Pascal Weckesser

The competition of different length scales in quantum many-body systems leads to phenomena such as correlated dynamics and nonlocal order. To investigate such effects in an itinerant lattice-based quantum simulator, it has been proposed to introduce tunable extended-range interactions using off-resonant optical coupling to Rydberg states, known as Rydberg dressing. In this work, we use this approach to realize an effective one-dimensional extended Bose-Hubbard model. Harnessing our quantum gas microscope, we probe the correlated out-of-equilibrium dynamics of extended-range repulsively bound pairs and “hard rods.” By contrast, operating near equilibrium, we observe density ordering when adiabatically turning on the extended-range interactions. Our results pave the way to realizing light-controlled extended-range interacting quantum many-body systems.

Hybrid systems of single, trapped ions embedded in quantum gases are a promising platform for quantum simulations and the study of long-range interactions in the ultracold regime. Feshbach resonances allow for experimental control over the character and strength of the atom-ion interaction. However, the complexity of atom-ion Feshbach spectra, e.g., due to second-order spin-orbit coupling, requires a detailed experimental understanding of the resonance properties—such as the contributing open-channel partial waves. In this work, we immerse a single barium ( Ba + ) ion in a bath of lithium (Li) atoms spin polarized in their hyperfine ground state to investigate the collision energy dependence of magnetically tunable atom-ion Feshbach resonances. We demonstrate fine control over the kinetic energy of the Ba + ion and employ it to explore three-body recombination in the transition from the many- to the few-partial wave regime, marked by a sudden increase of resonant loss. In a dense spectrum—with on average 0.58(1) resonances per Gauss—we select a narrow, isolated feature and characterize it as an s -wave resonance. We introduce a quantum recombination model that allows us to distinguish it from higher-partial-wave resonances. Furthermore, in a magnetic field range with no significant loss at the lowest collision energies, we identify a higher-partial-wave resonance that appears and peaks only when we increase the energy to around the s -wave limit. Our results demonstrate that hybrid atom-ion traps can reach collision energies well in the ultracold regime and that the ion’s kinetic energy can be employed to tune the collisional complex to resonance, paving the way for fast control over the interaction in settings where magnetic field variations are detrimental to coherence.

Hybrid atom-ion systems are a rich and powerful platform for studying chemical reactions, as they feature both excellent control over the electronic state preparation and readout as well as a versatile tunability over the scattering energy, ranging from the few-partial wave regime to the quantum regime. In this work, we make use of these excellent control knobs, and present a joint experimental and theoretical study of the collisions of a single ¹³⁸Ba⁺ ion prepared in the 5 d 2 D 3 / 2 , 5 / 2 metastable states with a ground state ⁶Li gas near quantum degeneracy. We show that in contrast to previously reported atom-ion mixtures, several non-radiative processes, including charge exchange, excitation exchange and quenching, compete with each other due to the inherent complexity of the ion-atom molecular structure. We present a full quantum model based on high-level electronic structure calculations involving spin-orbit couplings. Results are in excellent agreement with observations, highlighting the strong coupling between the internal angular momenta and the mechanical rotation of the colliding pair, which is relevant in any other hybrid system composed of an alkali-metal atom and an alkaline-earth ion.

The relaxation behaviour of isolated quantum systems taken out of equilibrium is among the most intriguing questions in many-body physics¹. Quantum systems out of equilibrium typically relax to thermal equilibrium states by scrambling local information and building up entanglement entropy. However, kinetic constraints in the Hamiltonian can lead to a breakdown of this fundamental paradigm owing to a fragmentation of the underlying Hilbert space into dynamically decoupled subsectors in which thermalization can be strongly suppressed^2–5. Here we experimentally observe Hilbert space fragmentation in a two-dimensional tilted Bose–Hubbard model. Using quantum gas microscopy, we engineer a wide variety of initial states and find a rich set of manifestations of Hilbert space fragmentation involving bulk states, interfaces and defects, that is, two-, one- and zero-dimensional objects. Specifically, uniform initial states with equal particle number and energy differ strikingly in their relaxation dynamics. Inserting controlled defects on top of a global, non-thermalizing chequerboard state, we observe highly anisotropic, subdimensional dynamics, an immediate signature of their fractonic nature^6–9. An interface between localized and thermalizing states in turn shows dynamics depending on its orientation. Our results mark the observation of Hilbert space fragmentation beyond one dimension, as well as the concomitant direct observation of fractons, and pave the way for in-depth studies of microscopic transport phenomena in constrained systems.

