A nonrelativistic variant of the quantum theory for the resonant inelastic-scattering process of two x-ray photons in the deep $1s$-shell ionization threshold energy region of a free many-electron atom is established. A quantitative estimate for the magnitude of the observed differential cross section is given.

We report on the realization of a mixture of fermionic ${}^{161}\mathrm{Dy}$ and fermionic ${}^{40}\mathrm{K}$ where both species are deep in the quantum-degenerate regime. Both components are spin polarized in their absolute ground states, and the low temperatures are achieved by means of evaporative cooling of the dipolar dysprosium atoms together with sympathetic cooling of the potassium atoms. We describe the trapping and cooling methods, in particular the final evaporation stage, which leads to Fermi degeneracy of both species. Analyzing cross-species thermalization we obtain an estimate of the magnitude of the interspecies $s$-wave scattering length at low magnetic field. We demonstrate magnetic levitation of the mixture as a tool to ensure spatial overlap of the two components. The properties of the Dy-K mixture make it a very promising candidate to explore the physics of strongly interacting mass-imbalanced Fermi-Fermi mixtures.

We present a strategy based on two-dimensional arrays of coupled linear optical resonators to investigate the two-body physics of interacting bosons in one-dimensional lattices. In particular, we want to address the bound pairs in topologically nontrivial Su-Schrieffer-Heeger arrays. Taking advantage of the driven-dissipative nature of the resonators, we propose spectroscopic protocols to detect and tomographically characterize bulk doublon bands and doublon edge states from the spatially resolved transmission spectra, and to highlight Feshbach resonance effects in two-body collision processes. We discuss the experimental feasibility using state-of-the-art devices, with a specific eye on arrays of semiconductor micropillar cavities.

Wave-particle duality is the most essential nature of light, and the fact that the detection apparatus determines the observed wave or particle behavior is one of the most counterintuitive features of quantum mechanics. Recent progress in this direction has revealed novel and peculiar phenomena of photonic wave-particle morphing and wave-particle superposition, which provide a deep understanding of the wave-particle duality for photons. However, it remains an open question whether this new conception can be extended to a more complex physical system, such as an atomic system. We here report an experimental realization of the wave-particle superposition of distinct atomic spin excitations in a cold atomic ensemble by using multiplexed quantum memory. The quantum behavior of wave-particle superposition for matter is observed and verified. Our scheme further provides a benchmark understanding of the wave-particle duality in matter systems.

The detuning-induced stimulated Raman adiabatic passage (D-STIRAP) in two-level systems with permanent dipole moments (PDMs) is investigated. Based on a transformed Schrödinger equation, we discuss the coherence generation under the one- and two-photon excitations, respectively. In the one-photon excitation, our analysis shows that the envelope of the pump pulse gets modulated in presence of the PDMs. The modulation becomes stronger while the difference between the PDMs of the two levels becomes larger. Gradually, the adiabatic process of the D-STIRAP fails to complete and the robustness of the technique disappears. Therefore, parameters have to be chosen with care if maximum coherence is desired in systems with large or giant difference between the PDMs. Similar results can also be found in the two-photon excitation, although the influence of the PDMs on the D-STIRAP seems weaker compared with that in the one-photon excitation case. Finally, we point out that the negative role of the PDMs can be effectively avoided by simply choosing longer pulses instead and the D-STIRAP can be safely applied in two-level systems with PDMs.

Single-mode or mode multiplexed free-space atmospheric optical channels have drawn increasingly more attention in the past decade. The scope of their possible applications spans from the compatibility with the telecom WDM technology, fiber amplifiers, and modal multiplexing for increasing the channel throughput to various quantum communication related primitives such as entanglement distribution, high-dimensional spatially encoded quantum key distribution, and relativistic quantum cryptography. Many research papers discuss application of specific mode sets, such as optical angular momentum modes, for communication in the presence of atmospheric turbulence. At the same time some basic properties and key relations for such channels exposed to the atmospheric turbulence have not been derived yet. In the current paper we present simple analytic expressions and a general framework for assessing probability density functions of channel transmittance as well as modal cross-talk coefficients. Under some basic assumptions the presented results can be directly used for estimation of the Fried parameter of the turbulent channel based on the measured statistics of the fundamental mode transmittance coefficient.

