Shear flow and vortex array instabilities in annular strongly-correlated atomic superfluids (FINESS)

• Giacomo Roati (CNR-INO)

At the interface between two fluid layers in relative motion, infinitesimal fluctuations can grow exponentially, generating vorticity and causing the laminar flow to break down. Here, we study this scenario by creating two counter-rotating flows in annular atomic Fermi superfluids across the BEC-BCS crossover [1]. Due to the continuity of the superfluid wavefunction and the quantisation of circulation, the superfluids cannot maintain a continuous vortex sheet. Instead, we observe the formation of a regular array of quantised vortices forming along the shear layer. This vortex array is unstable. We connect its dynamics to the instability of the counter-propagating flows, establishing a clear link between shear flow and vortex instabilities. Our work opens the prospects for exploring out-of-equilibrium phenomena such as vortex matter phase transitions and the spontaneous emergence and decay of two-dimensional quantum turbulence in strongly-correlated superfluids. [1] D. Hernández-Rajkov et al., Nature Physics 20, 939 (2024)

Reynolds similitude of a pure superfluid at low temperatures (FINESS)

• Hiromitsu Takeuchi (Osaka Metropolitan University)

The Reynolds similitude, a key concept in hydrodynamics, states that two phenomena of different length scales with a similar geometry are physically identical. Flow properties are universally determined in a unified way in terms of the Reynolds number ${\cal R}$ (dimensionless, ratio of inertial to viscous forces in incompressible fluids). For example, the drag coefficient $c_D$ of objects with similar shapes moving in fluids is expressed by a universal function of ${\cal R}$. Certain studies introduced similar dimensionless numbers, that is, the superfluid Reynolds number ${\cal R}_s$, to characterize turbulent flows in superfluids. However, the applicablity of the similitude to inviscid quantum fluids is nontrivial as the original theory is applicable to viscous fluids. This study proposes a method to verify the similitude using current experimental techniques in quantum liquid He-II. A highly precise relation between $c_D$ and ${\cal R}_s$ was obtained in terms of the terminal speed of a macroscopic body falling in He-II at finite temperatures across the Knudsen (ballistic) and hydrodynamic regimes of thermal excitations. The Reynolds similitude in superfluids proves the quantum viscosity of a pure superfluid and can facilitate a unified mutual development of classical and quantum hydrodynamics; the concept of quantum viscosity provides a practical correspondence between classical and quantum turbulence as a dissipative phenomenon.

Vortices on rotating shell-shaped Bose-Einstein condensates (FINESS)

• Angela White (The University of Queensland)

The recent realisation of hollow-core bubbles of Bose-Einstein condensates on the Cold Atom Lab aboard the International Space Station has rendered this intriguing geometry accessible, motivating study into the behaviour of vortices on curved surfaces [1]. In shell geometries, superfluid vortex behaviour promises interesting responses to rotation as the continuity of the velocity field required across the closed-curved surface of the shell imposes additional restrictions on the condensate phase. We study the response of a bubble condensate to an externally imposed rotation, demonstrating that for small rotation rates, a familiar triangular Abrikosov lattice of vortices is formed, with two aligned vortex lattices appearing in each hemispherical shell. An elliptical deformation of the spherically symmetric condensate shape occurs at larger rotation rates due to the centrifugal force. As the driving rotation frequency is increased, a multi-charge vortex and its anti-vortex pair is formed at the poles, surrounded by singly charged vortices in the bulk condensate density. strong text

(Non equilibrium) thermodynamics of classical Integrable models in their thermodynamic limit (FINESS)

• Leticia Cugliandolo

Motivated by recent experimental developments in atomic physics, a large theoretical effort has been devoted to the analysis of the dynamics of quantum isolated systems after a sudden quench. In this talk I will describe the evolution of a family of classical many-body integrable (Neumann) models after instantaneous quenches of the same kind. The asymptotic dynamics of these models can be fully elucidated, and the stationary properties (in the thermodynamic limit) compared to the ones obtained exactly using a Generalised Gibbs Ensemble. The latter can not only be built but also used to evaluate analytically all relevant observables, a quite remarkable fact for an interacting integrable system with a non-trivial phase diagram.

Universal dynamics in strongly interacting Bose gases far from equilibrium (FINESS)

• RuGway WU (Atominstitut, Technische Universität Wien)

We prepare gases of Feshbach molecules of lithium 6 far from equilibrium by introducing broad excitations using an optical speckle potential, and study the relaxation of the strongly interacting isolated many-body system going through the formation of a molecular Bose-Einstein condensates (mBEC). We report for the first time, universal spatio-temporal scaling behaviour in a strongly interacting Bose gas. By varying the interaction strength of the system and observing the duration of scaling evolution, we show how the dimensionality of the system affects the stability of soliton-like excitations, and the non-equilibrium evolution rate. Comparison of the observed dependence on interaction with the earlier rubidium experiment suggests a possible general scaling law governing the self-similar evolution.

