The notions of causality adopted within the quantum information and space-time physics communities are distinct. Although experience tells us that these notions play together in a compatible manner in physical experiments, their general interplay is little understood in theory. Therefore we develop a theoretical framework that connects the two causality notions, while also clearly distinguishing them. The framework describes a composition of quantum operations through feedback loops, and the embedding of the resulting, possibly cyclic information-theoretic structure in an acyclic space-time structure. Relativistic causality (which forbids superluminal communication) then follows as a graph-theoretic compatibility condition between the two structures. Demonstrating that indefinite causal order (ICO) processes, widely studied in the quantum information community, can be formulated within our framework, we shed light on the links between indefinite and cyclic causality, and on questions regarding their physicality. In particular, there are several experiments that claim to implement ICO processes in Minkowski space-time, presenting an apparent theoretical paradox: how can an indefinite information-theoretic causal structure be consistent with a definite space-time structure? We address this through no-go theorems, showing that as a consequence of relativistic causality, (a) realizations of ICO processes necessarily involve the nonlocalization of systems in space-time and (b) will nevertheless admit an explanation in terms of a definite and acyclic causal order process, at a more fine-grained level. These results are made possible by introducing the concept of fine-graining that allows causal structures to be analyzed at different levels of detail. This fully resolves the apparent paradox and bears implications for the physical interpretation of ICO experiments. Our work also sheds light on the limits of quantum information processing in space-time and offers concrete insights on the operational meaning of indefinite causality, both within and beyond the context of a fixed space-time.

We consider an exactly solvable model describing the interaction of a quantized strong laser field with free electrons produced in the ionization of a target gas by the field. The interaction is shown to strongly affect the quantum state of the field. It squeezes and displaces the coherent state of a free laser field, which under realistic experimental conditions can result in the formation of nonclassical and non-Gaussian field states with a ring-shaped Wigner function. Our results demonstrate that the effect of the interaction with free electrons on the quantum state of the ionizing laser field should be taken into account simultaneously with that of harmonic generation and above-threshold ionization studied recently.

We study Thouless pumps, i.e., adiabatic topological transport, in an interacting spin chain described by the dimerized $XXZ$ Hamiltonian. In the noninteracting case, quantized Thouless pumps can only occur when a topological singularity is encircled adiabatically. In contrast, here we show that, in the presence of interactions, such topological transport can even persist for exotic paths in which the system gets arbitrarily close to the noninteracting singularity. We illustrate the robustness of these exotic Thouless pumps through the behavior of the noninteracting singularity, which for sufficiently strong interactions splits into two singularities separated by a spontaneous antiferromagnetic insulator. We perform a numerical benchmark of these phenomena by means of tensor network simulations of ground-state physics and real-time adiabatic dynamics. Finally, we propose an experimental protocol with Floquet-driven Rydberg tweezer arrays.

We have experimentally studied dissociative recombination (DR) of electronically and vibrationally relaxed ${\mathrm{ArH}}^{+}$ in its lowest rotational levels, using an electron-ion merged-beams setup at the Cryogenic Storage Ring. We report measurements for the merged-beams rate coefficient of ${\mathrm{ArH}}^{+}$ and compare it to published experimental and theoretical results. In addition, by measuring the kinetic energy released to the DR fragments, we have determined the internal state of the DR products after dissociation. At low collision energies, we find that the atomic products are in their respective ground states, which are only accessible via nonadiabatic couplings to neutral Rydberg states. Published theoretical results for ${\mathrm{ArH}}^{+}$ have not included this DR pathway. From our measurements, we have also derived a kinetic temperature rate coefficient for use in astrochemical models.

We theoretically investigate nondipole effects in the reconstruction of attosecond beating by interference of two-photon transitions (RABBIT) of helium using linearly polarized extreme ultraviolet and infrared fields. By scanning the time delay between the two fields, we observe modulations in sidebands (SBs) both for angular-integrated photoelectron yield and forward-backward asymmetry in photoelectron distribution along the light-propagation direction. The SB modulations of the forward-backward asymmetry reveal Wigner and continuum-continuum time delays of the electron wave packets ionized via nondipole paths, different from the conventional RABBIT where only the dipole paths are involved. Furthermore, the time delays extracted from the forward-backward asymmetry show an abrupt jump as a function of polar emission angle of photoelectrons, due to the competition among continuum partial waves in nondipole laser-assisted photoionization.

