Ionic Bose polarons are quantum entities emerging from the interaction between an ion and a Bose-Einstein condensate, featuring long-ranged interactions that can compete with the gas healing length, resulting in strong interparticle correlations and enhancement of the gas density around the ion. Full numerical treatment of such systems is computationally very expensive and does not easily allow one to study the system dynamics. For this purpose, we study a mean-field-based description of such systems in the co-moving frame. We find that it captures a sizable change in the gas density and qualitatively reproduces the available results based on Monte Carlo simulations. We consider a couple of scenarios which consist of a single ion and two pinned ions, where it is possible to extract their effective interaction induced by the bath. This approach seems to be promising for studying transport and nonequilibrium dynamics of charged (bi)polarons in condensed media.

Multistability cannot be derived from any theoretical model that is based on a monostable master equation. On the other hand, multistability is experimentally observed in a variety of quantum systems. A master equation having a nonlinear term that gives rise to disentanglement has been recently proposed. The dynamics governed by this master equation is explored for a quantum system made of coupled spins. It is found that the added nonlinear term can give rise to multistability. The spins' response to an externally applied magnetic field is evaluated, and both a phase transition and a dynamical instability are found. These findings, which originate from disentanglement-induced multistability, indirectly support the hypothesis that spontaneous disentanglement occurs in quantum systems.

Amorphous solids, i.e., systems which feature well-defined short-range properties but lack long-range order, constitute an important research topic in condensed matter. While their microscopic structure is known to differ from their crystalline counterpart, there are still many open questions concerning the emergent collective behavior in amorphous materials. This is particularly the case in the quantum regime, where the numerical simulations are extremely challenging. In this paper, we instead propose to explore amorphous quantum magnets with an analog quantum simulator. To this end, we first present an algorithm to generate amorphous quantum magnets, suitable for Rydberg simulators of the Ising model. Subsequently, we use semiclassical approaches to get a preliminary insight of the physics of the model. In particular, for ferromagnetic interactions, we calculate mean-field phase diagrams, and use the linear spin wave theory to study localization properties and dynamical structure factors of the excitations. For antiferromagnetic interactions, we show that amorphous magnets exhibit a complex classical energy landscape by means of simulated annealing. Finally, we outline an experimental proposal based on Rydberg atoms in programmable tweezer arrays, thus opening the road towards the study of amorphous quantum magnets in regimes difficult to simulate classically.

Ion-neutral reactions are driving the formation of small molecules in the gas phase of interstellar clouds, where hydrogen molecules and their ions are by far the most important collision partners for any species in the astrochemical network. Here we present absolute rate coefficient measurements for the reactions ${\mathrm{HD}}^{+}+\mathrm{C}\to {\mathrm{CH}}^{+}/{\mathrm{CD}}^{+}+\mathrm{D}/\mathrm{H}$ and ${\mathrm{H}}_3^{+}+\mathrm{C}\to {\mathrm{CH}}^{+}/{\mathrm{CH}}_2^{+}+{\mathrm{H}}_2/\mathrm{H}$ obtained using a recently commissioned ion-neutral collision setup at the Cryogenic Storage Ring. Our measurements with vibrationally cold ions result in significantly higher rate coefficients when compared with previous studies using internally excited ions, bringing them in better agreement with classical capture theories. Moreover, we have performed detailed quasiclassical trajectory (QCT) calculations for the ${\mathrm{HD}}^{+}+\mathrm{C}$ reaction, using new potential energy surfaces. Our experimental results and the QCT calculations show very good agreement for the absolute cross section of the reactions, as well as for the isotope effect. These results have great potential relevance for the chemistry of the interstellar medium and the onset of organic chemistry in space.

Measured annihilation spectra are presented for aromatic and heterocyclic ring molecules resolved as a function of incident positron energy using a trap-based positron beam. Comparisons with the vibrational mode spectra yield positron-molecule binding energies. Ab initio many-body theory predictions, which take proper account of electron-positron correlations including virtual-positronium formation, are presented and are found to be in good to excellent agreement with the measured binding energies. The calculations elucidate the competition between permanent dipole moments and $\pi$ bonds in determining the spatial distribution of the bound-state positron density. The implications of these results and the role of multimode features in annihilation in these molecules, including Fermi resonances, are discussed.

