Papers

We identify a memory effect produced by phonon scattering in a superfluid: a localized scattering event generates a far-field pressure pulse with vanishing time integral but a nonzero first temporal moment, corresponding to a permanent shift in a prepotential for the far-field velocity. The effect is controlled by the soft factor for phonon emission, including corrections from nonlinear dispersion. In superfluid $^4$He, scattering macroscopic phonon wave packets produces a signal whose estimated magnitude may lie within experimental reach, thereby providing a laboratory analogue of electromagnetic and gravitational memory.

The strong, tunable dipolar interactions of ultracold molecules make them a powerful platform for quantum many-body physics, but reaching degeneracy by evaporative cooling requires suppressing inelastic collisions. Microwave collisional shielding has enabled the preparation of degenerate Fermi gases and Bose-Einstein condensates of polar molecules, whereas static electric field shielding has been limited to fermionic species, which are less prone to inelastic loss. We demonstrate static-field Förster shielding of bosonic $^{23}$Na$^{87}$Rb molecules, suppressing two-body loss by up to four orders of magnitude. Within a bound-state-free electric-field window, three-body loss is also strongly suppressed, enabling efficient evaporation. Evaporating with an efficiency of 2.09(9), we increase the phase-space density of the gas by two orders of magnitude, reaching degeneracy with 7200(1000) molecules. We observe self-bound droplets at the end of evaporation over a wide range of field strengths, emerging from either degenerate or non-degenerate parent gases. We perform Path Integral Monte Carlo simulations, which suggest that the observed droplets are filamentary in nature, and find good agreement with the experimentally observed droplet formation temperatures. Our results establish Förster shielding as a single-field route to prepare degenerate gases and self-bound droplets of bosonic polar molecules.

We extend the functional determinant approach (FDA), previously restricted to static impurities, to the case of finite-mass impurities by using the mass-gap description of Fermi polarons [Phys. Rev. Lett. 135, 193401 (2025)]. The quadratic structure of the mass-gap model enables the exact evaluation of many-body spectra and Ramsey dynamics for mobile impurities. We show that this mass-gap FDA smoothly interpolates between the Fermi-edge singularity for infinitely heavy impurities and the emergence of quasiparticle weight for finite impurity mass. Our results demonstrate that the mass-gap FDA provides a transparent and computationally efficient framework to describe the quantum dynamics of mobile impurities.

In the whole space of low spatial dimensions, namely $d \leq 2$, we study the minimizers of an energy functional with an attractive cubic nonlinearity and a repulsive quintic nonlinearity, which describes a quantum Bose gas with a two-body attraction and a three-body repulsion. We prove the existence of minimizers at fixed effective statistics parameter. In the limit of a large effective statistics parameter of the mass-critical nonlinearity (with respect to the kinetic energy), we derive an effective Thomas-Fermi-like model for the homogeneous Bose gas. We also consider other limit regimes depending on the mass-critical nonlinearity.

We consider a homogeneous Bose gas composed of $N$ identical bosons occupying an infinite two-dimensional space. This gas exhibits both an attractive two-body interaction and a repulsive three-body interaction. Via the intermediate Hartree theory, we rigorously derive the homogeneous cubic-quintic nonlinear Schroedinger functional as the mean-field limit of the model. Our investigation focuses on the system's behavior in relation to the two-body interaction.

We investigate the behavior of a homogeneous one-dimensional Bose gas composed of $N$ identical bosons experiencing attractive two-body and repulsive three-body interactions. Utilizing intermediate Hartree theory, we rigorously derive the homogeneous cubic-quintic nonlinear Schrödinger functional as the mean-field limit of the many-body Hamiltonian. Particular attention is given to the impact of the repulsive three-body term on ground state energy estimates and mass concentration properties

The discovery of the strongly-coupled quark-gluon plasma (sQGP) in high-energy heavy-ion collisions has revealed remarkable properties of matter at high temperature, with transport coefficients close to conjectured bounds from quantum field theory at strong coupling. The high sQGP collision rates imply very large energy uncertainties and the melting of quasiparticle structures. Deploying quantum many-body theory based on the self-consistent $T$-matrix approach for the sQGP at high temperature, we investigate its manifestation at high baryon density and low temperature, as present in neutron stars and their mergers. We constrain the chemical-potential dependence of the quark interaction kernel using first-principles information from Quantum Chromodynamics on baryon-number susceptibilities. Very large collisional widths persist at large density and are found to relegate superconducting phases to rather small temperatures. Instead, a strongly coupled diquark liquid prevails in the thermodynamics under the conditions relevant to neutron-star mergers. At lower densities, the diquarks take over from the single-quark contributions, suggesting a pathway toward hadronization. Our results are consistent with observational constraints on the equation of state of neutron stars, corroborating the presence of strongly coupled quark matter in the interior of these objects.

