We study quantum droplets emerging in a quasi-one-dimensional asymmetric mixture of two atomic species with different intracomponent coupling constants. We find that such mixtures support a rich variety of multipole quantum droplets, where the macroscopic wave function of one component changes its sign and features a distinctive multipole structure, while the wave function of another component does not have zeros. Such multipole droplets have no counterparts in the reduced single-component model frequently used to describe symmetric one-dimensional mixtures. We study transformations of multipole states upon variation of the chemical potential of each component and demonstrate that quantum droplets can split into separated fundamental states, transform into flattop multipoles, or into a multipole component coupled to a flattop state with several humps on it, akin to antidark solitons. Multipole quantum droplets described here are stable in a large part of their existence domain. Our findings essentially broaden the family of quantum droplet states emerging in the beyond-mean-field regime and open the way for observation of such heterostructured states in Bose-Bose mixtures.

We develop an approach to directly control the phase of an electron's sub-cycle motion in an intense laser field. The method is established by tuning a low-frequency electric field applied on a centrosymmetric gaseous target during its interaction with a few-cycle infrared laser pulse. We find a universal analytical relation between the low-frequency electric field and its induced harmonic frequency shift, derived by the strong-field approximation. This simple relation and its universality are confirmed numerically by directly solving the time-dependent Schrödinger equation. Moreover, we demonstrate the benefits of the discovered relation in in situ applications, including continuously and precisely tuning XUV waves and developing a method of comprehensively sampling the THz pulse.

Out of thermal equilibrium, bosonic quantum systems can undergo Bose condensation away from the ground state, featuring a macroscopic occupation of an excited state or even of multiple states in the so-called Bose-selection scenario. In previous work, a theory was developed that predicts in which states a driven-dissipative ideal Bose gas condenses. Here we address the inverse problem: Given a target state with desired condensate fractions in certain single-particle states, how can this configuration be achieved by tuning available control parameters? Which type of experimental setup allows for flexible condensation control? We solve these problems, on the one hand, by proposing a Bose condenser, experimentally implementable in a superconducting circuit, where targeted Bose condensation into eigenstates of a chain of resonators is driven through the coupling to artificial quantum baths, realized via auxiliary two-level systems. On the other hand, we develop a theory to solve the inverse problem based on linear-programming methods. We further discuss the engineering of transition points between different Bose condensation configurations, which may find application in amplification, heat-flow control, and the design of highly structured quantum baths.

Any physical theory aims to establish the relationship between physical systems in terms of the interaction between these systems. However, any known approach in the literature to infer this interaction is dependent on the particular modeling of the physical systems involved. Here, we propose an alternative approach where one does not need to model the systems involved but only assume that these systems behave according to quantum theory. We first propose a setup to infer a particular entangling quantum interaction between two systems from the statistics. For our purpose, we utilize the framework of Bell inequalities. We then extend this setup where an arbitrary number of quantum systems interact via some entangling interaction.

The search for ferromagnetism in the Hubbard model has been a problem of outstanding interest since Nagaoka's original proposal in 1966. Recent advances in quantum simulation have today enabled the study of tunable doped Hubbard models in ultracold atomic systems. Employing large-scale density-matrix renormalization group calculations, we establish the existence of high-spin ground states of the Hubbard model on finite-sized triangular lattices, analyze the microscopic mechanisms behind their origin, and investigate the interplay between ferromagnetism and other competing orders, such as stripes. These results explain (and shed light on) the intriguing observations of ferromagnetic correlations in recent optical-lattice experiments. Additionally, we examine a generalized variant of the Hubbard model, wherein any second electron on a single lattice site is weakly bound compared to the first one, and demonstrate how this modification can lead to enhanced ferromagnetism, at intermediate lengthscales, on the nonfrustrated square lattice as well.

