A gas of ultracold atoms probed with laser light is a nearly-ideal experimental realization of a medium of resonant point-like scatterers, a key problem from condensed matter to biology or photonics. Yet, several recent experiments have reported large discrepancies with theory. In this work, we measure the complex transmission through a slab of ultracold two-level atoms with an interferometric technique. We find good agreement with first-principles simulations of mutually-coupled, laser-driven dipoles, and provide an explanation for the discrepancies in earlier measurements.

We present a series of experimental investigations on binary mixtures of Bose-Einstein condensates. Our focus lies on the regime where the interaction parameters place the system at the threshold of miscibility. We demonstrate that the dynamics of such mixtures can be effectively reduced to a single nonlinear equation. This framework is illustrated through the discussion of stable solitonic solutions in one and two dimensions. Furthermore, we show that employing a binary mixture enables exploration beyond the dynamics governed by the nonlinear Schrödinger equation, allowing us to address other fundamental equations in nonlinear physics, such as the Landau-Lifshitz equation describing the motion of spin chains in ferromagnetic materials.

The motion of a quantum system subjected to an external force often defeats our classical intuition. A celebrated example is the dynamics of a single particle in a periodic potential, which undergoes Bloch oscillations under the action of a constant force. Surprisingly, Bloch-like oscillations can also occur in one-dimensional quantum fluids without requiring the presence of a lattice. The intriguing generalization of Bloch oscillations to a weakly-bounded ensemble of interacting particles has been so far limited to the experimental study of the two-particle case, where the observed period is halved compared to the single-particle case. In this work, we observe the oscillations of the position of a mesoscopic solitonic wave packet, consisting of approximately 1000 atoms in a one-dimensional Bose gas when subjected to a constant uniform force and in the absence of a lattice potential. The oscillation period scales inversely with the atom number, thus revealing its collective nature. We demonstrate the pivotal role of the phase coherence of the quantum bath in which the wave packet moves and investigate the underlying topology of the associated superfluid currents. Our measurements highlight the periodicity of the dispersion relation of collective excitations in one-dimensional quantum systems. We anticipate that our observation of such a macroscopic quantum phenomenon will inspire further studies on the crossover between classical and quantum laws of motion, such as exploring the role of dissipation, similarly to the textbook case of macroscopic quantum tunneling in Josephson physics.

We use Ramsey interferometry to study spin dynamics in the strongly interacting regime of spin-orbit-coupled quantum gases in one-dimensional optical lattices. We observe an intrinsic many-body dephasing mechanism immune to spin-echo in two-component Mott insulators. We ascribe the dephasing to the motion of hole-like defects in an otherwise inert Mott insulator, the spinless nature of the holes explaining the ineffectiveness of spin echo to restore it. We show that a model of spin-orbit-coupled hardcore bosons can explain quantitatively our experimental observations.

The properties of quantum matter arise from the combined effects of dimensionality, interactions and quantum statistics. An experiment now studies what happens to ultracold bosons when the dimensionality of the system changes continuously between one and two dimensions.

At zero temperature, a Galilean-invariant Bose fluid is expected to be fully superfluid. Here we investigate theoretically and experimentally the quenching of the superfluid density of a dilute Bose-Einstein condensate due to the breaking of translational (and thus Galilean) invariance by an external 1D periodic potential. Both Leggett's bound fixed by the knowledge of the total density and the anisotropy of the sound velocity provide a consistent determination of the superfluid fraction. The use of a large-period lattice emphasizes the important role of two-body interactions on superfluidity.

Cold quantum gases, when acted upon by electromagnetic fields, can give rise to samples where isolated atoms coexist with dimers or trimers, which raises the question of the interactions between these various constituents. Here we perform microwave photoassociation in a degenerate gas of $^{87}$Rb atoms to create weakly-bound dimers in their electronic ground state. From the density-induced shift of the photoassociation line, we measure the atom-dimer scattering length for the two least-bound states of the molecular potential. We also determine the complete energy diagram of one hyperfine manifold of the least-bound state, which we accurately reproduce with a simple model.