Quantized vortices are a hallmark of superfluidity. However, in a photon Bose–Einstein condensate, the photon-photon interaction is so weak that in a homogeneous system the healing length exceeds the experimentally achievable size of the condensate itself. Here we show that a harmonic confinement regularizes the vortex size to experimentally achievable scales. Moreover, such an external confinement closely resembles the standard experimental setup of a pumped dye-filled microcavity. We model the condensate via a complex Gross–Pitaevskii equation and obtain an approximate dynamical single-vortex solution by applying the recently proposed projection optimization method. The latter generalizes the variational approach of closed systems to open-dissipative systems without assuming a specific form for the condensate phase. The radial photon flow, which is characteristic for driven-dissipative systems, yields a definition of the vortex core size based on the competition of gain and loss. However, this condition reproduces the heuristic closed system definition now based on physical grounds. A subsequent linear stability analysis shows that interaction, as well as pumping and loss can drive the system to an unstable regime. In this way one can fundamentally distinguish between closed and open-dissipative systems.

We experimentally investigate quantum synchronization for one of the simplest possible quantum systems, namely an externally driven few-level system with equally spaced energy levels effectively acting as a spin-1 system. Coupling to excited auxiliary states, we realize additive effective Lindblad operators that are associated with non-conventional dissipative pathways, which are shown to enhance, in certain parameter regimes, quantum synchronization. The experimental set-up, which utilizes cold $^{87}$Rb atoms in a MOT, and associated synchronization extraction protocol are benchmarked carefully through dedicated simulations. Convincing agreement is found between experiment and simulations. The dissipation engineering approach established in our work can be readily extended to systems with more energy levels, such as effective spin-$3/2$ or spin-$2$ systems, and has implications for quantum synchronization studies in higher-spin systems as well as for a wide range of quantum science studies and technology applications.

We study the nonequilibrium response of a one-dimensional Bose-Hubbard medium to a two-magnon bound state propagating along an attractive XXZ chain. A Holstein-type displacement coupling makes the magnon density act both as a local chemical-potential perturbation and as a source of bosons. We derive a long-wavelength hydrodynamic description of the density and phase fluctuations and test it against matrix-product-state simulations of the full coupled dynamics. The theory predicts a comoving near-field deformation together with retarded density waves confined to a sound cone. In the Mott regime, the deformation remains localized around the moving pair. In the compressible regime, two counterpropagating fronts detach and approach the semiclassical sound velocity, recovered from the equilibrium density with no fitted parameters. The coupling also induces a dynamical backreaction that slows the bound state and broadens its magnon-density profile. The comparison delimits where hydrodynamics stays quantitative once the probe reacts back on the medium.

Exponentially decaying long-range hoppings are ubiquitous in realistic tight-binding models and are often truncated to obtain a finite-range description. We show that this approximation can fail dramatically in non-Hermitian systems under open boundary conditions: an infinitesimal long-range hopping can nonperturbatively reconstruct the spectrum and eigenstates of a short-range non-Hermitian system. The mechanism is controlled by a competition between the decay length of infinitesimal long-range hoppings and the localization length of non-Hermitian skin modes, leading to a sharp transition as the decay rate is tuned. In one dimension, we show that a squeezed generalized Brillouin zone (GBZ) replaces the original GBZ of the short-ranged Hamiltonian, yielding the reconstructed open-boundary spectrum. In two or higher dimensions, we formulate a squeezed amoeba formulation describing the reconstructed spectral density. We further show that long-range hoppings can qualitatively reshape Green's function, which can be readily detected in experiments.