Thermal fluctuations in multicomponent quantum gases (FINESS)

• Arko Roy (IIT Mandi)

In the study of various quantum phase transitions and excitations in Bose-Einstein condensates (BECs), several key findings emerge across different scenarios. For a spin-orbit-coupled quasi-one-dimensional BEC, temperature-induced transitions from a superfluid plane-wave phase to a supersolid phase are observed, contrasting with the behavior in homogeneous systems. The Hartree-Fock-Bogoliubov theory with the Popov approximation is utilized to analyze collective excitation spectra, particularly focusing on the softening of the spin-dipole mode near the quantum critical point. Furthermore, the collective excitations of a spin-orbit-coupled spin-1 BEC in a trapping potential are studied theoretically at both zero and finite temperatures. Density and spin excitations exhibit distinct behaviors with temperature variations, notably affected by spin-orbit coupling. Transitioning to a two-dimensional coherently-coupled Bose mixture, a paramagnetic-ferromagnetic quantum phase transition at zero temperature is identified, with subsequent investigations into its behavior at finite temperatures. Stochastic Gross-Pitaevskii formalism is employed, revealing a linear shift of the critical point with temperature and power-law scaling of critical quantities, consistent with thermal critical exponents. Finally, the role of thermal fluctuations in two-dimensional binary Bose mixtures is explored, particularly focusing on the miscible-immiscible transition. Mean-field Hartree-Fock theory predicts a transition instability at non-zero temperatures due to divergent behavior in spin susceptibility, which is partially confirmed by numerical simulations. However, discrepancies between theory and simulations suggest that mean-field approximations struggle to accurately describe the system near the transition, especially concerning thermal fluctuations.

Tweezers, trapped ions, and Rydbergs: a quantum simulation zoo (FINESS)

• Arghavan Safavi-Naini (University of Amsterdam)

Trapped-ions are one of the most mature platforms for quantum computation and quantum simulation. In this talk I will show that by adding other ingredients, such as optical tweezers, neutral atoms, and Rydberg atoms, we can engineer more flexible quantum simulation platforms, as well as new quantum computation architectures. In trapped-ion quantum simulators the spin-spin interactions mediated by the collective motion of the ions in the crystal (phonons) are tunable range power law interactions. I will show that additional optical tweezer potentials can be used to engineer the phonon spectrum, and thus tune the interactions and connectivity of the ion qubits beyond the power-law interactions accessible in current setups. Next, I will show that optical tweezers delivering qubit state-dependent local potentials allow us to create a new scalable architecture for trapped-ion quantum computing. Finally, I will discuss how adding one more ingredient, a gas of neutral atoms, allows us to explore ultracold chemistry and exploit ion mediated interactions for infinite range Rydberg blockade and facilitation.

Observation of Nonlinear Response and Onsager Regression in a Photon Bose-Einstein Condensate (FINESS)

• Nikolas Longen (Institut für Angewandte Physik, Universität Bonn, Wegelerstr. 8, 53115 Bonn, Germany)

The quantum regression theorem states that the correlations of a system at two different times are governed by the same equations of motion as the temporal response of the average values. Such a relation provides a powerful framework for the investigation of physical systems by establishing a formal connection between intrinsic microscopic behaviour and a macroscopic effect due to an external cause, allowing to determine e.g. structure factors. Here I report experiments demonstrating that the two-time second-order correlations of an photon Bose-Einstein condensate inside a dye-filled microcavity exhibit the same eigenvalues of the dynamics as the response of the condensate to a sudden perturbation of the dye molecule bath. This confirms an unconventional form of the regression theorem for a coupled many-body quantum system, where the perturbation acts on the bath and only the condensate response is monitored. For strong perturbations, we observe nonlinear relaxation dynamics back to the steady state which our microscopic theory relates to the equilibrium fluctuations, thereby extending the regression theorem beyond the regime of linear response. The demonstrated nonlinearity of the condensate-bath system paves the way for studies of novel elementary excitations in lattices of driven-dissipative photon condensates.

Photon BECs in dye-filled microcavities and VCSELs (FINESS)

• Axel Pelster (RPTU Kaiserslautern-Landau)

The talk provides an overview of current theoretical challenges for describing a photon Bose-Einstein condensate (BEC), which represents a modern prime example for an open dissipative quantum many-body system. In the original experimental platform of dye-filled microcavities [1] the technique of direct laser writing [2] allows to microstructure potentials with different geometries on the mirror surfaces. In this way soon lattices of coupled photon condensates containing hundreds of individual sites are realizable, which are expected to have spiral vortices [3]. We show that their shape can be approximately determined analytically with a projection optimization method, which extends the variational optimization method for BECs of closed systems to open-dissipative condensates [4]. Furthermore, quite recently photon BECs have also been observed in vertical cavity surface-emitting lasers (VCSELs) [5,6]. Here frequent photon absorption and emission processes occur due to the creation and annihiliation of excitons in the semiconductor device, yielding a thermalization of photons. But it was found experimentally that the extracted spectral temperatures are significantly lower than those of the device, which warrants a theoretical explanation.