This study analyzes the use of a Cs paraffin-coated vapor cell with a maximum 17-second Zeeman population lifetime to realize spontaneous polarization (SP) and spin bistability (SB). To our knowledge, this is the first time that hysteresis loop and time evolution results of spin bistable states are observed in coated cells when both the spin exchange rate and optical pumping rate are much larger than the longitudinal relaxation rate. After obtaining the characteristics and providing the figure of merit in SB, we validate the agreement between the experimental results and the theoretical prediction, thereby demonstrating the possibility of regulating SB using precise optical controls. Achieving stretched SB in vapor cells at room temperature will provide insights into the prospects of optical switches and flip-flops in integrated optics and quantum interface.

We experimentally realize an effective Hamiltonian with a continuously adjustable staggered gauge field for weakly interacting bosons in an optical lattice. Periodic driving realizes a staggered $\mathrm{\Phi }$-flux model, where $\mathrm{\Phi }$ can be continuously tuned around $\pi$ or $2\pi$. We study heating and prethermal relaxation in the modulated lattices following sudden changes in the effective fields. By quenching the system between different Floquet-induced fields with different ground states, we observe Bose recondensation of quench-excited atoms on time scales faster than heating due to the drive. These results are necessary steps towards the generation of correlated states by Hamiltonian modulation, but drive-induced heating (observed here in the weakly interacting limit) is poorly understood in the highly correlated limit and needs to be further studied to determine if such correlated states are realizable.

We discuss a generalization of the non-Hermitian skin effect to finite-size photonic structures with neither gain nor loss in the bulk and a purely real energy spectrum under periodic boundary conditions (PBCs). We show that despite being Hermitian in the bulk, such systems can still have significant portions of eigenmodes concentrated at the edges and that this edge concentration can be linked to the nontrivial point-gap topology of the size-dependent regularized PBC spectrum, accounting for the radiative losses due to photon emission through the edges. We focus on a particular example of an array of atoms chirally coupled to the waveguide, but our results are also applicable to other systems with losses through the edges.

We have observed the spontaneous appearance of currents in a ring of ultracold fermionic atoms $\left({}^6\mathrm{Li}\right)$ with attractive interactions, following a quench to a BCS-like pair superfluid. We have measured the winding number probability distribution for a range of quench rates, with a quench protocol using simultaneous forced evaporation and interaction ramps to achieve faster effective quench rates with less atom loss than a purely evaporative quench. We find that for the fastest quenches the mean-square winding number of the current follows a scaling law in the quench rate with exponent $\sigma =0.24\left(2\right)$, which is somewhat lower than that predicted by the Kibble-Zurek mechanism for the three-dimensional $XY$ model (1/3) and unexpectedly closer to the value obtained from mean-field theory (1/4). For slower quenches nonuniversal effects become significant and we observe a lower rate of spontaneous current formation that does not follow a simple scaling law.

Analog quantum simulation is emerging as a powerful tool for uncovering classically unreachable physics such as many-body real-time dynamics. A complete quantification of uncertainties is necessary in order to make precise predictions using simulations on modern-day devices. Therefore, the inherent physical limitations of the device on the parameters of the simulation must be understood. This paper examines the interplay of errors arising from simulation of approximate time evolution with those due to practical, real-world device constraints. These errors are studied in Heisenberg-type systems on analog quantum devices described by the Ising Hamiltonian. A general framework for quantifying these errors is introduced and applied to several proposed time evolution methods, including Trotter-like methods and Floquet-engineered constant-field approaches. The limitations placed on the accuracy of time evolution methods by current devices are discussed. Characterization of the scaling of coherent effects of different error sources provides a way to extend the presented Hamiltonian engineering methods to take advantage of forthcoming device capabilities.

We have applied a machine learning algorithm to predict the emergence of environment-induced spontaneous synchronization between two qubits in an open system setting. In particular, we have considered three different models, encompassing global and local dissipation regimes, to describe the open system dynamics of the qubits. We have utilized the $k$-nearest-neighbor algorithm to estimate the long-time synchronization behavior of the qubits only using the early time expectation values of qubit observables in these three distinct models. Our findings clearly demonstrate the possibility of determining the occurrence of different synchronization phenomena with high precision even at the early stages of the dynamics using a machine learning-based approach. Moreover, we show the robustness of our approach against potential measurement errors in experiments by considering random errors in the qubit expectation values, initialization errors, as well as deviations in the environment temperature. We believe that the presented results can prove to be useful in experimental studies on the determination of quantum synchronization.

