Session G05: Novel Phases of Cold Atoms, Ions, and Molecules

A negative temperature state of bosons in an optical kagome lattice

The kagome lattice exhibits a high degree of geometric frustration giving rise to a flat band. Since kinetic energy plays no role in a flat band, it is interaction and topology that determine the behaviour of the system. The many-body physics of atoms in this system is however not fully understood and the frustration makes simulations with classical computers challenging. We instead use neutral atoms in optical lattices as an analogue quantum simulator. Because the flat band is the highest in energy of the three touching lowest bands it will normally not be populated when ultracold atoms are loaded to the lattice. In our case this problem is solved by preparing atoms in a state with negative absolute temperature, for which the highest energies are preferentially occupied. I will present our work creating, for the first time, a negative temperature state of bosonic ³⁹K atoms in the kagome lattice and report on our observations of the melting of the negative temperature Mott Insulator into the flat band.   

Dimension-Dependent, Tunable Spin Dynamics with Itinerant Ultracold Polar Molecules

Ultracold molecules enable exploration of many-body physics due to their long-range, anisotropic dipolar interactions. With a microwave-addressable spin encoded in the two lowest rotational states of the molecules, motion controlled with optical lattices, and dipolar interactions tuned with d.c. electric fields, we realized a coupled spin-motion system of interacting molecules. In the motionless case, when molecules are pinned by a 3D optical lattice, their Ramsey contrast dynamics are well described by the XXZ Heisenberg spin model. When the molecules are free to move in either 1D tubes or 2D layers, molecular collisions modify the contrast decay rate differently for small and large electric fields. We attribute this effect to the difference between two collision channels of aligned and anti-aligned molecular pairs. Tuning between the fully pinned and the free moving extremes by allowing tunneling between lattice sites reveals further exciting behavior with a decoherence resonance appearing in 2D. This research marks the first time that t-J models have been explored with polar molecules, paving the way for future studies of kinetic spin models with the tunability of molecular platforms.   

A Coherence Microscope Based on the Matter-Wave Talbot Effect

Imaging is crucial for gaining insight into physical systems. In the case of ultracold atoms in optical lattices, quantum gas microscopes have revolutionized the access to quantum many-body systems by detecting and addressing single atoms on single lattice sites. The novel technique of quantum gas magnification uses matter-wave optics to magnify the density distribution before the optical imaging and therefore allows to directly image the Talbot carpet that forms when releasing the atoms from an optical lattice. 

We realize this for a BEC of Lithium-7 atoms in a triangular lattice and map out the spatial coherence by analyzing the contrast of successive Talbot copies. The technique should also allow to reconstruct the fluctuating phase profile of individual samples imaged at a Talbot copy. This will realize a coherence microscope with spatially resolved access to phase information, which allows to study domain walls, thermally activated vortex-pairs, or to locally evaluate coherence in inhomogeneous quantum many-body systems.   

Experimental exploration of the 1D anyon-Hubbard model via adiabatic state preparation

Anyons are indistinguishable particles whose exchange statistics are neither bosonic nor fermionic. While anyons are known to exist in two dimensions, a one-dimensional (1D) system can also host fractional statistics. In this work, we focus on the 1D anyon-Hubbard model (AHM) which describes anyons on a 1D lattice and has been theorized to host many phenomena. We experimentally realize the AHM with ultracold rubidium-87 atoms in an optical lattice via periodic driving [1]. Leveraging the ability to independently tune Hubbard parameters, we engineer the two-particle ground states of the AHM via adiabatic state preparation, by connecting a Fock state to the ground state of the AHM with arbitrary statistical phase. As the statistical phase increases from 0 to 𝜋, we observe smooth change from bosonic, through anyonic, to pseudo-fermionic density profiles, which manifests a continuous build up of Friedel oscillations. Moreover, we probe the properties of the AHM via expansion dynamics, revealing the existence of chiral bound states induced by quantum statistics. Our work lays a foundation to study many-body phenomena of the AHM, such as a statistically-induced Mott insulator to superfluid phase transition and novel two-component superfluids. 

[1] J Kwan, et al., Realization of 1D anyons with arbitrary statistical phase. Preprint on arxiv (2023).      