Super Fermi polaron and Nagaoka ferromagnetism in a two-dimensional square lattice (FINESS)

• Xia-Ji LIU (Swinburne)

We address the Fermi polaron physics of an impurity hopping around a two-dimensional square lattice and interacting with a sea of fermions at given filling factor. When the interaction is attractive, we find standard Fermi polaron quasiparticles, categorized as attractive polarons and repulsive polarons. When the interaction becomes repulsive, interestingly, we observe an unconventional highly-excited polaron quasiparticle, sharply peaked at the corner of the first Brillouin zone. This super Fermi polaron branch arises from the dressing of the impurity’s motion with holes, instead of particles of fermions. We show that super Fermi polarons become increasingly well-defined with increasing impurity-fermion repulsions and might be considered as a precursor of Nagaoka ferromagnetism, which would appear at sufficiently large repulsions and at large filling factors. We also investigate the temperature-dependence of super Fermi polarons and find that they are thermally robust against the significant increase in temperature.

The Statistical Mechanics of Scalar Dark Matter (FINESS)

• Chanda Prescod-Weinstein (University of New Hampshire)

Scalar dark matter is all the rage these days in particle cosmology. In this talk, I will describe efforts to understand scalar dark matter systems that may display Bose-Einstein condensate-like behavior. Is it an analogy? How far does it go? I show that this BEC-like state has implications for astrophysical observations, and I will discuss challenges associated with trying to understand out of equilibrium dynamics in these systems.

Galactic-Scale Superfluidity: True Macroscopic Condensation with Long-Range Interactions? (FINESS)

• Nick Proukakis (Newcastle University)

Dark matter, an integral component of the perceived mass-energy content of the Universe, is usually modelled as a collection of collisionless particles through the established cold dark model (CDM). Despite its impressive success in reproducing large-scale features, increasing evidence is indicating potential shortfalls on shorter (< galactic) scales. An alternative model, “Fuzzy Dark Matter” (FDM), has been gaining increasing attention in the cosmological community: this model postulates the existence of an ultralight bosonic particle exhibiting galactic-size de Broglie wavelengths, facilitating a wave description: central to this model is the suppression of small-scale gravitational collapse due to quantum pressure, which leads to galaxies containing “solitonic cores”. Here I will outline the links between such a cosmological model, laboratory condensates and astrophysical observations, critically discussing implications and open questions. Using established tools from finite-temperature non-equilibrium condensates [1], I will present a picture of a coherent self-bound galactic-scale solitonic condensate (balancing gravitational attraction against quantum pressure), surrounded by a halo of partially-incoherent particles resembling a quasi-condensate state with spatiotemporally-localised regions of enhanced coherence and a quasi-equilibrium turbulent vortex tangle [2]. Drawing on the standard two-fluid model and atomic bimodal distributions, I will present an extended theory which allows both incoherent and coherent degrees of freedom to be fully self-consistently coupled, in a manner incorporating both the cosmological CDM and FDM models, and also existing cold-atom kinetic and stochastic models [3]. Moreover, by contrasting our findings to astrophysical observations, I will critically analyse the viability of such models [4,5]. Funding: Leverhulme Trust, Horizon 2020.

False vacuum decay in an ultracold spin-1 Bose gas (FINESS)

• Thomas Billam

Cold atomic gases offer multiple prospects for simulating the physics of the very early universe in the laboratory, and an ultracold atom analogue of early universe false vacuum decay was recently observed in a ferromagnetic superfluid [1]. In this talk I will discuss theoretical modelling of false vacuum decay analogues using c-field methods [2]. Specifically, I will describe truncated-Wigner and stochastic projected Gross-Pitaevskii simulations of false vacuum decay, modelling zero and finite temperatures respectively, in a spin-1 Bose gas analogue [3, 4]. I discuss the comparison of these simulations to the bubble nucleation rates predicted by the non-perturbative instanton method, and areas of potential future work to refine stochastic simulations of these non-equilibrium superfluid systems.

Observation of stationary turbulence in spinor Bose-Einstein condensates (FINESS)

• Yong-il Shin (Seoul National University)

Spinor Bose-Einstein condensates (BECs) of atomic gases represent a quantum fluid characterized by multiple symmetry breaking, providing an interesting platform for the exploration of quantum turbulence. In this talk, I will report our observation of a stationary turbulent state in a spin-1 atomic BEC driven by a radio-frequency magnetic field. The magnetic driving injects energy into the system through spin rotation, leading to the emergence of an irregular spin texture in the condensate. As the driving persists, the spinor condensate evolves into a nonequilibrium steady state marked by distinctive spin turbulence. Remarkably, under specific driving conditions, the turbulence attains its maximum intensity, accompanied by an isotropic spin composition. Through numerical simulations and experimental validation, we find that the turbulence in the BEC is sustained by a mechanism rooted in the chaotic nature of internal spin dynamics induced by the magnetic driving.