We explore the formation, stability, and propagation dynamics of light bullets carrying orbital angular momentum (vorticity) in Bessel-structured nonlinear graded-index waveguides. While these waves are prone to the collapse in uniform three-dimensional media, we demonstrate that their stabilization is provided by a squared-Bessel index profile in the plane orthogonal to the propagation direction, which may play the role of the waveguiding or antiwaveguiding potential. Different solution families are obtained, revealing a variety of power-vs.-propagation-constant diagrams, characterizing the coexistence of multiple families. We analyze the propagation dynamics in a wide experimentally accessible parameter range. Our results contribute to the understanding of nonlinear light localization in multidimensional settings and suggest new possibilities for using structured light in advanced photonic applications.

We consider an ultracold Bose gas in a half-open potential well, otherwise confined to a quasi-one-dimensional geometry. The Bogoliubov equations for its elementary excitations are solved in the continuous spectrum to explore the particle and energy content near the edge of the gas, beyond the often applied local density approximation. In particular, gradients in the condensate density enhance density-dominated excitations in the border region. We discuss excess (missing) particles and their energy by comparing to suitable reference solutions for quasi-homogeneous systems. The density profile near the edge shows no Friedel oscillations, but a dipolar feature from the spill-out of thermally excited particles. The calculations are performed in the grand-canonical ensemble and in the thermodynamic limit.

Sound propagation in quantum droplets differs from that in conventional Bose-Einstein condensates (BECs) because of their self-bound nature and the role of quantum fluctuations. We investigate sound propagation in one-dimensional quantum droplets formed by a symmetric Bose-Bose mixture, focusing on finite-size and confinement effects. Using the extended Gross-Pitaevskii equation, we extract the sound velocity from the real-time propagation of localized density perturbations and compare it with the low-energy excitation spectrum. We find that, unlike in a conventional BEC, the sound velocity of a finite droplet is strongly affected by its density profile and quantum-pressure contribution. It decreases with increasing particle number as the droplet evolves from a Gaussian-like to a flat-top profile, approaching the bulk quantum-droplet value. In contrast, external harmonic confinement compresses the droplet and enhances the sound velocity, driving the system toward the acoustic behavior of a trapped BEC. Our results establish sound propagation as a sensitive probe of finite-size effects and the crossover between self-bound quantum droplets and conventional Bose gases, and suggest a feasible route for experimental observation in ultracold $^{39}$K droplets.

Multi-dimensional coherent spectroscopy (MDCS) goes beyond standard linear-response probes and provides a powerful tool for investigating correlations between quasiparticles such as excitons (bound electron-hole pairs). Here we present a microscopic theory of MDCS that accounts for the delocalized nature of excitons in two-dimensional semiconductors. In contrast to the more phenomenological few-level approaches typically employed for modelling MDCS, our theory features mobile excitons with continuous momentum degrees of freedom. We find that the energy continuum associated with exciton momenta is crucial for producing interaction-induced decoherence, as well as capturing the interference between different exciton-polaron quasiparticles in the case of charge-doped semiconductors. Crucially, our calculated MDCS spectra agree well with recent experiments on doped monolayer MoSe$_2$ [Hao et al., Nature Communications 8, 15552 (2017)], and they suggest that the interactions between exciton polarons depend strongly on phase-space filling effects, where exciton polarons compete for electrons. Our results demonstrate that microscopic approaches allow one to gain new insights from the fine structure of MDCS on two-dimensional semiconductors, and they illustrate the utility of microscopic approaches to modelling MDCS experiments more generally.

We investigate the non-equilibrium dynamics of atomic-molecular Bose-Einstein condensates coupled via a Feshbach resonance, with Gaussian white noise acting on both the coupling strength and the detuning. Using the bosonic Josephson-junction and Bloch-sphere formalisms, we examine how coherence (coupling) noise and imbalance (detuning) noise modify the coherence-resonance point as functions of the initial polarization and the Feshbach detuning. Noise plays a more coherent role when its characteristic timescale matches the intrinsic timescales of the dynamics, leading to extrema in the time-averaged purity and the Husimi-Q distribution.