Sounds waves and fluctuations in one-dimensional supersolids

We consider the low energy excitations of a dilute supersolid state of matter with a one-dimensional crystal structure. We develop a hydrodynamic description based on a Lagrangian with generalized elastic parameters determined from ground state calculations. We are able to compare the predictions of this hydrodynamic theory to the direct calculation of the excitation spectrum obtained by solving the Bogoliubov-de Gennes equations. We make detailed comparisons of the speeds of sound, the various types of density fluctuations and the phase fluctuations of the gapless bands. Our results are presented for two contrasting modes: (i) a dipolar Bose-Einstein condensate in an infinite tube and (ii) a dilute Bose gas of atoms with soft-core interactions.   

Entanglement in dipolar synthetic dimension platforms with ultracold Rydberg atoms or polar molecules

Synthetic dimension platforms offer pathways to study unique quantum matter. Recent studies (arXiv: 2307.16269) have revealed a rich phase diagram of a quantum many-body system of ultracold atoms (or polar molecules) with a set of Rydberg states (or rotational states) as synthetic dimension where the real particles are arranged in optical microtrap or optical tweezer arrays and interact via dipole-dipole exchange interaction. The Rydberg or the rotational states are coupled via microwaves which are easily tuned to the desired tunneling scheme. Three non-trivial symmetry-broken phases were characterized: two of which are string (1 synthetic dimension) or membrane (2 synthetic dimension) -like phase wherein the only a few synthetic states are populated, and one non-localized but ordered. In this talk, we discuss this system and their quantum phases. The string or membrane-like phase is highly entangled and for carefully chosen real-space geometries and number of synthetic sites, they seem to exhibit a large degeneracy, much like in a quantum liquid. The degeneracy is lifted in the presence of strong synthetic tunneling amplitudes. We will discuss the effects of real-space geometry, different synthetic tunneling schemes and the number of synthetic sites on the ground-state properties. Through this talk, we hope to elucidate that synthetic dimension platforms are very versatile and highly tunable which allows one to generate unique quantum matter by minimal tweaking of the system parameters.   

Studying frustration with ultracold bosons in a triangular optical lattice

In the lowest band of the triangular lattice, the inherent geometric frustration of the lattice gives rise to two degenerate, inequivalent maxima with different wavevectors.  When tunnelling is inverted, such that these maxima become the minima, the superfluid ground state becomes chiral and chooses one of the two minima, breaking a discrete symmetry. The nature of the quantum phase transition from this chiral superfluid to a Mott insulator (SF-MI) at strong interactions is not settled, as the lattice frustration hinders quantum Monte-Carlo simulation. 

We experimentally study this transition in an optical lattice. Instead of actively changing the sign of the tunnelling, we utilize negative absolute temperatures where atoms predominantly occupy the highest-energy states. We find that due to the inherent geometric frustration, the critical interaction for the bosonic SF-MI transition is strongly reduced. Furthermore, by dynamically crossing the SF-MI transition in variable time we observe a smooth emergence of coherence, reminiscent of the Kibble-Zurek mechanism. This is consistent with a continuous phase transition and thereby suggests the existence of an intervening chiral Mott insulator phase.    

The Attractive SU(N) Fermi-Hubbard Model in Ultracold Molecules

The SU(N) Fermi-Hubbard model has seen a surge of interest due to its realization in alkaline-earth atoms in optical lattices. It has a rich, exotic phase diagram resulting from its high symmetry. However, the alkaline-earth atoms have limitations: in particular, they lack magnetic Feshbach resonances, so can realize only repulsive SU(N) Fermi-Hubbard models. Motivated by our (unpublished) work showing that shielded ultracold molecules enjoy an SU(N) symmetry with more flexible interactions, we study the attractive SU(N) Fermi-Hubbard model. By using the Determinant Quantum Monte Carlo (DQMC) method with an appropriate Hubbard-Stratonovich transformation, we can accurately calculate properties of this model. While our method works for arbitrary N, we have focused in particular on the square-lattice SU(3) model. We have studied the formation and ordering of three-particle bound states of SU(3) singlets with density, temperature, and interaction strength. We make predictions for experiments and suggest the most exciting directions forward.