Universal coarsening in 2D and 3D Bose gases (FINESS)

• Martin Gazo

Coarsening of an isolated far-from-equilibrium quantum system is a paradigmatic many-body phenomenon, relevant from subnuclear to cosmological lengthscales, and predicted to feature universal dynamic scaling. It is hypothesised that the associated scaling exponents would allow for the classification of nonequilibrium phenomena into an out-of-equilibrium analogue of equilibrium universality classes. In this talk, I will present our recent observations of universal scaling in the coarsening of isolated homogenous two- and three-dimensional Bose gases. We start by preparing a degenerate gas in a far-from-equilibrium state, and then observe the relaxation towards an equilibrium condensate. We reveal universal scaling in the experimentally accessible finite-time dynamics by elucidating and accounting for initial-state-dependent prescaling effects. The observed scaling exponents match analytical predictions, and are independent of both the initial state and the strength of interparticle interactions. The methods we introduce establish a direct comparison between cold-atom experiments and non-equilibrium field theory, and are applicable to any study of universality far from equilibrium. Finally, we also investigate the timescales associated with coarsening. While stronger interactions generally speed up the thermalisation dynamics, we find that at sufficiently high interactions the coarsening dynamics becomes interaction-independent, hinting at the existence of a universal 'speed limit' for coarsening.

Ferrodark solitons in a spinor superfluid: exact solutions, novel speed limit and anomalous dynamics (FINESS)

• Xiaoquan Yu (Graduate School of China Academy of Engineering Physics)

Exact propagating topological solitons are found in the easy-plane phase of ferromagnetic spin-1 Bose-Einstein condensates, manifesting themselves as kinks in the transverse magnetization. Propagation is only possible when the symmetry-breaking longitudinal magnetic field is applied. Such solitons have two types: a low energy branch with positive inertial mass and a higher energy branch with negative inertial mass. Both types become identical at the maximum speed, a new speed bound that is different from speed limits set by the elementary excitations. The physical mass, which accounts for the number density dip, is negative for both types. In a finite one-dimensional system subject to a linear potential, the soliton undergoes oscillations caused by transitions between the two types occurring at the maximum speed.

Hamiltonian engineering using Bragg matter-wave interferometers in an optical cavity (FINESS)

• Ana Rey (JILA)

Cavity-QED systems have emerged as a powerful platform for generating highly entangled states, with significant implications for both quantum metrology and quantum simulation. A particular setting that has gained great interest for Bragg matterwave interferometry, and thus inertial navigation and fundamental science, is an array of free-falling atoms inside a cavity where one can encode a pseudo-spin ½ degree of freedom using a pair of selected momentum states, which are coupled by the cavity photons. I will show this system not only is suitable for the generation of a spin exchange Hamiltonians via two photon processes, but also arbitrary collective tunable Heisenberg XYZ models without the need of Floquete engineering. This can be done via cavity-mediated four-photon or higher photon interactions with two dressing lasers in a high finesse cavity. In particular I will report on how to dynamically generate the so-called two-axis counter-twisting model, a special type of collective XYZ model proposed more than 30 years ago for the fast generation of spin squeezed states that saturate the Heisenberg bound. In general I will discuss why this system opens a unique pathway for the use of momentum states for quantum enhanced interferometry and quantum simulation.

Self organisation and metastability of cavity bosons at very long times, beyond the adiabatic elimination approximation (FINESS)

• Piotr Deuar (Insititute of Physics, Polish Academy of Sciences)

Phase-space formulations of quantum mechanics like the positive-P and truncated Wigner can give access to the full quantum behaviour of very large systems. In particular, the full distribution of single-shot configurations can be obtained from a stochastic simulation. This is particularly useful for dissipative systems for which direct simulation is harder but phase space methods become stable [1]. In a recent work [2] we have looked at the very long-time behaviour and self-organisation of weakly interacting bosons in a 2d optical lattice coupled to a lossy cavity, in the regime of high filling similar to experiments at ETH. The truncated Wigner representation allows us to go orders of magnitude longer in time compared to earlier numerical work. It takes into account the dynamics and correlation of the cavity mode, quantum fluctuations, and self-organization of individual runs. We observe metastability at very long times and superfluid quasi-long range order, in sharp contrast with the true long range order found in the ground state of the Bose-Hubbard model with extended interactions obtained by adiabatically eliminating the cavity. The metastability appears to be dependent on the relaxation of the adiabatic elimination constraint. As the strength of the cavity coupling increases in a superfluid, the system first becomes (lattice) supersolid at the superradiant transition and then turns into a charge-density wave via the BKT mechanism. Notably, experimental preparation times have often been comparable with the very long times simulated here, so the metastable effects may be relevant in practice.