When two Bose-Einstein condensates-labelled 1 and 2-overlap spatially, the equilibrium state of the system depends on the miscibility criterion for the two fluids. Here, we theoretically focus on the non-miscible regime in two spatial dimensions and explore the properties of the localized wave packet formed by the minority component 2 when immersed in an infinite bath formed by component 1. We address the zero-temperature regime and describe the two-fluid system by coupled classical field equations. We show that such a wave packet exists only for an atom number N (2) above a threshold value corresponding to the Townes soliton state. We identify the regimes where this localized state can be described by an effective single-field equation up to the droplet case, where component 2 behaves like an incompressible fluid. We study the near-equilibrium dynamics of the coupled fluids, which reveals specific parameter ranges for the existence of localized excitation modes.

We investigate the loss of spatial coherence of one-dimensional bosonic gases in optical lattices illuminated by a near-resonant excitation laser. Because the atoms recoil in a random direction after each spontaneous emission, the atomic momentum distribution progressively broadens. Equivalently, the spatial correlation function (the Fourier-conjugate quantity of the momentum distribution) progressively narrows down as more photons are scattered. Here we measure the correlation function of the matter field for fixed distances corresponding to nearest-neighbor (n-n) and next-nearest-neighbor (n-n-n) sites of the optical lattice as a function of time, hereafter called n-n and n-n-n correlators. For strongly interacting lattice gases, we find that the n-n correlator C 1 decays as a power-law at long times, C 1 a 1/ t alpha , in stark contrast with the exponential decay expected for independent particles. The power-law decay reflects a non-trivial dissipative many-body dynamics, where interactions change drastically the interplay between fluorescence destroying spatial coherence, and coherent tunnelling between neighboring sites restoring spatial coherence at short distances. The observed decay exponent alpha approximate to 0.54(6) is in good agreement with the prediction alpha = 1/2 from a dissipative Bose-Hubbard model accounting for the fluorescence-induced decoherence. Furthermore, we find that the n-n correlator C 1 controls the n-n-n correlator C 2 through the relation C 2 approximate to C 1 2 , also in accordance with the dissipative Bose-Hubbard model.

We report on experiments exploring the non-Hermitian dynamics of pairs of two-level atoms tightly confined in an optical lattice and driven by a near-resonant laser. Although spontaneous emission is negligible for the long-lived excited state, two-body dissipation arises from strong inelastic collisions between two atoms in the excited state. We demonstrate quasi-adiabatic control of the internal state of the pairs in the quantum Zeno regime where inelastic losses dominate. Preparing each atom pair in the longest-lived eigenstate of the non-Hermitian Hamiltonian describing the dissipative dynamics, we measure a lifetime enhanced by more than two orders of magnitude in comparison to the bare two-body lifetime. The measured enhancement factor is in quantitative agreement with the expected lifetime of the prepared ``non-Hermitian dressed state''.

We consider strong two-body losses in bosonic gases trapped in one-dimensional optical lattices. We exploit the separation of timescales typical of a system in the many-body quantum Zeno regime to establish a connection with the theory of the time-dependent generalized Gibbs ensemble. Our main result is a simple set of rate equations that capture the simultaneous action of coherent evolution and two-body losses. This treatment gives an accurate description of the dynamics of a gas prepared in a Mott insulating state and shows that its long-time behavior deviates significantly from mean-field analyses. The possibility of observing our predictions in an experiment with $^{174}\mathrm{Yb}$ in a metastable state is also discussed.

We demonstrate the arbitrary control of the density profile of a two-dimensional Bose gas by shaping the optical potential applied to the atoms. We use a digital micromirror device (DMD) directly imaged onto the atomic cloud through a high resolution imaging system. Our approach relies on averaging the response of many pixels of the DMD over the diffraction spot of the imaging system, which allows us to create an optical potential with an arbitrary intensity profile and with micron-scale resolution. The obtained density distribution is optimized with a feedback loop based on the measured absorption images of the cloud. Using the same device, we also engineer arbitrary spin distributions thanks to a two-photon Raman transfer between internal ground states.

Most experimental observations of solitons are limited to one-dimensional (1D) situations, where they are naturally stable. For instance, in 1D cold Bose gases, they exist for any attractive interaction strength g and particle number N. By contrast, in two dimensions, solitons appear only for discrete values of gN, the so-called Townes soliton being the most celebrated example. Here, we use a two-component Bose gas to prepare deterministically such a soliton: Starting from a uniform bath of atoms in a given internal state, we imprint the soliton wave function using an optical transfer to another state. We explore various interaction strengths, atom numbers, and sizes and confirm the existence of a solitonic behavior for a specific value of gN and arbitrary sizes, a hallmark of scale invariance.

