We study a weakly interacting one-dimensional Bose gas with two impurities coupled locally to the boson density. We derive analytical results for the induced interaction between the impurities at arbitrary positive coupling and separation $r$. At $r\lesssim \xi$, where $\xi$ denotes the healing length of the Bose gas, the interaction is well described by the mean-field contribution. Its form changes as the coupling is increased, approaching a linear function of $r$ at short distances in the regime of strong coupling. The mean-field contribution decays exponentially at arbitrary coupling for $r\gg \xi$. At such long distances, however, the effect of quantum fluctuations becomes important, giving rise to a long-ranged quantum contribution to the induced interaction. At longest distances it behaves as $1/r^3$, while at strong coupling we find an intermediate distance regime with a slower decay, $1/r$. The quantum contribution in the crossover regime is also calculated. The induced interaction between impurities (i.e., polarons) is attractive and leads to the formation of their bound state, known as bipolaron. We discuss its binding energy.

Dark solitons in quantum fluids are well-known nonlinear excitations that are usually characterized by a single length scale associated with the underlying background fluid. We show that in the presence of spin-orbit coupling and a linear Zeeman field, superfluid Fermi gases support two different types of nonlinear excitations featured by corresponding length scales related to the existence of two Fermi surfaces. Only one of these types, which occurs for finite spin-orbit coupling and a Zeeman field, survives to the topological phase transition, and is therefore capable of sustaining Majorana zero modes. At the point of the emergence of this soliton for varying Zeeman field, the associated Andreev bound states present a minigap that vanishes for practical purposes, in spite of lacking the reality condition of Majorana modes.

There is a growing interest in searching for topology in fractal dimensions with the aim of finding different properties and advantages compared to the integer dimensional case. Here we construct a local Hamiltonian on a fractal lattice whose ground state exhibits topological braiding properties. The fractal lattice is obtained from a second-generation Sierpinski carpet with Hausdorff dimension 1.89. We use local potentials to trap and exchange anyons in the model, and the numerically obtained results for the exchange statistics of the anyons are close to the ideal statistics for quasiholes in a bosonic Laughlin state at half filling. For the considered system size, the energy gap is about three times larger for the fractal lattice than for a two-dimensional square lattice, and we find that the braiding results obtained on the fractal lattice are more robust against disorder. We propose a scheme to implement both fractal lattices and our proposed local Hamiltonian with ultracold atoms in optical lattices.

The intrinsically nonlinear rotation dynamics of rigid bodies offer unprecedented ways to exploit their quantum motion. In this Letter, we devise a rotational analog of Mach-Zehnder interferometry, which allows steering symmetric rotors from fully aligned to completely antialigned. The scheme uses a superposition of four distinct orientations, emerging at the eighth of the quantum revival time, whose interference can be controlled by a weak laser pulse. We develop a semiclassical model of the effect and demonstrate that it persists even in presence of imperfections and decoherence.

We investigate photoelectron momentum distribution (PMD) from strong-field ionization of Ar by a polarization-gated (PG) pulse constructed by a left circularly polarized and a time-delayed right circularly polarized carrier-envelope phase stabilized few-cycle laser pulse. We experimentally demonstrate the feasibility of using a PG pulse to precisely control the interferences between electron wave packets (EWPs) released at different times within one optical cycle. In contrast to previous works where the various types of intracycle interferences are usually hardly resolved and separated, our work shows that, with this specific PG control scheme, it becomes possible to create a highly distinct interference pattern composed of well-separated structures produced by different types of intracycle interferences in the measured PMD. Quantum trajectory Monte Carlo simulations reproduce well the experimental findings and uncover the physical mechanism behind this subcycle control of EWPs with a PG pulse.

We analyze the structure of the space of temporal correlations generated by quantum systems. We show that the temporal correlation space under dimension constraints can be nonconvex. For the general case, we provide the necessary and sufficient dimension of a quantum system needed to generate a convex correlation space for a given scenario. We further prove that this dimension coincides with the dimension necessary to generate any point in the temporal correlation polytope. As an application of our results, we derive nonlinear inequalities to witness the nonconvexity for qubits and qutrits in the simplest scenario, and present an algorithm which can help to find the minimum for a certain type of nonlinear expressions under dimension constraints.