Observation of Shapiro Steps in an Atomic Superfluid (FINESS)

• Herwig Ott (RPTU Kaiserslautern-Landau)

Shapiro steps occur in the reverse AC Josephson effect, which is one of the three fundamental effects in superconducting Josephson junctions. When a DC and an AC current are applied simultaneously to a Josephson junction, finite voltage steps are generated across the junction. The voltage is directly linked to the applied frequency via V= h/(2e) x f, where f is the frequency of the alternating current. The series connection of several such junctions in one device corresponds to the current voltage standard. We have observed Shapiro steps in a Bose-Einstein condensate of rubidium atoms. Following the protocol proposed by Singh et al [1], we move a narrow barrier through the superfluid at a constant velocity, which corresponds to a DC particle current through the barrier. At the same time, we perform a sinusoidal modulation of the barrier velocity with frequency f, which corresponds to an additional AC current through the barrier. When the instantaneous velocity of the barrier exceeds the critical velocity of the superfluid, a finite particle imbalance occurs between the two sides of the barrier. We find that the corresponding chemical potential difference takes on discrete values corresponding to Shapiro steps. We characterize the Shapiro steps and investigate their microscopic dynamics. [1] V. P. Singh, J. Polo, L. Mathey, and L. Amico. arXiv:2307.08743 (2023)

Designing Atomtronic Circuits via Superfluid Dynamics (FINESS)

• Sarah Jährling

We propose an implementation concept for atomtronic circuit elements based on the criticality of superfluid dynamics in specially designed Bose-Einstein condensates (BECs). Specifically, to obtain a logical 2-input AND-gate, we employ a T-shaped BEC together with two mobile and one stationary Gaussian barrier, functioning as Josephson junctions. The transistor-like behavior of the AND-gate can be identified by studying the resulting non-equilibrium density distributions around the stationary barrier for different scenarios of the deployable mobile barriers. Extending the original setup, we present a logical 4-input AND-gate in an attempt to realize an advanced connected atomtronic circuit. In addition, we discuss the possibility of a universal set of logical gates by establishing a connection to a logical NOT-gate motivated by the recent studies of Singh et al. [1] by exploiting Josephson oscillations. Lastly, we illustrate the potential by merging NOT- and AND-gate elements into an atomtronic NAND-gate.

Driven-dissipative spinor superfluids: a compact Kardar-Parisi-Zhang dynamics of the phase

• Marzena Szymanska (University College London)

Driven-dissipative quantum fluids can differ substantially from their equilibrium counterparts. The long-wavelength phase dynamics of a polariton/photon condensate has been shown to obey Kardar-Parisi-Zhang (KPZ) equation. Since the phase is a compact variable, vortices in 2D and phase slips in 1D can proliferate destroying the KPZ scaling. The interplay between KPZ physics and topological defects is currently subject of great interest, especially in polariton context [1,2,3]. Here, we consider multicomponent system relevant to polariton condensate with different polarisations. The effective theory for Z2 degenerate coupled condensates with U(1)xU(1) symmetries maps onto coupled multicomponent KPZ equations. We perform dynamical renormalisation group analysis as well as exact numerical simulations to place polariton condensates in the subspace of a generally rich flow diagram.

Exact Results of Fermi Polarons with Ultracold Atoms (FINESS)

• Hui Hu

The behaviour of an impurity immersed in a many-body Fermi sea – the so-called Fermi polaron problem – is a long-standing challenge in condensed matter physics and many-body physics. Over the last two decades, there are numerous efforts from ultracold atom community to quantitatively understand the Fermi polaron physics. To date, the ground state of the attractive Fermi polaron has been theoretically predicted and experimentally measured to a great accuracy. However, describing the excited states of Fermi polarons proves to be notably difficult and current theoretical works fail to explain the latest spectral measurement at finite temperature. In this talk, we present two exact results for the finite-temperature spectral function of Fermi polarons. On the one hand, we propose an exactly solvable model in the immobile heavy polaron limit, which exactly establishes various salient quasiparticle features in the spectral function. On the other hand, we derive an exact set of equations of the spectral function for mobile Fermi polarons, by using the diagrammatic theory and by including particle-hole excitations of the Fermi sea shake-up to arbitrarily high orders. This provides a very rare case that a quantum many-body system can be exactly solved by working out the complete Feynman diagrams. Our exact results of Fermi polarons might be used to better understand the intriguing polaron dynamical responses in two or three dimensions, whether in free space or within lattices.