We derive a microscopic theory of drag for a Bose-Einstein condensate interacting with a moving reservoir. Starting from interactions between a condensate and a drifting fermionic bath, we integrate out the bath degrees of freedom within the Born–Markov approximation to obtain an effective Gross–Pitaevskii equation with a drag potential. From this potential we derive an effective drag force and obtain a closed expression for the drag coefficient, determined by the reservoir density fluctuations and the condensate density profile. As an application, we simulate the drag for an exciton–polariton condensate interacting with a drifting electron gas.

Rydberg entangling gates driven by a two-photon transition in alkali atoms suffer from an adverse scaling of light-shift-induced errors. Robustness to such detuning errors is known to be impossible to achieve in the design of conventional Rydberg gate protocols, where only one of the qubit states is coupled to the Rydberg state. Here, we show that in a more general framework, in which both qubit states take part in the gate, full or partial robustness to these errors can be realized. We present two gate constructions, which either cancel the errors outright or convert them into single-qubit errors that can be corrected locally. We map the regimes – in terms of light-shift strength, intensity inhomogeneity, and Rydberg decay rate – in which these protocols outperform the widely used time-optimal Rydberg gate, and find that they already include the conditions of state-of-the-art experiments. Finally, we show the existence of a Rydberg `fly-by' entangling gate, an important primitive for an emerging class of neutral-atom quantum computing architectures.

Spontaneous breaking of a continuous symmetry produces a massless Goldstone mode that can evolve across a degenerate manifold at zero energy cost. Goldstone modes have been identified primarily through excitation spectra, mode softening or collective oscillations. However, their time-domain transport under intrinsic fluctuations and dissipation has remained largely unexplored. Here we directly track the stochastic transport of a Goldstone mode in a self-organized atomic crystal inside an optical ring cavity. The ring cavity maps the order-parameter phase onto the real-space position of the emergent crystal. Without any external perturbation, fundamental photon-scattering recoil drives the collective transport, while cavity dissipation generates friction. We monitor individual trajectories of the atoms and their self-generated optical lattice by measuring the cavity output phase. We find that the diffusion constant decreases as $1/N$, indicating that all atoms move collectively as a rigid object rather than independently. By tuning the Langevin driving force and cavity-mediated damping, we show that the normalized diffusion constant collapses onto a single universal curve. This work extends the study of continuous symmetry breaking from excitation-frequency measurements to real-time tracking of transport, and opens routes for studying non-equilibrium collective transport, phonon dynamics, and defect formation in driven-dissipative quantum matter.

We present details of an experimental platform for studying Fermi polarons in a quantum-gas mixture. The system consists of about a thousand bosonic $^{133}$Cs impurities immersed in a deeply degenerate Fermi gas ($T/T_F \sim 0.2$) of approximately $2\times 10^5$ $^6$Li atoms in a single hyperfine state, with interspecies interactions tunable via a Feshbach resonance. Using optical Raman spectroscopy without relative momentum transfer, we perform injection spectroscopy and thereby create the Fermi polaron. Owing to the large mass imbalance between the two species, the setup provides access to previously unexplored regimes of Fermi polarons.

Field sensing with state-of-the-art atom interferometers is restricted to the use of uncorrelated devices op- erating in parallel. We can overcome this limitation by using distributed sensing protocols where quantum correlations among spatially-separated devices are engineered in the spatial mode carrying the signal. We show that a tweezer array in a cavity offers an ideal testbed to engineer quantum states for distributed sensing, with the possibility to generate entanglement both within and between the clouds. The competition between local and intercloud cavity-mediated exchange allows the sign and spatial pattern of the intercloud couplings to select the squeezed mode. For two ensembles, positive coupling produces uniform collective squeezing, whereas negative coupling generates strong staggered, nonlocal squeezing. A semiclassical analysis reveals a counter-twisting- like phase-space flow, qualitatively distinct from standard one-axis twisting. The analysis and results can be further generalized to a larger number of ensembles. We apply the scheme to differential Ramsey interferometry with common phase noise spanning the full 2 \pi range, the resulting staggered states reduce the phase uncertainty below the standard quantum limit, with an ellipse estimator approaching the Cramèr-Rao bound. These results establish programmable cavity interactions as a scalable route to entanglement tailored to distributed signals.