The coupling of an isolated quantum state to a continuum is typically associated with decoherence and decreased lifetime. For coupling rates larger than the bandwidth of the associated continuum, decoherence can be mitigated, and new stable eigenstates emerge. Here, we laser-couple diatomic molecules of highly excited Rydberg atoms, so-called Rydberg macrodimers, to a continuum of free motional states. Enabled by their small vibrational eigenfrequencies, we achieve the regime of strong continuum couplings and observe the appearance of new resonances. We explain the observed spectroscopic features as molecular states emerging in the presence of the light field using a Fano model. For atoms arranged on a lattice, we predict the strong continuum coupling to even stabilize triatomic molecules and find the first signatures of these by observing three-atom loss correlations using quantum gas microscopy. Our results present a mechanism to control decoherence and bind polyatomic molecules using strong light-matter interactions. Published by the American Physical Society 2024

The Mott-insulating phase of the two-dimensional (2D) Bose-Hubbard model is expected to be characterized by a nonlocal brane parity order. Parity order captures the presence of microscopic particle-hole fluctuations and entanglement, whose properties depend on the underlying lattice geometry. We realize 2D Bose-Hubbard models in dynamically tunable lattice geometries, using neutral atoms in a passively phase-stable tunable optical lattice in combination with programmable site-blocking potentials. We benchmark the performance of our system by single-particle quantum walks in the square, triangular, kagome, and Lieb lattices. In the strongly correlated regime, we microscopically characterize the geometry dependence of the quantum fluctuations and experimentally validate brane parity as a proxy for the nonlocal order parameter signaling the superfluid–to–Mott-insulating phase transition.

Enhancing light–matter coupling at the level of single quanta is essential for numerous applications in quantum science. The cooperative optical response of subwavelength atomic arrays has been found to open new pathways for such strong light–matter couplings, while simultaneously offering access to multiple spatial modes of the light field. Efficient single-mode free-space coupling to such arrays has been reported, but spatial control over the modes of outgoing light fields has remained elusive. Here, we demonstrate such spatial control over the optical response of an atomically thin mirror formed by a subwavelength array of atoms in free space using a single controlled ancilla atom excited to a Rydberg state. The switching behaviour is controlled by the admixture of a small Rydberg fraction to the atomic mirror, and consequently strong dipolar Rydberg interactions with the ancilla. Driving Rabi oscillations on the ancilla atom, we demonstrate coherent control of the transmission and reflection of the array. These results represent a step towards the realization of quantum coherent metasurfaces, the demonstration of controlled atom–photon entanglement and deterministic engineering of quantum states of light.

The fields of ultracold atoms and trapped ions are important pillars of experimental quantum optics. Recently, the expertise of both fields has b |

Measurement-based quantum computing relies on the rapid creation of large-scale entanglement in a register of stable qubits. Atomic arrays are well suited to store quantum information, and entanglement can be created using highly-excited Rydberg states. Typically, isolating pairs during gate operation is difficult because Rydberg interactions feature long tails at large distances. Here, we engineer distance-selective interactions that are strongly peaked in distance through off-resonant laser coupling of molecular potentials between Rydberg atom pairs. Employing quantum gas microscopy, we verify the dressed interactions by observing correlated phase evolution using many-body Ramsey interferometry. We identify atom loss and coupling to continuum modes as a limitation of our present scheme and outline paths to mitigate these effects, paving the way towards the creation of large-scale entanglement.