Lamata et al. have presented an entanglement classification of four qubits under stochastic local operation and classical communication (SLOCC) based on an inductive approach [Phys. Rev. A 75, 022318 (2007)]. While it works well for three qubits, we show that some of the families for four qubits have overlaps and hence this approach is not valid for entanglement classification of four or more qubits under SLOCC.

Bell's theorem shows that local measurements on entangled states give rise to correlations incompatible with local-hidden-variable models. The degree of quantum nonlocality is not maximal though, as there are even more nonlocal theories beyond quantum theory still compatible with the nonsignaling principle. In spite of decades of research, we still have a very fragmented picture of the whole geometry of these different sets of correlations. Here we employ both analytical and numerical tools to ameliorate that. First, we identify two different classes of Bell scenarios where the nonsignaling correlations can behave very differently: In one case, the correlations are generically quantum and nonlocal while in the other quite the opposite happens as the correlations are generically classical and local. Second, by randomly sampling over nonsignaling correlations, we compute the distribution of a nonlocality quantifier based on the trace distance to the local set. With that we conclude that the nonlocal correlations can show a concentration phenomenon: Their distribution is peaked at a distance from the local set that increases both with the number of parts or measurements being performed.

In 1985, Leggett and Garg formulated a class of inequalities for testing the compatibility between macrorealism and quantum mechanics. In this paper, we point out that, based on the same assumptions of macrorealism that are used to derive Leggett-Garg inequalities (LGIs), there is a scope of formulating another class of inequalities different from the standard LGIs. By considering the three-time measurement scenario in a dichotomic system, we first propose an interesting variant of the standard LGIs and show that its quantum violation is larger than the standard LGIs. By extending this formulation to the $n$-time measurement scenario, we found that the quantum violations of variants of the LGIs for a qubit system increase with $n$, and, for a sufficiently large $n$, an algebraic maximum can be reached. We then compare the violations of the standard and variants of the LGIs in an unsharp measurement scenario and show that, for any arbitrary $n$, the violation of the latter is more robust to unsharpness than the former. Furthermore, we examine the relation between the quantum violations of the variants of the LGIs and an another formulation macrorealism, known as no-signaling in time conditions.

Quantum circuit Born machines are generative models which represent the probability distribution of classical dataset as quantum pure states. Computational complexity considerations of the quantum sampling problem suggest that the quantum circuits exhibit stronger expressibility compared to classical neural networks. One can efficiently draw samples from the quantum circuits via projective measurements on qubits. However, similar to the leading implicit generative models in deep learning, such as the generative adversarial networks, the quantum circuits cannot provide the likelihood of the generated samples, which poses a challenge to the training. We devise an efficient gradient-based learning algorithm for the quantum circuit Born machine by minimizing the kerneled maximum mean discrepancy loss. We simulated generative modeling of the BARS-AND-STRIPES dataset and Gaussian mixture distributions using deep quantum circuits. Our experiments show the importance of circuit depth and the gradient-based optimization algorithm. The proposed learning algorithm is runnable on near-term quantum device and can exhibit quantum advantages for probabilistic generative modeling.

Total solar eclipses, as the recent one seen across North America, are rare opportunities for optical spectroscopy of the corona. In view of the dearth of accurate rest-frame wavelength data, we measured 11 of the strongest optical coronal lines belonging to Fe X-XIV thereby proving the existence of the Fe XII line at 290.385(8) nm. Four lines, such as the green coronal line at 530.28113(13) nm, were measured with unprecedented precision, allowing in principle for absolute velocity determinations of plasmas with uncertainties of 0.08 km ${\mathrm{s}}^{-1}$. These results furthermore stringently benchmark the theory of complex open-$3p$-shell ions.