Molecules embedded in superfluids provide an experimentally controllable platform for investigating impurity physics. Here, we investigate a driven molecule rotating in a superfluid environment, including helium and Bose–Einstein condensates, at rotation frequencies similar to the bath dynamics. Within the experimentally relevant platform of an optical centrifuge, we show that the rotor remains localized up to a characteristic harmonic frequency that can realistically exceed the excitation energies of the bath, enabling access to ultrafast rotating impurities. In the co-rotating frame, the bath excitations experience a rotational Doppler shift, generating angular-momentum-resolved resonances absent in equilibrium angulon theory. We identify a dissipative rotational sound barrier at which the molecule resonantly emits bath excitations and undergoes strong angular momentum exchange with the surrounding medium. Overall, we establish the dynamical phase diagram of the driven rotor in a quantum solvent and introduce a generic platform for investigating fast driven rotating impurities in quantum many-body systems.

We investigate the mean-field ground states of coupled one-dimensional Bose-Einstein condensates subject to a synthetic magnetic field. The resulting interacting coupled-wire model has one continuous and one discrete spatial direction, providing a controlled way to interpolate between the physics of few-leg ladders and extended vortex lattices. For two wires, we study the vortex-like, biased-density, and Meissner-like states, exploring how the finite longitudinal size of the system modifies the transitions between them. Increasing the number of wires, the ground state evolves towards an extended vortex lattice. We find numerically that periodic boundary conditions in the discrete direction can favour staggered arrays of like-signed vortices resembling an Abrikosov lattice, while open boundaries in small finite-size systems confine the vortices into rows near the centre of the synthetic direction. Our results explore how finite size effects and boundary conditions govern the emergence and spatial organisation of vortices in continuous-discrete quantum fluids, with relevance to future experiments in tunnel-coupled atomic wires or with synthetic dimensions.

Circular states of strontium are promising platforms for quantum technologies. They exhibit much longer lifetimes than laser-accessible Rydberg states and possess an optically active ionic core that can be manipulated with laser light. Here we trap circular states in optical tweezers by exploiting the dynamical polarizability of the ionic core. We measure the decay of the orbital component of a n=51 circular state and observe singlet-triplet spin oscillations arising from the interplay of vector light shifts induced by the optical tweezers, spin-orbit coupling associated with the Rydberg electron, and the difference in Landé $g$-factor between the ionic core and the Rydberg electrons. By observing the two electron spin dynamics, we measure the spin-orbit interaction and the correction $\delta g \sim$ 10^(-5) of the Landé $g$-factor of the Sr$^+$ 5s$_{1/2}$ ionic core electron with respect to that of a free electron. This provides a first high-precision measurement of $g_{\textrm{Sr}^+, 5s_{1/2}}$ = 2.002290(1).

We study the emptiness formation probability (EFP) in the ground state of the repulsive one-dimensional Lieb-Liniger Bose gas. For a macroscopic empty interval of length $2R$, its leading asymptotic behavior is described by a rate function $f(\gamma_0)$, defined by $-\log P(R)\sim(\rho_0R)^2f(\gamma_0)$, where $\rho_0$ is the mean density and $\gamma_0$ is the dimensionless interaction strength. We propose a parameter-free integral equation for $f(\gamma_0)$. Starting from the exact dual-field Fredholm-determinant representation of the EFP, we show how the conjectured kernel arises formally and identify the uniform asymptotic statement that remains to be proven for a rigorous derivation. The conjecture reproduces the Tonks-Girardeau and weak-coupling limits, as well as the first correction obtained independently in both limits. It also agrees at the few-percent level with numerical minimization of the Lieb-Liniger hydrodynamic action over more than four orders of magnitude in coupling. The numerical calculation yields the corresponding emptiness instantons and their astroid-like vacuum regions.

We consider a non-interacting Bose gas governed by a general one-particle Hamiltonian in the canonical ensemble. Our main results are large-deviation estimates for the number of particles in the Bose–Einstein condensate. We show that the left- and right-tail probabilities decay at different exponential rates as the number of particles tends to infinity.