Tan's contact is a quantity that unifies many different properties of a low-temperature gas with short-range interactions, from its momentum distribution to its spatial two-body correlation function. Here, we use a Ramsey interferometric method to realize experimentally the thermodynamic definition of the two-body contact, i.e. the change of the internal energy in a small modification of the scattering length. Our measurements are performed on a uniform two-dimensional Bose gas of $^{87}$Rb atoms across the Berezinskii-Kosterlitz-Thouless superfluid transition. They connect well to the theoretical predictions in the limiting cases of a strongly degenerate fluid and of a normal gas. They also provide the variation of this key quantity in the critical region, where further theoretical efforts are needed to account for our findings.

In atomic systems, clock states feature a zero projection of the total angular momentum and thus a low sensitivity to magnetic fields. This makes them widely used for metrological applications like atomic fountains or gravimeters. Here, we show that a mixture of two such nonmagnetic states still displays magnetic dipole-dipole interactions comparable to the one expected for the other Zeeman states of the same atomic species. Using high-resolution spectroscopy of a planar gas of ^{87}Rb atoms with a controlled in plane shape, we explore the effective isotropic and extensive character of these interactions and demonstrate their tunability. Our measurements set strong constraints on the relative values of the s-wave scattering lengths a_{ij} involving the two clock states.

A fluid is said to be scale invariant when its interaction and kinetic energies have the same scaling in a dilation operation. In association with the more general conformal invariance, scale invariance provides a dynamical symmetry which has profound consequences both on the equilibrium properties of the fluid and its time evolution. Here we investigate experimentally the far-from-equilibrium dynamics of a cold two-dimensional rubidium Bose gas. We operate in the regime where the gas is accurately described by a classical field obeying the Gross-Pitaevskii equation, and thus possesses a dynamical symmetry described by the Lorentz group SO(2,1). With the further simplification provided by superfluid hydrodynamics, we show how to relate the evolutions observed for different initial sizes, atom numbers, trap frequencies, and interaction parameters by a scaling transform. Finally, we show that some specific initial shapes-uniformly filled triangles or disks-may lead to a periodic evolution corresponding to a novel type of breather for the two-dimensional Gross-Pitaevskii equation.

Decoherence is ubiquitous in quantum physics, from the conceptual foundations to quantum information processing or quantum technologies, where it is a threat that must be countered. While decoherence has been extensively studied for simple, well-isolated systems such as single atoms or ions, much less is known for many-body systems where inter-particle correlations and interactions can drastically alter the dissipative dynamics. Here we report an experimental study of how spontaneous emission destroys the spatial coherence of a gas of strongly interacting bosons in an optical lattice. Instead of the standard momentum diffusion expected for independent atoms, we observe an anomalous sub-diffusive expansion, associated with a universal slowing down ∝ 1/t^1/2 of the decoherence dynamics. This algebraic decay reflects the emergence of slowly-relaxing many-body states, akin to sub-radiant states of many excited emitters. These results, supported by theoretical predictions, provide an important benchmark in the understanding of open many-body systems.

Decoherence plays a major role in our current understanding of the conceptual foundations of quantum physics1. In many instances, decoherence is also a threat that must be countered (for instance, in quantum information processing or quantum technologies). While decoherence has been extensively studied for simple, well-isolated systems such as single atoms or ions2, much less is known for many-body systems where interparticle correlations and interactions can drastically alter the dissipative dynamics3–6. Here, we study experimentally the decoherence of a gas of strongly interacting bosons in an optical lattice exposed to near-resonant light and spontaneous emission. We observe an anomalous subdiffusion in momentum space, associated with a universal slowing down ∝1/t1/2 of the loss of spatial coherence. This algebraic decay reflects the emergence of slowly relaxing many-body states5, akin to the subradiant states of many excited emitters4. These results, supported by theoretical predictions, provide an important benchmark in the understanding of open many-body systems. Strongly interacting bosons in an optical lattice exhibit anomalous subdiffusive evolution when subjected to a dissipative process. The experimental observations are attributed to a mechanism termed ‘interaction-impeding of decoherence’.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Topological phenomena in quantum gases Jérôme Beugnon