Applying the semiclassical model, we identify two connected effects in intraband high-order harmonic generation (HHG): (1) propagation time from the point of emission at the source to the point of detection, and (2) beyond-electric-dipole corrections to the light-matter interaction. These effects inherit information regarding the dispersion and cause specific features in the spectra including even-ordered harmonics in systems with space- and time-inversion symmetry. They can for certain experimental geometries be measured free of the dipole background.

We use a convolutional neural network to retrieve the internuclear distance in the two-dimensional ${\mathrm{H}}_2^{+}$ molecule ionized by a strong few-cycle laser pulse based on the photoelectron momentum distribution. We show that a neural network trained on a relatively small dataset consisting of a few thousand images can predict the internuclear distance with an absolute error less than $0.1$ a.u. Deep learning allows us to retrieve more than one parameter from a given momentum distribution. Specifically, we used a convolutional neural network to retrieve both the internuclear distance and the laser intensity. We study the effect of focal averaging, and we find that the convolutional neural network trained using the focal averaged electron momentum distributions also shows a good performance in reconstructing the internuclear distance.

Collective excitations in regular structures of strongly interacting driven dipoles have been found to lead to strongly antibunched statistics of the emitted photons $[g^{\left(2\right)}\left(0\right)\ll 0.5]$, but precise control of the number of emitters and their spatial distribution is in general a complex experimental task. We overcome this limitation by showing quantum light emission from densely packed driven dipoles does not require their arrangement in any specific structure, pointing strongly towards possible single-photon sources based on hot thermal vapors confined within nanocells, or on irregular quantum dot ensembles.

The gas-phase ion-neutral reaction of ${\mathrm{CCl}}^{+}$ $+$ benzene $\left({\mathrm{C}}_6{\mathrm{H}}_6\right)$ is investigated using a linear Paul ion trap coupled to a time-of-flight mass spectrometer. Low collision energies are achieved by sympathetically cooling ${\mathrm{CCl}}^{+}$ reactant ions with cotrapped laser-cooled ${\mathrm{Ca}}^{+}$. The observed products include the astrochemically relevant carbocations ${\mathrm{C}}_7{\mathrm{H}}_5^{+}$, ${\mathrm{C}}_5{\mathrm{H}}_3^{+}$, and ${\mathrm{C}}_3{\mathrm{H}}_3^{+}$, as well as ${\mathrm{C}}_3{\mathrm{H}}_2{\mathrm{Cl}}^{+}$. Branching ratios of these products are measured, and ${\mathrm{C}}_7{\mathrm{H}}_5^{+}$, a carbon-growth species, is favored. Complementary electronic structure calculations provide thermodynamic limits for the reaction and allow for assignment of reaction products to specific structural isomers. Only one exoergic isomer is identified for each observed product with the exception of ${\mathrm{C}}_7{\mathrm{H}}_5^{+}$ where many identified structural isomers are exoergic. The results from this Letter broaden our understanding of the reactivity and possible role of ${\mathrm{CCl}}^{+}$ and ${\mathrm{C}}_6{\mathrm{H}}_6$ in interstellar chemistry. Furthermore, this Letter provides insight into a potential pathway to larger carbocations that may be precursors to more complex polycyclic aromatic hydrocarbons.

In the demonstrations of slow light in photorefractive (PR) materials, the delays are only reported for long pulse durations (ms to s) that are in discrepancy with typical ns-pulse length in data processing applications. Here we demonstrate that the slowdown of light in a PR crystal can be also achieved in the nanosecond regime at room temperature. We experimentally show that the use of a pulsed laser with a high peak intensity may indeed reduce the ${\mathrm{Sn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ crystal response time to a scale of ns and achieve a two-wave mixing (TWM) gain of $7.9{\text{cm}}^{-1}$. We show the slowdown of light pulses whose duration ranges from 10 to 100 ns with a fractional delay up to 0.3. These results are similar to those obtained in optical fibers, except that the slow light with PR TWM is achieved through smaller propagation lengths and at wide wavelength ranges. This could reveal potential applications of PR slow light systems and have a significant impact for future optical communications systems.

