in quantum information processing and in elucidating fundamental phenomena in many-body physics. Following impressive advances in cooling and trapping techniques, ensembles of ultracold polar molecules have emerged as a promising synthetic system that combines several advantageous properties. These include a large set of internal states for encoding quantum information, long nuclear and rotational coherence times and long-range, anisotropic interactions. The latter are expected to allow the exploration of intriguing phases of correlated quantum matter, such as topological superfluids, quantum spin liquids, fractional Chern insulators and quantum magnets. Probing correlations in these phases is crucial to understand their microscopic properties, necessitating the development of new experimental techniques. Here we use quantum gas microscopy to measure the site-resolved dynamics of quantum correlations in a gas of polar molecules in a two-dimensional optical lattice. Using two rotational states of the molecules, we realize a spin-1/2 system where the particles are coupled via dipolar interactions, producing a quantum spin-exchange model. Starting with the synthetic spin system prepared far from equilibrium, we study the evolution of correlations during the thermalization process for both spatially isotropic and anisotropic interactions. Furthermore, we study the correlation dynamics in a spin-anisotropic Heisenberg model engineered from the native spin-exchange model using Floquet techniques. These experiments push the frontier of probing and controlling interacting systems of ultracold molecules, with prospects for exploring new regimes of quantum matter and characterizing entangled states useful for quantum computation and metrology. 

/pdf/probing-site-resolved-correlations-in-a-spin-system-of-2uj9xq23.pdf

Probing site-resolved correlations in a spin system of ultracold molecules

In this paper we consider the massive scalar perturbation on the top of a small spinning-like black hole in context of Einstein-bumblebee modified gravity in order to probe the role of spontaneous Lorentz symmetry breaking on the superradiance scattering and corresponding instability. We show that at the low-frequency limit of the scalar wave the superradiance scattering will be enhanced with the Lorentz-violating parameter $\ensuremath{\alpha}l0$ and will be weakened with $\ensuremath{\alpha}g0$. Moreover, by addressing the black hole bomb issue, we extract an improved bound in the instability regime indicating that $\ensuremath{\alpha}l0$ increases the parameter space of the scalar field instability, while $\ensuremath{\alpha}g0$ decreases it.

Black hole superradiance in the presence of Lorentz symmetry violation

Strongly interacting spins underlie many intriguing phenomena and applications1–4 ranging from magnetism to quantum information processing. Interacting spins combined with motion show exotic spin transport phenomena, such as superfluidity arising from pairing of spins induced by spin attraction5,6. To understand these complex phenomena, an interacting spin system with high controllability is desired. Quantum spin dynamics have been studied on different platforms with varying capabilities7–13. Here we demonstrate tunable itinerant spin dynamics enabled by dipolar interactions using a gas of potassium-rubidium molecules confined to two-dimensional planes, where a spin-1/2 system is encoded into the molecular rotational levels. The dipolar interaction gives rise to a shift of the rotational transition frequency and a collision-limited Ramsey contrast decay that emerges from the coupled spin and motion. Both the Ising and spin-exchange interactions are precisely tuned by varying the strength and orientation of an electric field, as well as the internal molecular state. This full tunability enables both static and dynamical control of the spin Hamiltonian, allowing reversal of the coherent spin dynamics. Our work establishes an interacting spin platform that allows for exploration of many-body spin dynamics and spin-motion physics using the strong, tunable dipolar interaction. Tunable itinerant spin dynamics enabled by dipolar interactions are demonstrated with polar molecules, establishing an interacting spin platform that allows for exploration of many-body spin dynamics and spin-motion physics using strong, tunable dipolar interaction. 

/pdf/tunable-itinerant-spin-dynamics-with-polar-molecules-1xi45x3z.pdf

Tunable itinerant spin dynamics with polar molecules

Collisional resonances are important tools that have been used to modify interactions in ultracold gases, for realizing previously unknown Hamiltonians in quantum simulations1, for creating molecules from atomic gases2 and for controlling chemical reactions. So far, such resonances have been observed for atom–atom collisions, atom–molecule collisions3–7 and collisions between Feshbach molecules, which are very weakly bound8–10. Whether such resonances exist for ultracold ground-state molecules has been debated owing to the possibly high density of states and/or rapid decay of the resonant complex11–15. Here we report a very pronounced and narrow (25 mG) Feshbach resonance in collisions between two triplet ground-state NaLi molecules. This molecular Feshbach resonance has two special characteristics. First, the collisional loss rate is enhanced by more than two orders of magnitude above the background loss rate, which is saturated at the p-wave universal value, owing to strong chemical reactivity. Second, the resonance is located at a magnetic field where two open channels become nearly degenerate. This implies that the intermediate complex predominantly decays to the second open channel. We describe the resonant loss feature using a model with coupled modes that is analogous to a Fabry–Pérot cavity. Our observations provide strong evidence for the existence of long-lived coherent intermediate complexes even in systems without reaction barriers and open up the possibility of coherent control of chemical reactions. Observations of a pronounced and narrow Feshbach resonance in collisions between two triplet ground-state NaLi molecules are described, providing evidence for the existence of long-lived coherent intermediate complexes even in systems without reaction barriers. 