Bose polarons in a box: universal features and the effects of finite temperature (FINESS)

• Jiri Etrych (University of Cambridge)

An impurity immersed in a quantum bath is a fundamental setting in many-body physics that, in spite of its apparent simplicity, features complex emergent behaviour. I will present our recent experiments in which we measure the spectral properties and real-time dynamics of mobile impurities injected into a homogeneous Bose–Einstein condensate (BEC), using two Feshbach resonances to tune both the impurity-bath and intrabath interactions. We map out the attractive and repulsive branches of polaron quasiparticles and explore the breakdown of the quasiparticle picture for near-resonant interactions. On the repulsive side of the resonance, we resolve both the repulsive polaron and the molecular state associated with the Feshbach resonance in the strongly interacting regime and show that the latter also has a many-body character. Our measurements reveal remarkably universal behavior, controlled by the bath density and a single dimensionless interaction parameter, with no significant dependence on the intrabath interactions. Finally, I will also present an extension of our study to finite temperatures both below and above the BEC phase transition of the bath. In particular, we find that many-body effects are suppressed as the temperature of the bath is increased, which can lead to counterintuitive narrowing of spectral features near the resonance.

Non-Hermitian band geometry and dynamics of exciton-polaritons (FINESS)

• Eliezer Estrecho (The Australian National University)

Losses are ubiquitous in exciton-polariton systems, resulting in a short lifetime compared to thermalisation, which makes polariton condensates a good platform for investigating non-equilibrium physics. Additionally, losses can lead to intriguing non-Hermitian effects in systems with non-Hermitian effective Hamiltonians. In this talk, I will present the rich features that can arise in exciton-polariton systems when the losses depend on polarization and momentum [1]. A novel topological winding number can exist in momentum space around exceptional points, which are degeneracies where the eigenstates coalesce. This has direct consequences for the band geometry, resulting in the generalization of the quantum geometric tensor. We propose a method to experimentally measure the non-Hermitian tensor using the polarization of exciton polaritons [2]. The non-Hermiticity also leads to nontrivial dynamics in space, momentum, and pseudospin. For example, a wavepacket can split in both momentum and real space and exhibit self-acceleration without any external potential. Pseudospin defects can also form in momentum space along arcs where the imaginary parts of the energy eigenvalues cross [3]. The pseudospin dynamics are further modified due to an imbalance in loss rates, which directly impacts the zitterbewegung effect for exciton-polaritons.

Bose-Einstein condensation and lasing of low-dimensional semiconductor materials (FINESS)

• Zheng Sun (East China Normal University)

Exciton-polaritons are unique quasiparticles formed by the interaction of excitons and optical modes, offering promising applications in coherent light sources and optical control devices. Our presentation focuses on the observation of Bose-Einstein condensation of upper polariton branch in a WS2 monolayer microcavity. As the condensation threshold is reached, we note a nonlinear increase in upper polariton intensity, reduced linewidth, and enhanced temporal coherence, characteristic of the condensation phenomenon. Through simulations, we determine the specific particle density range necessary for this condensation based on excitonic properties and pumping conditions. This discovery opens avenues for exploring condensate competition and its practical use in polaritonic lasers. Additionally, we explore the potential of Van der Waals homostructures consisting of stacked WS2 layers for enhancing optical properties. Our experiments demonstrate ultra-low threshold laser emission from triple WS2 layers separated by hBN, indicating efficient laser operation possibilities with such structures.

Coherent fraction of an equilibrium condensate (FINESS)

• Hassan Alnatah (University of Pittsburgh)

We report recent progress on the measurement of the coherent fraction of a two-dimensional Bose gas in thermal equilibrium. We have created a homogeneous exciton-polariton gas in equilibrium, realizing the textbook paradigm of a uniform Bose Gas in two-dimensions. Under these conditions, we have measured the coherent fraction of this Bose gas from very low density up to density well above the condensation threshold. These measurements reveal a consistent power law for the coherent fraction over nearly three orders of its magnitude. The same power law is seen in numerical simulations solving the two-dimensional Gross-Pitaevskii equation for the equilibrium coherence; these simulations also show that the power law corresponds to the coherence length in the system growing with a power law of 1.6 as a function of the total density. This power law has not been predicted by prior analytical theories. This work has been supported by the National Science Foundation through Grant DMR-2306977.

Wave breaking and multisoliton fission in a chip-scale superfluid waveflume

• Christopher Baker (University of Queensland)