The vibrational branching fractions from the lowest excited electronic state for ${\text{SrOCH}}_3$, ${\text{SrNH}}_2$, and $\text{SrSH}$ are measured at the $<0.1\%$ level. Spectra are obtained by driving the $\tilde{X}\text{−}\tilde{A}$ transitions and dispersing the fluorescence on a grating spectrometer. We also perform ab initio calculations for the energies of vibrational levels relevant for laser cooling, as well as branching fractions to support the interpretations of all molecular spectra. Symmetry-group analysis is applied in conjunction with our data to study rotational closure in these molecules. These analyses indicate favorable prospects for laser cooling ${\text{SrNH}}_2$ and other similar alkaline-earth-metal(-like) amides for future beyond the Standard Model physics searches using polyatomic molecules with long-lived parity doublets.

Optically pumped magnetometers (OPMs) are revolutionizing the task of magnetic-field sensing due to their extremely high sensitivity combined with technological improvements in miniaturization which have led to compact and portable devices. OPMs can be based on spin-oriented or spin-aligned atomic ensembles which are spin polarized through optical pumping with circular or linear polarized light, respectively. Characterization of OPMs and the dynamical properties of their noise is important for applications in real-time sensing tasks. In our work, we experimentally perform spin noise spectroscopy of an alignment-based magnetometer. Moreover, we propose a stochastic model that predicts the noise power spectra exhibited by the device when, apart from the strong magnetic field responsible for the Larmor precession of the spin, white noise is applied in the perpendicular direction aligned with the pumping-probing beam. By varying the strength of the noise applied as well as the linear-polarization angle of incoming light, we verify the model to accurately predict the heights of the Larmor-induced spectral peaks and their corresponding linewidths. Our work paves the way for alignment-based magnetometers to become operational in real-time sensing tasks.

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 study the dynamics of the pump mode in the down-conversion Hamiltonian using the cumulant-expansion method, perturbation theory, and the full numerical simulation of systems with a pump mean photon number of up to $100000$. We particularly focus on the properties of the pump mode such as depletion, entanglement, and squeezing for an experimentally relevant initial state in which the pump mode is initialized in a coherent state. Through this analysis, we obtain the short-time behavior of various quantities and derive timescales at which the above-mentioned features, which cannot be understood through the parametric approximation, originate in the system. We also provide an entanglement witness involving moments of bosonic operators that can capture the entanglement of the pump mode. Finally, we study the photon-number statistics of the pump and the signal and idler modes to understand the general behavior of these modes for experimentally relevant timescales.

Quantum behavior of the electromagnetic field in unbounded macroscopic media displaying absorption is properly described by the Langevin noise formalism (macroscopic quantum electrodynamics), where the field is assumed to be entirely produced by medium fluctuating sources via the dyadic Green's function. On the other hand, such formalism is able to deal with the case of finite-size lossy objects placed in vacuum only as a limiting situation where the permittivity limit $\mathrm{Im}\left(\varepsilon \right)\to 0^{+}$ pertaining the regions filled by vacuum is taken at the end of the calculations. Strictly setting $\mathrm{Im}\left(\varepsilon \right)=0$ is forbidden in the Langevin noise formalism since the field would vanish in the lossless regions, and this is physically due to the fact that the contribution of the scattering modes to the field is not separated from the contribution produced by the medium fluctuating sources. Recently, a modified Langevin noise formalism has been proposed to encompass the scattering modes and accordingly, it is able to describe the structured lossless situations by strictly setting $\mathrm{Im}\left(\varepsilon \right)=0$. However, such modified formalism has been numerically validated only in a few specific geometries. In this paper we analytically derive the modified Langevin noise formalism from the established canonical quantization of the electromagnetic field in macroscopic media, thus proving that it models any possible scenario involving linear, inhomogeneous, and magnetodielectric samples. The derivation starts from quantum Maxwell equations in the Heisenberg picture together with their formal solution as the superposition of the medium assisted field and the scattering modes. We analytically prove that each of the two field parts can be expressed in term of particular bosonic operators, which in turn diagonalize the electromagnetic Hamiltonian and whose associated quasiparticles are medium assisted and scattering polaritons, respectively. The key ingredient underpinning our reasoning is a peculiar integral relation linking the far-field amplitude of the dyadic Green's function and the scattering modes, a relation we rigorously derive and physically explain by identifying the scattering modes as fields generated by infinitely far dipole point sources.

Squeezing is a nonclassical feature of quantum states that is a useful resource, for example, in quantum sensing of mechanical forces. Here, we show how to use optimal control theory to maximize squeezing in an optomechanical setup with two external drives and determine how fast the mechanical mode can be squeezed. For the autonomous drives considered here, we find the inverse cavity decay to lower-bound the protocol duration. At and above this limit, we identify a family of protocols leveraging a two-stage control strategy, where the mechanical mode is cooled before it is squeezed. Identification of the control strategy allows for two important insights: to determine the factors that limit squeezing and to simplify the time dependence of the external drives, making our protocol readily applicable in experiments.

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.