Ionization and electron capture in collisions of bare carbon ions with atomic hydrogen has been studied using the wave-packet continuum discretization approach. The three-body Schrödinger equation governing the collision process is solved using the two-center expansion of the total scattering wave function. Calculations have been performed for the projectile energy range from 1 keV/amu to 10 MeV/amu. While there is excellent agreement with experimental data for the total electron-capture cross section over the entire energy range, the calculated total ionization cross section slightly overestimates the only available measured point. The singly and doubly differential ionization cross sections at 1 and 2.5 MeV/amu are in good agreement with experiment. The differential cross section calculations are extended to lower energies where perturbative methods are expected to fail. At 100 keV/amu impact energy the present singly differential cross section in the ejected angle of the electron shows a pronounced peak in the forward direction. It is concluded that at low incident energies electron capture into the continuum of the projectile strongly enhances electron ejection in the forward direction.

We employ mixed quantum-classical molecular dynamics simulations to investigate the fragmentation of ${\mathrm{N}}_2$ molecules after core-level photoionization by an x-ray laser, subsequent Auger decay, and followed by a femtosecond IR pulse that interacts with ${{\mathrm{N}}_2}^{2+}$. The delayed IR pulse couples the dissociative electronic states of ${{\mathrm{N}}_2}^{2+}$ with electronic states that can support long-lived vibrational resonances. We compare our simulations with previous quantum dynamics calculations in a quasidiabatic representation, which employed a small number of electronic states. Good agreement for both the Auger spectrum as well as the influence of the delayed IR pulse is found. By employing the mixed quantum-classical treatment, we can greatly reduce the computational cost to simulate the fragmentation dynamics compared to the quantum dynamics simulations. Furthermore, we reinvestigate the title process by employing an extended set of adiabatic potential energy surfaces and also investigate the role of nonadiabatic coupling in the process. The use of the full set of adiabatic potentials increases the dissociation probability and changes the details of the interaction with the IR pulse, but no effect due to the nonadiabatic coupling is found.

Optically induced collective conduction-electron oscillations can generate intense plasmonic fields near metallic nanoparticles. We suggest a method for reconstructing such nanoplasmonic fields with nanometer spatial and sub-femtosecond temporal resolution from streaked photoemission spectra. Applying this imaging scheme to Au nanospheres, we demonstrate the accurate spatiotemporal reconstruction of the plasmonic near-field distribution in comparison with the directly calculated plasmonic field.

We propose a set of schemes to create and probe fractionally charged excitations of a fractional Chern insulator state in an optical lattice. This includes the creation of localized quasiparticles and quasiholes using both static local defects and the dynamical local insertion of synthetic flux quanta. Simulations of repulsively interacting bosons on a finite square lattice with experimentally relevant open boundary conditions show that already a four-particle system exhibits signatures of charge fractionalization in the quantum-Hall-like state at the filling fraction of $1/2$ particle per flux quantum. This result is favorable for the prospects of adiabatic preparation of fractional Chern insulators. Our work is inspired by recent experimental breakthroughs in atomic quantum gases: the realization of strong artificial magnetic fields in optical lattices, the ability of single-site addressing in quantum gas microscopes, and the preparation of low-entropy insulating states by engineering an entropy-absorbing metallic reservoir.

We study a special dynamical regime of a Bose-Einstein condensate in a ring-shaped lattice where the populations in each site remain constant during the time evolution. The states in this regime are characterized by equal occupation numbers in alternate wells and nontrivial phases, while the phase differences between neighboring sites evolve in time yielding persistent currents that oscillate around the lattice. We show that the velocity circulation around the ring lattice alternates between two values determined by the number of wells and with a specific time period that is only driven by the on-site interaction energy parameter. In contrast to the self-trapping regime present in optical lattices, the occupation number at each site does not show any oscillation and the particle imbalance does not possess a lower bound for the phenomenon to occur. These findings are predicted with a multimode model and confirmed by full three-dimensional Gross-Pitaevskii simulations using an effective on-site interaction energy parameter.