Ultracold dipolar particles pinned in optical lattices or tweezers provide an excellent platform for the study of the intriguing equilibration dynamics of spin models with dipolar exchange. Starting with an initial state in which spins of opposite orientation are prepared in each of the legs of a ladder lattice, we show that spin relaxation displays an unexpected dependence on interleg distance and dipole orientation. This dependence, stemming from the interplay between intra- and interleg interactions, results in three distinct relaxation regimes: (i) ergodic, characterized by the fast relaxation towards equilibrium of correlated pairs of excitations generated at exponentially fast rates from the initial state; (ii) metastable, in which the state is quasilocalized in the initial state and only decays in exceedingly long timescales, resembling false-vacuum decay; and, surprisingly, (iii) partially relaxed, with coexisting fast partial relaxation and partial quasilocalization. The realization of this intriguing dynamics is at hand in current state-of-the-art experiments in dipolar gases.

We demonstrate the use of coherent-state quantum process tomography (csQPT) for a bosonic-mode superconducting circuit. We have enhanced our methodology over previous implementations of csQPT by leveraging Kraus operators and constrained gradient descent to learn the underlying process. We show the results of our method by characterizing a logical quantum gate implemented using displacement and selective number-dependent arbitrary phase operations on an encoded qubit. Our use of csQPT allows for the reconstruction of Kraus operators for the larger Hilbert space rather than being limited to the logical subspace. This approach enables us to more accurately identify and quantify the various error mechanisms that can lead to gate infidelity, including those occurring outside of the computational subspace. We showcase the potential of our approach by demonstrating the ability to quantify leakage outside of the computational subspace, a key factor for developing more robust and reliable quantum gates in high-dimensional systems.

We report numerically exact quantum scattering calculations on magnetic Feshbach resonances in ultracold, strongly anisotropic atom-molecule [Rb(${}^{2}S$) $+$ SrF(${}^2{\mathrm{\Sigma }}^{+}$)] collisions based on state-of-the-art ab initio potential energy surfaces. We find broad resonances mediated by the intermolecular spin-exchange interaction, as well as narrow resonances due to the intramolecular spin-rotation interaction, which are unique to atom-molecule collisions. Remarkably, the density of resonances in atom-molecule collisions is not much higher than that in atomic collisions despite the presence of a dense manifold of molecular rotational states, which can be rationalized by analyzing the adiabatic states of the collision complex.

Understanding the dissociative electron attachment (DEA) in low-temperature ammonia plasma is crucial for many technological applications, in particular, for alternative energy sources. However, theoretical modeling of this process has been challenging due to the complex interplay of electronic motion and multidimensional nuclear dynamics. This Letter presents a theoretical approach to investigate the process, applying it to the breakup of the ${\mathrm{NH}}_3$ molecule by low-energy electrons. The potential energy surface of the ${\mathrm{NH}}_3{}^{-}$ 5.5 eV resonant state is computed to elucidate the mechanism of the dissociative process. The cross section of DEA via the 5.5 eV resonance is then calculated for both ${\mathrm{NH}}_3$ and ${\mathrm{ND}}_3$. The positions of the peaks in both cross sections and the ratio between them are in excellent agreement with previous experiments. Our findings suggest that the developed theoretical model accurately describes the DEA process and could serve as a useful tool for providing DEA cross sections for other molecules to model low-temperature plasma.

Electronic structure methods based on the Kohn-Sham (KS) formalism of density-functional theory are ubiquitously and highly successfully used in physics, chemistry, and materials science. Whether the KS formalism is universally applicable is, however, an open question because it is not known whether a KS model system exists for all physical electron systems. This is the question of whether electron densities of real physical systems are always noninteracting $v$-representable. Here we give indications that this is not always the case. To that end, we calculated highly accurate electron densities of first and second row atoms with a partially filled $p$ shell by high-level quantum chemistry methods and by full configuration interaction and then tried to determine the effective KS potential by KS inversion. Except for nitrogen and phosphorus this was only possible for an occupation pattern violating the Aufbau principle. This means that the resulting wave function is not a ground state of the KS Hamiltonian operator and therefore not a valid KS wave function, which indicates that the KS formalism is not always applicable. Strategies to avoid the presented $v$-representability problem are discussed.