Boson sampling is a sampling task proven to be hard to simulate efficiently using classical computers under plausible assumptions, which makes it an appealing candidate for quantum advantage. However, due to a large noise rate for near-term quantum devices, it is still unclear whether those noisy devices maintain the quantum advantage for much larger quantum systems. Since the noise rate typically grows with the circuit depth, an alternative is to find evidence of simulation hardness at the shallow-depth quantum circuit. To find the evidence, one way is to identify the minimum depth required for the average-case hardness of approximating output probabilities, which is considered a necessary condition for the state-of-the-art technique to prove the simulation hardness of boson sampling. In this work, we analyze the output probability distribution of shallow-depth boson sampling for Fock states and Gaussian states and examine the limitation of the average-case hardness argument at this shallow-depth regime for geometrically local architectures. We propose a shallow-depth linear optical circuit architecture that can overcome the problems associated with geometrically local architectures. Our numerical results suggest that this architecture demonstrates possibilities of average-case hardness properties in a shallow-depth regime through its resemblance to the global Haar-random boson sampling circuit. This result implies that the corresponding architecture has the potential to be utilized for scalable quantum advantage with its shallow-depth boson sampling.

We derive analytically the leading beyond-mean-field contributions to the zero-temperature equation of state and to the fermionic quasiparticle residue and effective mass of a dilute Bose-Fermi mixture in two dimensions. In the repulsive case, we perform quantum Monte Carlo simulations for two representative bosonic concentrations and equal masses, extending a method for correcting finite-size effects in fermionic gases to Bose-Fermi mixtures. We find good agreement between analytic expressions and numerical results for weak interactions, while significant discrepancies appear in the regime close to mechanical instability, above which we provide evidence of phase separation of the bosonic component.

The Gottesman-Kitaev-Preskill (GKP) code offers the possibility to encode higher-dimensional qudits into individual bosonic modes with, for instance, photonic excitations. Since photons enable the reliable transmission of quantum information over long distances and since GKP states subject to photon loss can be recovered to some extent, the GKP code has found recent applications in theoretical investigations of quantum communication protocols. While previous studies have primarily focused on GKP qubits, the possible practical benefits of higher-dimensional GKP qudits are hitherto widely unexplored. In this paper, we carry out performance analyses for three quantum repeater protocols based on GKP qudits including concatenations with a multiqudit quantum polynomial code. We find that the potential data transmission gains for qudits are often hampered by their decreased GKP error-correcting capabilities. However, we also identify parameter regimes in which having access to an increased number of quantum levels per mode can enhance the theoretically achievable secret-key rate of the quantum repeater. Some of our protocols share the attractive feature that local processing and complete error syndrome identification are realizable without online squeezing. Provided a supply of suitable multimode GKP states is available, this can be realized with a minimal set of passive linear optical operations, even when the logical qudits are composed of many physical qudits.

The long-range electrostatic interactions between molecules depend strongly on their relative orientation, which manifests as a rotational-state dependence. Interactions between molecules in the same rotational quantum state are well-known attractive rotational van der Waals interactions. Interactions in rotational states that differ by one quantum show resonant dipole-dipole interactions. We show that where molecules are in rotational states that differ by more than one quantum, they exhibit repulsive van der Waals interactions. At temperatures below a millikelvin, this effect can reduce collisional loss by multiple orders of magnitude. These repulsive interactions lead to applications in quantum simulation and impurity physics with ultracold polar molecules.