Strong particle-plasmon interaction in electronic systems can lead to composite hole-plasmon excitations. We investigate the emergence of similar composite quasiparticles in ultracold dipolar Fermi liquids originating from the long-range dipole-dipole interaction. We use the ${\text{G}}_0\text{W}$ technique with an effective interaction obtained from the static structure factor to calculate the quasiparticle properties and single-particle spectral function. We first demonstrate that within this formalism a very good agreement with the quantum Monte Carlo results could be achieved over a wide range of coupling strengths for the renormalization constant and effective mass. The composite quasiparticle-zero sound excitations which are undamped at long wavelengths emerge at intermediate and strong couplings in the spectral function and should be detectable through the radio frequency spectroscopy of nonreactive polar molecules at high densities.

Interesting experimental signatures of quantum cavity optomechanics arise because the quantum back-action induces correlations between incident quantum shot noise and the cavity field. While the quantum linear theory of optomechanics (QLT) has provided vital understanding across many experimental platforms, in certain new setups it may be insufficient: analysis in the time domain may be needed, but QLT obtains only spectra in frequency space; and nonlinear behavior may be present. Direct solution of the stochastic equations of motion in time is an alternative, but unfortunately standard methods do not preserve the important optomechanical correlations. We introduce two-timescale stochastic Langevin (T2SL) propagation as an efficient and straightforward method to obtain time traces with the correct correlations. We show that T2SL, in contrast to standard stochastic simulations, can efficiently simulate correlation phenomena such as ponderomotive squeezing and reproduces accurately cavity sideband structures on the scale of the applied quantum noise and even complex features entirely submerged below the quantum shot noise imprecision floor. We investigate nonlinear regimes and find that, where comparison is possible, the method agrees with analytical results obtained with master equations at low temperatures and in perturbative regimes.

We propose an alternative formulation of the sensor method presented in Phys. Rev. Lett. 109, 183601 (2012) for the calculation of frequency-filtered and time-resolved photon correlations. Our approach is based on an algebraic expansion of the joint steady state of quantum emitter and sensors with respect to the emitter-sensor coupling parameter $\epsilon$. This allows us to express photon correlations in terms of the open quantum dynamics of the emitting system only and ensures that computation of correlations are independent on the choice of a small value of $\epsilon$. Moreover, using time-dependent perturbation theory, we are able to express the frequency- and time-resolved second-order photon correlation as the addition of three components, each of which gives insight into the physical processes dominating the correlation at different time scales. We consider a bioinspired vibronic dimer model to illustrate the agreement between the original formulation and our approach.

It has been shown that any generalized measurement can be decomposed into a sequence of weak measurements corresponding to a stochastic process. However, the weak measurements may require almost arbitrary unitaries, which are unlikely to be realized by any real measurement device. Furthermore, many measurement processes are destructive, like photon counting procedures that terminate once all photons are consumed. One cannot expect to have full control of the evolution of a state under such destructive measurements, and the possible unitaries allow only a limited set of weak measurements. In this paper, we consider a qubit model of destructive weak measurements, which is a toy version of an optical cavity, in which the state of an electromagnetic field mode inside the cavity leaks out and is measured destructively while the vacuum state $|0\rangle$ leaks into the cavity. At long times, the state of the qubit inevitably evolves to be $|0\rangle$, and the only available control is the choice of measurement on the external ancilla system. Surprisingly, this very limited model can still perform an arbitrary projective measurement on the qubit in any basis, where the probability of getting an outcome satisfies the usual Born rule. Combining this method with probabilistic postprocessing, the result can be extended to any generalized measurement with commuting positive-operator-valued measurement (POVM) elements. This implies, among other results, that any two-outcome POVM on a qubit can be decomposed into a sequence of destructive weak measurements by this restricted measurement device.