Session X10: Long-Ranged Interactions in Quantum Gases

Exploring the supersolid phase of matter with dipolar quantum gases

Supersolids are a paradoxical quantum phase of matter intermediate between superfluids and crystals. I will show how the fundamental properties of supersolids can be explored in Bose-Einstein condensates of strongly dipolar atoms. I will in particular show how one of their key properties, the sub-unity superfluid fraction, can be measured by studying the Josephson effect that naturally exists in supersolids.

    

Observation of vortices in a dipolar supersolid

Supersolids represent a unique class of matter characterized by the spontaneous breaking of two continuous symmetries: translational invariance, resulting from the emergence of a crystal structure, and phase invariance, attributed to the phase locking of single-particle wave functions that underlie superfluid phenomena. Initially hypothesized in solid helium, the observation of supersolids has found a pioneering platform in ultracold quantum gases, particularly with notable success in dipolar atoms. While coherent phase locking in dipolar supersolids has been extensively explored through measurements of phase coherence and gapless Goldstone modes, the identification of quantized vortices, a key hydrodynamic fingerprint of superfluidity, has remained elusive.

 

In this talk, we present our recent results in generating vortices in dipolar supersolid gases with two-dimensional crystalline order. Our theoretical investigation and experimental observations reveal a fundamental difference in vortex seeding dynamics between unmodulated and modulated quantum fluids. This research not only marks a significant advancement in our understanding of dipolar supersolids but also opens new avenues for studying the hydrodynamic properties of exotic quantum systems exhibiting multiple spontaneously broken symmetries.   

Spin Squeezing of Itinerant Dipoles in an Erbium Quantum Gas Microscope

Long-range interactions promise new developments in the field of quantum enhanced sensing. The past two decades have seen significant interest in the creation of scalable spin squeezing and exploring squeezing generation with alternatives to the paradigmatic one-axis-twisting model. Here we experimentally demonstrate metrological squeezing using novel microwave clock transitions in the ground hyperfine manifold of 167Er that harbor appreciable XY interactions. Typically spin squeezing has been achieved with all-to-all interactions, which in the context of trapped atom experiments requires complex state and interaction engineering. Neutral atoms in a sufficiently tight-spacing lattice offer a different pathway to achieving spin-squeezing via the intrinsic magnetic dipole-dipole interaction. We find that by taking advantage of the hyperfine coupling between J and I every neutral atom can find a magnetically insensitive transition with appreciable dipolar exchange. These transitions enable coherent many body spin physics on the second timescale. Following recent theory work, we use one of these transitions to generate spin squeezing in a site-resolved optical lattice quantum simulator. Our lattice experiment enables tunneling allowing us to investigate the question of how itinerant particles impact spin-squeezing. This demonstration establishes a novel method for preparing a metrologically useful coherent spin-squeezed state in a variety of cold atom experimental platforms.   

Towards superconducting mmwave cavity-induced spin squeezing in a neutral cold atom ensemble

Using a cavity to enhance interactions between light and neutral atoms has been investigated for two decades. Its cooperativity, the quantity characterizing the quantum-level interaction strength compared with the decoherence process, however, is surpassed by and is now at least six orders of magnitude behind the superconducting circuit counterparts. Here, we report our recent progress in merging the highly developed superconducting technique and the advantageous properties of neutral atoms, like homogeneity and long coherence time. Specifically, we dress an ensemble of cold Rb atoms by the Rydberg level, which strongly interacts with a superconducting mmwave cavity, to engineer the collective spin state. We showcase this platform by generating spin squeezing, which can be generalized to many highly non-classical states such as the GHZ state, enabled by the strong and unitary interaction offered by the superconducting mmwave cavity and Rydberg levels of neutral Rb atoms.   

Quantifying the impact of dipolar interactions on the accuracy of ultracold quantum simulators and the stability of their many-body phases

Rapid experimental advances in controlling ultracold magnetic atoms and dipolar molecules are making it possible to realize and study enticing quantum states of matter that encapsulate novel properties conferred by long-range interactions. This progress, however, comes with several caveats. A first, more fundamental question is how long-range interactions impact the physics of paradigmatic models that have been studied with short-range interactions thus far. Another, more technical question revolves around how accurately such models can actually be realized in experiments. In this talk, I will present recent results that address both questions. First, I will show how dipolar interactions can strongly enrich many-body phases of matter and their thermodynamic properties with examples from interacting quasicrystals and super-Tonks-Girardeau states. In these systems, dipolar interactions can either strengthen or weaken localization and prethermalization phenomena depending on their magnitude, even leading to new phases. I will then provide a quantitative blueprint for the realization of near-term dipolar quantum simulators in one and two dimensions by systematically comparing their physics to that of the lattice models they purportedly quantum simulate. I will demonstrate that in regimes of high filling and high interactions, strong quantitative discrepancies inevitably arise. This is due to higher band occupation caused by the long-range nature of dipolar interactions, and can have striking consequences for the correct prediction of ground-state properties.   

A Long Range Interacting Erbium Hubbard Quantum Simulator

Long-range interactions play an important role in nature; however, quantum simulations of lattice systems have largely not been able to realize such interactions. We report on recent developments of our Erbium dipolar quantum gas microscope. Our 266 nm lattice spacing magnifies various long range interactions, specifically magnetic dipole interactions, as well as optical near field effects. We recently resolved dipolar quantum solids at half filling generated by the magnetic dipole interaction. Adding finite onsite interaction would, at half filling, allow for preparation of a topological Haldane insulator. We explore the properties of various magnetic Fano-Feschbach resonances to operate in the soft-core extended Hubbard model. In addition, our deeply sub-wavelength lattice allows for super and sub radiance on our 841 nm transition. Combined with our tunable spacing accordion lattice we are able to explore the radiance phase diagram of extended optical emitter arrays with single site resolution. This work demonstrates that novel strongly correlated quantum phases and dynamics can be studied using dipolar interaction in optical lattices, opening the door to quantum simulations of a wide range of lattice models with long-range interactions.   

Exploration of dipolar quantum phases with a degenerate gas of NaCs molecules

Quantum gases of polar molecules have been proposed as a rich system in which long-range interactions can be tuned from the weakly interacting to the unexplored strongly interacting regime. Starting from a BEC of polar molecules [1], we tune the dipolar and s-wave interactions while retaining a long-lived gas through double microwave shielding. Utilizing this control, we set out to explore different quantum phases including droplets and droplet arrays.

[1] “Observation of Bose-Einstein condensation in a gas of dipolar molecules,” Bigagli, N., Yuan, W., Zhang, S., Bulatovic, B., Karman, T., Stevenson, I., Will, S., arXiv:2312.10965 (2023).