In this talk I will present research interfacing cavity optomechanics and superfluid physics for the study of nonlinear wave phenomena. Building upon our previous work in superfluid optomechanics [1], I will present a novel sensor architecture formed by covering nanofabricated silicon photonic crystal beams with a thin superfluid helium-4 film. This creates an optically addressable quasi-one-dimensional wave tank containing a few femtoliters of superfluid helium, upon which waves can be generated, propagate and be readout. Superfluid helium’s characteristics present a unique opportunity for the study of nonlinear wave propagation. Indeed, thanks to superfluid helium’s vanishing viscosity, the depth of the film h can readily be made as small as a few nanometers without wave attenuation—something impossible to do with classical fluids. Our platform thus enables us to generate waves whose aspect ratio (defined as the wavelength over depth λ/h) exceeds 10,000:1, two orders of magnitude larger than that achievable in the world’s largest wave tanks and exceeding that of the most extreme terrestrial phenomena such as tsunamis. This, combined with our recently developed ability to engineer strong fountain pressure forces [2], now allows us to combine within a single device high spatial and temporal resolution along with strong actuation capabilities. Leveraging these unique characteristics, I will show how our superfluid wave tank enables us to generate and measure (within a sub-millimetre-sized device in a laboratory setting) a rich variety of superfluid nonlinear wave phenomena for the first time, including wavebreaking, multisoliton fission and optomechanical dissipative solitons [3] —opening up the way for the study of extreme regimes of nonlinear hydrodynamics on a chip.

Quartet superfluid in mass-imbalanced ultracold Fermi mixtures

• Xiaoling Cui (Institute of Physics, Chinese Academy of Sciences)

In this talk, I will introduce our recent works on universal few-body clusters and the resulted high-order fermion superfluid in mass-imbalanced Fermi mixtures. First, we exactly solve the (N+1) problems in 2D with N=3 and 4, where a light atom interacts with N heavy fermions via contact potentials. It is found that the critical heavy-light mass ratios to support a (3+1) tetramer and a (4+1) pentamer are sufficient low to be accessible by a number of mass-imbalanced Fermi mixtures now available in cold atoms laboratories. Further, we study the associated few-body correlations in modifying the Fermi polaron properties and fermion superfluidity of a many-body heavy-light system. In particular, we identify a new fermion superfluidity, called the quartet superfluid (QSF), well beyond the conventional pairing framework in such simple two-component Fermi system. This superfluid phase corresponds to the condensation of quartet (or tetramer) clusters and thus features high-order correlations, as manifested in the momentum-space crystallization of pairing field and the density distribution of heavy fermions. Finally, we explore the dimensional crossover of various universal clusters from 3D to 2D, and show that these clusters are very robust against the presence of an axial confinement and a finite effective range. This suggests the detectability of few-body clusters and quartet superfluid in realistic quasi-2D ultracold Fermi mixtures.

Odd-frequency superfluidity from a particle-number-conserving perspective (FINESS)

• Joachim Brand (Massey University)

We investigate odd-in-time—or *odd-frequency*—pairing of fermions in equilibrium systems within the particle-number-conserving framework of Penrose, Onsager and Yang, where superfluid order is defined by macrosocopic eigenvalues of reduced density matrices. We show that odd-frequency pair correlations are synonymous with even fermion-exchange symmetry in a time-dependent correlation function that generalises the two-body reduced density matrix [1]. Macroscopic even-under-fermion-exchange pairing is found to emerge from conventional Penrose-Onsager-Yang condensation in two-body or higher-order reduced density matrices through the symmetry-mixing properties of the Hamiltonian. We identify and characterise a *transformer* matrix responsible for producing macroscopic even fermion-exchange correlations that coexist with a conventional Cooper-pair condensate, while a *generator* matrix is shown to be responsible for creating macroscopic even fermion-exchange correlations from hidden orders such as a multi-particle condensate. The transformer scenario is illustrated using the spin-imbalanced Fermi superfluid as an example. The generator scenario is demonstrated by the composite-boson condensate arising for itinerant electrons coupled to magnetic excitations. Structural analysis of the transformer and generator matrices is shown to provide general conditions for odd-frequency pairing order to arise in a given system.

Phase transitions and nonequilibrium dynamics in driven quantum matter (FINESS)

• David Weld (UC Santa Barbara)

Subjecting a quantum system to a time-dependent Hamiltonian can generate a rich array of dynamics and phases of matter. I will discuss results from a sequence of recent cold-atom experiments on kicked and driven quantum matter, highlighting data on anomalous transport, nonequilibrium phase diagrams, quantum thermodynamics, and the interplay between dynamical localization and Anderson localization.

Imaginary gauge potentials in a non-Hermitian spin-orbit coupled quantum gas (FINESS)

• Ian Spielman

In 1996 Hatano and Nelson proposed a non-Hermitian lattice model containing an imaginary Peierls phase [Phys.~Rev.~Lett.~{\bf 77}~570--573 (1996)], and subsequent analyses revealed it that is an instance of a new class of topological systems. We experimentally realize a continuum analog to this model containing an imaginary gauge potential in a homogeneous spin-orbit coupled Bose-Einstein condensate (BEC). The base spin-orbit coupled Hamiltonian is made non-Hermitian by adding tunable spin-dependent loss by microwave coupling to a subspace with spontaneous emission. We find that the Heisenberg equations of motion for position and momentum with an imaginary gauge potential depend explicitly on the system's phase-space distribution. In our experiment we first observed the non-Hermitian skin effect by localizing an initial state at the trap boundary, and verifying that it was stationary. We then revealed collective nonreciprocal transport in real space, where the acceleration is a decreasing function of BECs spatial extent in agreement with non-Hermitian Gross-Pitaevskii equation simulations.