When three particles in three dimensions interact with a short-range potential fine-tuned to an infinite scattering length, they form an infinite sequence of loosely bound states obeying discrete scale invariance known as Efimov states. Here we show that analogous states are formed by three charged particles carrying two equal charges and one opposite charge in one, two, and three dimensions without any fine-tuning. Our finding is based on the Born-Oppenheimer approximation, where an effective inverse-square attraction is induced as a consequence of the dipole-charge interaction between a hydrogenlike heavy-light atom and a far-separated heavy particle. Because the resulting Efimovian states emerge toward the second or higher dissociation threshold, they are to be realized as quasibound states and may be observed by exciting hydrogen molecular ions and trions in excitonic systems. We also consider the same system but with a logarithmic Coulomb potential relevant to quantum vortices in two-dimensional superfluids, where the Efimovian states are shown to emerge as genuine bound states toward the first dissociation threshold.

We present a supervised learning model to calibrate the photon collection rate during the fluorescence imaging of cold atoms. The linear regression model finds the collection rate at each location on the sensor such that the atomic population difference equals that of a highly precise optical cavity measurement. This 192 variable regression results in a measurement variance 27% smaller than our previous single variable regression calibration. The measurement variance is now in agreement with the theoretical limit due to other known noise sources. This model efficiently trains in less than a minute on a standard personal computer's CPU and requires less than 10 min of data collection. Furthermore, the model is applicable across a large change in population difference and across data collected on different days.

We analyze the dynamics of one-dimensional quantum gases with strongly attractive contact interactions. We specify a class of initial states for which attractive forces effectively act as strongly repulsive ones during the time evolution. Our findings extend the theoretical results on the super-Tonks-Girardeau gas to a highly nonequilibrium dynamics. The mechanism is illustrated on the prototypical problem of the domain stability in a two-component Fermi gas. We also discuss finite-range interactions and analyze the universality of the presented results. Moreover, we use our conclusions to argue for the existence of metastable quantum droplets in the regime of strongly attractive contact and attractive dipolar interactions.

Non-Hermitian systems with parity-time reversal ($\mathcal{PT}$) or anti-$\mathcal{PT}$ symmetry have attracted a wide range of interest owing to their unique characteristics and counterintuitive phenomena. One of the most extraordinary features is the presence of an exceptional point, across which a phase transition with spontaneously broken $\mathcal{PT}$ symmetry takes place. We implement a Floquet Hamiltonian of a single qubit with anti-$\mathcal{PT}$ symmetry by periodically driving a dissipative quantum system of a single trapped ion. With stroboscopic emission and quantum state tomography, we obtain the time evolution of density matrix for an arbitrary initial state, and directly demonstrate information retrieval, eigenstate coalescence, and topological energy spectra as unique features of non-Hermitian systems.

We demonstrate real-time magnetometry by detecting fluorescence from a nitrogen-vacancy center in the setting of coherent population trapping and by estimating the magnetic field from the time series of the observed photon counts, which are correlated with the underlying field variation. The proof-of-principle experiment uses an external time-varying magnetic field that follows an Ornstein-Uhlenbeck (OU) process. A conventional estimator based on the use of average photon counts per relevant time interval is susceptible to photon number shot noise and performs poorly at relatively low average photon counts. In comparison, by taking into consideration the statistical properties of the OU process, a Bayesian inference-based estimator can effectively update dynamical information of the field in real time with the detection of just a single photon.

Temporal cavity solitons in ring microresonators provide broad and controllable generation of frequency combs with applications in frequency standards and precise atomic clocks. Three-level media in the $\mathrm{\Lambda }$ configuration inside microresonators displaying electromagnetically induced transparency can be used for the generation of temporal cavity solitons and frequency combs in the presence of anomalous dispersion and two external driving fields close to resonance. Here domain walls separating regions of two dark states due to quantum interference correspond to realizations of stimulated Raman adiabatic passage without input pulses. With no need of modulational instabilities, bright temporal cavity solitons and frequency combs are formed when these domain walls lock with each other. Wide stability ranges, close to resonance operation, and the optimal shape of the cavity solitons due to three-level quantum interference can make them preferable to those in two-level media.

The state of a quantum system may be steered towards a predesignated target state, employing a sequence of weak blind measurements (where the detector's readouts are traced out). Here we analyze the steering of a two-level system using the interplay of a system Hamiltonian and weak measurements and show that any pure or mixed state can be targeted. We show that the optimization of such a steering protocol is underlain by the presence of Liouvillian exceptional points. More specifically, for high-purity target states, optimal steering implies purely relaxational dynamics marked by a second-order exceptional point, whereas for low-purity target states, it implies an oscillatory approach to the target state. The dynamical phase transition between these two regimes is characterized by a third-order exceptional point.