We study the dynamics of a Bose-Hubbard model coupled to an engineered environment which in the noninteracting limit induces an effective nonreciprocal hopping as described by the Hatano-Nelson model. At strong interactions, two bosons occupying the same site form a so-called repulsively bound pair, or doublon. Using tensor-network simulations, we clearly identify a distinct doublon light cone and show that the doublon inherits nonreciprocity from that of single particles. Applying the idea of reservoir engineering at the level of doublons, we introduce a new set of dissipators and we analytically show that then the doublon dynamics are governed by the Hatano-Nelson model. This brings about a two-particle non-Hermitian skin effect and nonreciprocal doublon motion. Combining features of the two models we study, we show that single particles and doublons can be made to spread with opposite directionality, opening intriguing possibilities for the study of dynamics in interacting nonreciprocal models.

We develop a simple model and propose a scheme that allows the production of twisted atoms in free space using the absorption of twisted photons by a bound electron. We show that in the inelastic collision of a photon and an atom, the twisted state of the photon is transferred to the center-of-mass state, so that the projection of the orbital momentum of the atom becomes $m_{\gamma }-\mathrm{\Delta }{m}_e$. We also show that, depending on the experimental conditions, the twistedness of the photon is either transferred to the atomic center-of-mass quantum state or modifies the selection rule for the bound electron transition. The proposed scheme is general and enables complex shaping of the atomic wavefront.

We present measurement schemes that do not rely on photon-number-resolving detectors but are nevertheless optimal for estimating a differential phase shift in interferometry with either an entangled coherent state or a qubit which-path state (where the path taken by a coherent-state wave packet is entangled with the state of a qubit). The homodyning schemes analyzed here achieve optimality (saturate the quantum Cramér-Rao bound) by maximizing the sensitivity of measurement outcomes to phase-dependent interference fringes in a reduced Wigner distribution. In the presence of photon loss, the schemes become suboptimal, but we find that their performance is independent of the phase to be measured. They can therefore be implemented without any prior information about the phase and without adapting the strategy during measurement, unlike strategies based on photon-number parity measurements or direct photon counting.

We report the experimental demonstration of laser cooling of bunched Li-like ${}^{16}\mathrm{O}^{5+}$ ion beams at ∼64% of the speed of light in the storage ring CSRe. The longitudinal dynamics of laser-cooled bunched ion beams were investigated by observing the Schottky signals in real time with a high-sensitivity Schottky resonator. Furthermore, a multiparticle phase-space tracking code was developed to simulate the longitudinal Schottky spectrum of the ions inside the bucket and to interpret the experimental observations and derive the momentum distribution of the laser-cooled beams. We demonstrate that the relative longitudinal momentum spread of ${\mathrm{O}}^{5+}$ ion beams reached $\mathrm{\Delta }p/p\approx 2.0\times {10}^{-6}$ with laser cooling, which is one order of magnitude smaller than what can be reached by electron cooling. The laser-cooling technique and the simulation method developed in this work can be applied to study phase-transition effects of bunched ion beams at storage rings and also pave the way for laser-cooling and precision laser spectroscopy experiments at the future large heavy-ion accelerator facilities HIAF (China) and FAIR (Germany), and the proposed Gamma Factory at CERN.

The radiative lifetime of the $A{}^2\mathrm{\Pi }_{1/2}$ $(v=0)$ state in radium monofluoride (RaF) is measured to be 35(1) ns. The lifetime of this state and the related decay rate $\mathrm{\Gamma }=2.86\left(8\right)\times {10}^7$ ${\text{s}}^{-1}$ are of relevance to the laser cooling of RaF via the optically closed $A{}^2\mathrm{\Pi }_{1/2}\leftarrow X{}^2\mathrm{\Sigma }_{1/2}$ transition, which makes the molecule a promising probe to search for new physics. RaF is found to have a comparable photon-scattering rate to homoelectronic laser-coolable molecules. Owing to its highly diagonal Franck-Condon matrix, it is expected to scatter an order of magnitude more photons than other molecules when using just three cooling lasers, before it decays to a dark state. The lifetime measurement in RaF is benchmarked by measuring the lifetime of the $8P_{3/2}$ state in Fr to be 83(3) ns, in agreement with literature.