Creating and Manipulating Dirac Strings in Spinor Condensate (FINESS)

• Han Pu (Rice University)

Artificial monopoles have been engineered in various systems, yet there has been no systematic study of the singular vector potentials associated with the monopole field. We show that the Dirac string, the line singularity of the vector potential associated with the monopole field, can be engineered, manipulated, and made manifest in a spinor atomic condensate. We elucidate the connection among spin, orbital degrees of freedom, and the artificial gauge, and show that there exists a mapping between the vortex filament and the Dirac string. We also devise a proposal where preparing initial spin states with relevant symmetries and then adiabatically turn on the effective monopole field can result in different vortex patterns, revealing an underlying correspondence between the internal spin states and the spherical vortex structures [1]. Such a mapping also leads to a new way of constructing spherical Landau levels, and monopole harmonics. Our observation provides insights into the behavior of quantum matter possessing internal symmetries in curved spaces. [1] arXiv:2402.14705

Medium-enhanced repulsion between polaron quasiparticles in a quantum gas (FINESS)

• Meera Parish

The problem of mobile quantum impurities immersed in a quantum gas [1] has attracted much attention recently owing to its clean realisation in cold atomic gases, as well as its relevance to a variety of systems spanning a range of energy scales, from semiconductors to neutron stars. Of particular interest is the interactions between polaron quasiparticles—impurities that are dressed by excitations of the surrounding gas—since this has ramifications for the phases of matter that emerge from such impurities. However, there is currently much debate about the nature of the interactions, with the latest experiments even disagreeing on the sign of the interaction strength [2,3]. In this talk, I will focus on bosonic impurities in a Bose-Einstein condensate and I will reveal that the medium actually enhances the existing repulsive interactions between the bosonic impurities. Furthermore, one can show that this is the *dominant* effect at zero temperature in the regime of weak interactions, in contrast to the prevailing wisdom in the field.

An Efficient Quantum Phase-Space Method for Simulating Feedback Control of Interacting Many-Body Quantum Systems (FINESS)

• Simon Haine

Accurately modelling measurement and control of ultracold Bose gases has so far proved unfeasible due the prohibitively large size of the numeric simulations, and problems with under-sampling. We present a new field-theoretic technique based on existing phase-space methods, and use it model feedback cooling of a Bose gas subject to measurements via phase-contrast imaging. is developed for scalable numerical simulations of controlled quantum systems, and used to model feedback cooling of a Bose gas subject to periodic, non-destructive measurements via phase-contrast imaging. We check the validity of our approach in a two-mode system, which permits an exact solution due to its low-dimensional nature, and observe exceptional agreement across various moments of pseudospin operators. In addition, we benchmark our approach with existing techniques such as the Number-Phase Wigner particle filter, which has been the leading choice for existing simulations of controlled quantum systems. Finally, we present preliminary results demonstrating successful cooling of a thermal state with low condensate fraction to condensate formation in both quasi-1D and 2D geometries, correctly accounting for measurement induced backaction and spontaneous emission effects. It is shown that the final achievable condensate fraction is dependent upon experimental parameters such as the measurement strength, rate, and detector resolution, and a simple model is constructed to derive optimal values for the parameters above.

How to deduce the entropy from atom-atom correlations (FINESS)

• Karen Kheruntsyan

We derive a thermodynamic Maxwell relation by which the entropy of an ultracold atomic gas can be deduced from the local (same point) atom-atom correlation function. The Maxwell relation in question is applicable to many-body systems with short-range interactions that can be characterised by $s$-wave scattering. For such systems, the local atom-atom correlation function represents a thermodynamic quantity that can be calculated from the Helmholtz free energy using the Hellmann-Feynman theorem [1]. Here, we exploit this property to derive a Maxwell relation that relates the atom-atom correlation to the thermodynamic entropy of the system [2]. As a practical application of this Maxwell relation, we utilise it to calculate the entropy of a weakly interacting one-dimensional (1D) Bose gas from its atom-atom correlations in the context of the c-field approach of the stochastic projected Gross-Pitaevskii equation (SPGPE). The SPGPE is a well established and widely used numerical approach for computing thermal equilibrium and dynamical properties of finite temperature Bose gases, such as partially condensed Bose-Einstein condensates in 2D and 3D, or phase-fluctuating quasicondensates in 1D. Despite its wide applicability to ultracold quantum gas systems, computing the entropy of such systems within the SPGPE has not been accomplished prior to this work. Our calculations can also be viewed as a numerical experiment that serves as a proof-of-principle demonstration of an experimental method to deduce the entropy of an ultracold quantum gas from the measurements of atom-atom correlations.