/pdf/a-feshbach-resonance-in-collisions-between-triplet-ground-3qqko86w.pdf

A Feshbach resonance in collisions between triplet ground-state molecules

Superradiance and stability of Kerr black hole enclosed by anisotropic fluid matter

Ultracold polar molecules offer strong electric dipole moments and rich internal structure, which makes them ideal building blocks to explore exotic quantum matter, implement novel quantum information schemes, or test fundamental symmetries of nature. Realizing their full potential requires cooling interacting molecular gases deeply into the quantum degenerate regime. However, the complexity of molecules which makes their collisions intrinsically unstable at the short range, even for nonreactive molecules, has so far prevented the cooling to quantum degeneracy in three dimensions. Here, we demonstrate evaporative cooling of a three-dimensional gas of fermionic sodium-potassium molecules to well below the Fermi temperature using microwave shielding. The molecules are protected from reaching short range with a repulsive barrier engineered by coupling rotational states with a blue-detuned circularly polarized microwave. The microwave dressing induces strong tunable dipolar interactions between the molecules, leading to high elastic collision rates that can exceed the inelastic ones by at least a factor of 460. This large elastic-to-inelastic collision ratio allows us to cool the molecular gas down to 21 nanokelvin, corresponding to 0.36 times the Fermi temperature. Such unprecedentedly cold and dense samples of polar molecules open the path to the exploration of novel many-body phenomena, such as the long-sought topological p-wave superfluid states of ultracold matter. 

https://www.nature.com/articles/s41586-022-04900-0.pdf

Evaporation of microwave-shielded polar molecules to quantum degeneracy

We study the dynamics of charged massless scalar fields in asymptotically anti–de Sitter (AdS) dilatonic black holes. The instability of the scalar field, which is triggered by the charged superradiant effect and Dirichlet boundary condition at AdS spatial infinity, can be notably described by exponential growth of the scalar field amplitude in time evolving and unstable quasinormal modes of scalar field perturbation. The numerical computation to demonstrate the superradiant instability of the charged scalar field are performed in the time domain and frequency domain, respectively. We confirm the mode that dominates long time behavior of scalar field matches with quasinormal mode obtained from frequency domain analysis quite well. It is well known the small Reissner-Nordstrom-anti–de Sitter (RN-AdS) black holes are superradiant unstable, while the large RN-AdS black holes are stable against the charged scalar perturbations. According to our numerical results, it is observed that the large dilatonic AdS black holes are also superradiant unstable. At last, we conclude that there exist rapid exponential growing superradiant modes in charged dilatonic AdS black holes, which is significant for the nonlinear dynamical evolution of charged superradiant instability.

Superradiance and dynamical evolution of a charged scalar field in an asymptotically anti-de-Sitter dilatonic black hole

Scattering resonances are an essential tool for controlling the interactions of ultracold atoms and molecules. However, conventional Feshbach scattering resonances1, which have been extensively studied in various platforms1-7, are not expected to exist in most ultracold polar molecules because of the fast loss that occurs when two molecules approach at a close distance8-10. Here we demonstrate a new type of scattering resonance that is universal for a wide range of polar molecules. The so-called field-linked resonances11-14 occur in the scattering of microwave-dressed molecules because of stable macroscopic tetramer states in the intermolecular potential. We identify two resonances between ultracold ground-state sodium-potassium molecules and use the microwave frequencies and polarizations to tune the inelastic collision rate by three orders of magnitude, from the unitary limit to well below the universal regime. The field-linked resonance provides a tuning knob to independently control the elastic contact interaction and the dipole-dipole interaction, which we observe as a modification in the thermalization rate. Our result provides a general strategy for resonant scattering between ultracold polar molecules, which paves the way for realizing dipolar superfluids15 and molecular supersolids16, as well as assembling ultracold polyatomic molecules. 

/pdf/field-linked-resonances-of-polar-molecules-3ieoqu8l.pdf

Field-linked resonances of polar molecules

We study the nonequilibrium condensation process of a holographic $p$-wave superconductor model, which is described by Einstein-Maxwell theory coupled with the charged complex vector field in asymptotic anti-de Sitter (AdS) spacetime. By solving the gravitational dynamics in the asymptotic AdS spacetime numerically, we obtain the full time dependent solution of the system, and observe a dynamical process from the perturbed Reissner-Nordstr\"om-AdS black hole to the anisotropic black hole with vector hair when the initial black hole temperature is smaller than the critical temperature ${T}_{c}$. By holography, this nonlinear dynamical process is then dual to the phase transition process from a normal state to a $p$-wave superconducting state on the boundary theory. We also investigate the time evolution of the superconducting condensate operator and horizon area.

Nonequilibrium condensation process of a holographic $p$-wave superconductor

We analytically show that the effective interaction potential between microwave-shielded polar molecules consists of an anisotropic van der Waals-like shielding core and a modified dipolar interaction. This effective potential is validated by comparing its scattering cross sections with those calculated using intermolecular potential involving all interaction channels. It is shown that a scattering resonance can be induced under microwave fields reachable in current experiments. With the effective potential, we further study the Bardeen-Cooper-Schrieffer pairing in the microwave-shielded NaK gas. We show that the superfluid critical temperature is drastically enhanced near the resonance. As the effective potential is suitable for exploring the many-body physics of molecular gases, our results pave the way for studies of the ultracold gases of microwave-shielded molecular gases.

Xingliang Chen

Papers

Evaporation of microwave-shielded polar molecules to quantum degeneracy

Superradiance and dynamical evolution of a charged scalar field in an asymptotically anti-de-Sitter dilatonic black hole

Field-linked resonances of polar molecules

Nonequilibrium condensation process of a holographic $p$-wave superconductor

Effective Potential and Superfluidity of Microwave-Shielded Polar Molecules.