We develop and study an atom-only description of the Dicke model with time-periodic couplings between atoms and a dissipative cavity mode. The cavity mode is eliminated, giving rise to effective atom-atom interactions and dissipation. We use this effective description to analyze the dynamics of the atoms that undergo a transition to a dynamical superradiant phase with macroscopic coherences in the atomic medium and the light field. Using Floquet theory in combination with the atom-only description we provide a precise determination of the phase boundaries and of the dynamical response of the atoms. From this we can predict the existence of dissipative time crystals that show a subharmonic response with respect to the driving frequency. We show that the atom-only theory can describe the relaxation into such a dissipative time crystal and that the damping rate can be understood in terms of a cooling mechanism.

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.

We study the detection of beyond-quantum nonlocal states that can exist in a theoretical model whose local systems are standard quantum theory in the framework of general probabilistic theories (GPTs). We find that device-dependent detections are possible for beyond-quantum nonlocal states in GPTs even though device-independent detections are not valid. We give a device-dependent detection based on local observables to distinguish any beyond-quantum nonlocal state from all standard quantum states. In particular, we give a way to detect any beyond-quantum nonlocal state of the two-qubit system by observing only spin observables on local systems. Our results will help in the experimental detection of beyond-quantum nonlocality or justification of standard quantum theory.

Arrays of coupled dipole emitters support collective single- and multiphoton states that can preserve quantum excitations. One of the crucial characteristics of these states is the lifetime, which is fundamentally limited due to spontaneous emission. Here, we present a mechanism of the external coupling of two states via the radiation continuum, which allows for an increase in the lifetime of both single and double excitations. As an illustrative example, we consider a ringlike ensemble of quantum emitters, demonstrating that upon slight optimization of the structure geometry, one can increase the lifetime of singly and doubly excited states with nonzero orbital momentum by several orders of magnitude. The proposed mechanism of multiphoton excitations lifetime control has a universal nature and might be applied for a wide class of open quantum systems and quantum ensembles besides the particular geometry considered in this Letter.

One of the most counterintuitive aspects of quantum theory is its claim that there is “intrinsic” randomness in the physical world. Quantum information science has greatly progressed in the study of intrinsic, or secret, quantum randomness in the past decade. With much emphasis on device-independent and semi-device-independent bounds, one of the most basic questions has escaped attention: how much intrinsic randomness can be extracted from a given state $\rho$, and what measurements achieve this bound? We answer this question for three different randomness quantifiers: the conditional min-entropy, the conditional von Neumann entropy, and the conditional max-entropy. For the first, we solve the min-max problem of finding the projective measurement that minimizes the maximal guessing probability of an eavesdropper. The result is that one can guarantee an amount of conditional min-entropy $H_{\text{min}}^{*}=-{log}_2{P}_{\text{guess}}^{*}\left(\rho \right)$ with $P_{\text{guess}}^{*}\left(\rho \right)=\frac{1}{d}{\left(\mathrm{tr}\sqrt{\rho }\right)}^2$ by performing suitable projective measurements. For the conditional von Neumann entropy, we find that the maximal value is $H^{*}={log}_{2}d-S\left(\rho \right)$, with $S\left(\rho \right)$ the von Neumann entropy of $\rho$, while for the conditional max-entropy, we find the maximal value $H_{\text{max}}^{*}={log}_{2}d+{log}_2{\lambda }_{\text{max}}\left(\rho \right)$, where ${\lambda }_{\text{max}}\left(\rho \right)$ is the largest eigenvalue of $\rho$. Optimal values for $H_{\text{min}}^{*}, H^{*}$ and $H_{\text{max}}^{*}$ are achieved by measuring in any basis that is unbiased with respect to the eigenbasis of $\rho$, as well as by other, less intuitive, measurements.