Time-periodic systems allow engineering new effective Hamiltonians from limited physical interactions. For example, the inverted position of the Kapitza pendulum emerges as a stable equilibrium with rapid drive of its pivot point. In this work we propose the Kapitzonium: a Floquet qubit that is the superconducting circuit analog of a mechanical Kapitza pendulum. Under periodic driving, the bit- and phase-flip rates of the emerging qubit states are exponentially suppressed with respect to the ratio of the effective Josephson energy to charging energy. However, we find that dissipation causes leakage out of the Floquet qubit subspace. We engineer a passive cooling scheme to stabilize the qubit subspace, which is crucial for high-fidelity quantum control under dissipation. Furthermore, we introduce a hardware-efficient fluorescence-based method for qubit measurement and discuss the experimental implementation of the Floquet qubit. Our work provides the fundamental steps to develop more complex Floquet quantum systems from the ground up to realize large-scale protected engineered dynamics.

Recent advancements in abilities to create and manipulate the electron's transverse wave function within the transmission electron microscope (TEM) and scanning TEM have enabled vectorially resolved electron-energy-loss (EEL) and -gain (EEG) measurements of nanoscale and quantum material responses using pre- and postselected free-electron states. This newfound capability is prompting renewed theoretical interest in quantum-mechanical treatments of inelastic-electron-scattering observables and the information they contain. Here we present a quantum-mechanical treatment of the inelastic scattering of free electrons between pre- and postselected transverse states with fully retarded electron-sample interactions for both spontaneous EEL and continuous-wave laser-stimulated EEG measurements. General expressions for the state-resolved energy-loss and -gain rates are recast in forms amenable to numerical calculation using the method of coupled dipoles. We numerically implement our theory within the electron-driven discrete dipole approximation code and use it to investigate specific examples that highlight its versatility regarding the number, size, geometry, and material composition of the target specimen as well as its ability to describe matter-wave diffraction from finite nanoscopic targets.

Unless constrained by symmetry, measurement of an observable on an ensemble of identical quantum systems returns a distribution of values which are encoded in the full counting statistics. While the mean value of this distribution is important for determining certain properties of a system, the full distribution can also exhibit universal behavior. In this paper we study the full counting statistics of particle number in one-dimensional interacting Bose and Fermi gases which have been quenched far from equilibrium. In particular, we consider the time evolution of the Lieb-Liniger and Gaudin-Yang models quenched from a Bose-Einstein condensate initial state and calculate the full counting statistics of the particle number within a subsystem. We show that the scaled cumulants of the charge in the initial state and at long times are simply related and, in particular, the latter are independent of the model parameters. Using the quasiparticle picture we obtain the full time evolution of the cumulants and find that although their end points are fixed, the finite-time dynamics depends strongly on the model parameters. We go on to construct the scaled cumulant generating functions and from this determine the limiting charge probability distributions at long time which are shown to exhibit distinct, nontrivial, and non-Gaussian fluctuations and large deviations.

In classical optics, there is a well-known resolution limit called Rayleigh's curse in the separation of two incoherent optical sources in close proximity. Recently, Tsang et al. [Phys. Rev. X 6, 031033 (2016)] revealed that this difficulty may be circumvented in the framework of quantum theory. Following their work, various estimation methods have been proposed to overcome Rayleigh's curse, but none of them enables us to estimate the positions of two point sources simultaneously based on single-photon measurements with high accuracy. In this study, we propose a method to simultaneously estimate the positions of two point sources with the highest accuracy using an adaptive quantum state estimation scheme.

Sympathetic cooling of ${\mathrm{Th}}^{3+}$ ions is demonstrated in an experiment where ${}^{229}\mathrm{Th}$ and ${}^{230}\mathrm{Th}$ are extracted from uranium recoil-ion sources and are confined in a linear Paul trap together with laser-cooled ${}^{88}\mathrm{Sr}^{+}$ ions. Because of their similar charge-to-mass ratios the ions are closely coupled and arrange themselves into two-species Coulomb crystals containing up to a few tens of ${\mathrm{Th}}^{3+}$ ions. To show the suitability of the sympathetically cooled ${\mathrm{Th}}^{3+}$ ions for high-resolution laser spectroscopy, the absolute frequencies and isotope shifts of $5F_{5/2}\to 6D_{5/2}$ and $5F_{7/2}\to 6D_{5/2}$ transitions of ${}^{230}\mathrm{Th}^{3+}$ are measured. The system is developed for hyperfine spectroscopy of electronic transitions of nuclear ground and isomeric states in ${}^{229}\mathrm{Th}^{3+}$.