How does the Bose or Fermi statistics of microscopic particles survive when confinement binds them into emergent bosonic composites? We address this question in the strong-coupling limit of a $2+1$D $\mathbb{Z}_2$ lattice gauge theory, where charges are confined into tightly bound pairs that can be described by an effective dimer model. We find that the statistics of the underlying matter is encoded entirely in a single local hopping phase $\varphi$ -$0$ for bosons, $\pi$ for fermions. Treating $\varphi$ as a continuous parameter that interpolates between the two, we map the ground-state phase diagram with the help of tensor-network methods. The angle $\varphi$ itself drives a transition between a dimer-superfluid and a dimer-charge density wave state, while the magnetic coupling binds neighboring dimers into resonating pairs, in competition with the inter-dimer repulsion. We identify a novel gapped phase in which dimer pairs crystallize into an ordered pattern of resonating plaquettes. Finally, we propose a quench protocol under which identical dimer configurations evolve in markedly different ways depending on the statistics of their constituents. This provides a dynamical probe of the internal structure of dimers, and allows to detect ordered phases through real-time signatures, within reach of simulators that natively realize bosonic degrees of freedom.

Quantum geometry has emerged as a guiding principle across atomic and condensed-matter physics, shaping the topological responses of Bloch bands and the stability of the correlated phases they host. Sublattice symmetry, though common among bipartite lattice models, has not yet been exploited to obtain closed-form quantum geometry in multiband systems. For this purpose, we derive a general expression for the QGT of sublattice-symmetric systems in terms of contributions from the individual sublattice sectors, and show that this symmetry renders the Bloch Hamiltonian of a paradigmatic four-band model, the quarter-flux Harper-Hofstadter model, anti-block-diagonal, analytically yielding the spectrum, eigenstates, and full quantum geometric tensor (QGT), including the Berry curvature and quantum metric, for all four bands. The model, describing charged particles on a two-dimensional square lattice subjected to a uniform magnetic field, has recently been realized experimentally with ultracold atoms, photons, and superconducting circuits. Finally, we evaluate fractional-Chern-insulator stability criteria analytically and quantify the lowest band of the quarter-flux Harper-Hofstadter model to be a nearly ideal Chern band. Our approach opens a route for studying also the quantum geometry of other sublattice-symmetric multiband systems.

Cavity quantum electrodynamics (QED) modifies many-body systems by combining cavity-mediated collective interactions with microscopic short-range interactions. The resulting dynamics lies between the local and fully collective limits, such that neither locality nor collectivity alone provides a complete organizing principle. In the clean conceptual limit of dominant collective coupling, we show that the effective dynamics within each energetically isolated subspace is generically controlled by whether that subspace admits a local product-state basis, and identify exceptions imposed by angular-momentum selection rules. Product-state subspaces generically retain the spatial structure of the microscopic interaction. Entangled subspaces instead generically dress local processes with global operators, generating non-local but highly structured dynamics. We illustrate this by deriving the corresponding effective Hamiltonians in two representative cavity-QED spin models. The cavity-isolated subspace thus becomes a resource for generating competing short-range interactions or globally conditioned local processes, opening a class of many-body dynamics without local or fully collective counterparts.

Motivated by recent experimental advances toward electrical control of the effective mass and the realization of anisotropic mass distributions, we investigate the impact of mass anisotropy on driven-dissipative polariton condensates, focusing on modulational instability, pattern formation, and the collective dynamics of the emergent patterns. We show that unequal effective masses along the two orthogonal directions induce stripe pattern formation in an exciton-polariton condensate pumped by a non-resonant Gaussian laser beam above a critical mass ratio. A Bogoliubov analysis of the homogeneously pumped system reveals that mass anisotropy leaves the modulational-instability criteria unchanged but reshapes the unstable-mode dispersion by reducing the most unstable wave- length along the direction of larger effective mass with increasing mass ratio. This introduces an anisotropy-controlled length scale for pattern selection. When the pump width becomes comparable to the most unstable wavelength, the condensate becomes modulationally unstable and develops stripe patterns along the direction of larger effective mass, accompanied by particle-number fluctua- tions and center-of-mass oscillations along the same direction. As the pump width increases further, the stripes become distorted and develop spatial irregularities during the time evolution, indicative of modulational instability. Nevertheless, the anisotropic mass distribution preserves a pronounced directional preference, in contrast to the fully disordered patterns observed in the isotropic-mass case. Our results identify mass anisotropy as a key control parameter for directional modulational instability and pattern formation in nonequilibrium polariton condensates.