Understanding electron-electron correlations in matter ranging from atoms to solids represents a grand challenge for both experiment and theory. These correlations occur on attosecond timescales and have only recently become experimentally accessible. In the case of highly excited systems, the task of understanding and probing correlated interactions is even greater. In this work, we combine state-of-the-art light sources and advanced detection techniques with ab initio calculations to unravel the role of electron-electron correlation in ${\mathrm{D}}_2$ photoionization by mapping the dissociation of a highly excited ${{\mathrm{D}}_2}^{+}$ molecule. Correlations between the two electrons dictate the pathways along which the molecule dissociates and lead to a superposition of excited ionic states. Using 3D Coulomb explosion imaging and electron-ion coincidence techniques, we assess the relative contribution of competing parent ion states to the dissociation process for different orientations of the molecule with respect to the laser polarization, which is consistent with a shake-up ionization process. As a step toward observing coherent superposition experimentally, we map the relevant nuclear potentials using Coulomb explosion imaging and show theoretically that such an experiment could confirm this coherence via two-path interference.

Attosecond interferometry relies on the application of a train of attosecond extreme-ultraviolet (XUV) pulses and a synchronized low-intensity infrared (IR) field to measure the dynamics of photoemission via the interference signal in the spectrum of emitted electrons. So far, all condensed-phase experiments relying on this technique have investigated metallic, i.e. nontransparent, systems and have revealed the importance of transport and final-state effects. Here, we demonstrate the existence of a nonlocal mechanism of attosecond interferometry general to all condensed-matter systems and particularly important for those that are transparent to the IR. Key to the process is that, after XUV absorption, additional emission pathways result from the absorption or emission of an IR photon at remote positions due to the potential of neighboring atoms (or molecules). By solving the time-dependent Schrödinger equation (TDSE) in one dimension, we show that interference of the resulting local and remote pathways leads to a mapping of the atomic or molecular environment onto the attosecond interference signal. We derive an analytical theory for the local and remote pathways as well as the resulting response and find excellent agreement with the TDSE. Our analytical theory shows that the nonlocal mechanism generally encodes both mean-free paths and scattering delays into the experimental observables. We generalize the analytical theory to the case of multiple collisions and study the effect of path-length distributions typical of electron scattering in condensed matter.

We study the control of ionization dynamics of aligned ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ molecules in orthogonally polarized two-color (OTC) fields and calculate the photoelectron momentum distributions of molecules by solving the time-dependent Schrödinger equation and with a semiclassical model. In contrast with that of atoms in the OTC field, we show that the photoelectron momentum distribution is essentially dependent on the molecular outmost orbital, the alignment angle, and the phase of the OTC field. We illustrate the controlling of the molecular frame photoelectron angular distributions and the temporal and spatial double-slit interference effect for strong-field ionization of ${\mathrm{N}}_2$ molecules. For ${\mathrm{O}}_2$ molecules, it is shown that the ionization wave packets are splitting. The temporal and spatial double-slit interference effect is always present for ${\mathrm{O}}_2$, which is not able to be turned off using the OTC field. This work would have implications for probing molecular structures with a two-color synthesized laser field.

We investigate the nonlinear propagation of multiterawatt femtosecond laser pulses at 800 nm wavelength in air, under different external focusing conditions. We profile the laser beam in the vicinity of the nonlinear focus using a technique based on the dependence of the single-shot ablation threshold for gold on the angle of incidence of the laser beam on the sample. Under very tight focusing conditions ($\mathit{f}$ number $\sim 15$) we observe the propagation regime reminiscent of the nanosecond optical breakdown. No clear individual filaments are formed across the beam, and the estimated peak intensity surges to at least $200\mathrm{TW}/{\mathrm{cm}}^2$. As the external focusing is loosened to $\mathit{f}$ number $\sim 125$, we observe the transition to the multifilamentation regime. Distinct individual filaments are formed before the linear focus while the peak intensity reaches $\sim 80\mathrm{TW}/{\mathrm{cm}}^2$. Once formed, the filaments do not coalesce into a single or few superfilaments as they pass through the focus zone. Our experimental observations are supported by numerical simulations.

In contrast to the ordinary spin Hall effect (SHE), which is a single-body phenomenon caused by the spin-orbit interaction (SOI), we propose a many-body SHE induced by the dipole-dipole interaction (DDI) between particles and demonstrate it in a system of ultracold magnetic atoms. While the SOI usually requires the breaking of space-inversion symmetry, the DDI preserves it. The many-body SHE can, in principle, be observed in a wide range of systems with large dipole moments and offers a powerful tool to generate spin currents, an essential ingredient in spintronics and atomtronics.

A theory of random-matrix bases is presented, including expressions for orthogonality, completeness, and the random-matrix synthesis of arbitrary matrices. This is applied to ghost imaging as the realization of a random-basis reconstruction, including an expression for the resulting signal-to-noise ratio. Analysis of conventional direct imaging and ghost imaging leads to a criterion which, when satisfied, implies a reduced dose for computational ghost imaging. We also propose an experiment for x-ray phase contrast computational ghost imaging, which enables differential phase contrast to be achieved in an x-ray ghost-imaging context. We give a numerically robust solution to the associated inverse problem of decoding differential phase contrast x-ray ghost images, to yield a quantitative map of the projected thickness of the sample.

We investigate the gauge invariance of the dynamic (ac) Stark shift under “hybrid” gauge transformations from the “length” ($\vec{E}·\vec{r}$) to the “velocity” ($\vec{A}·\vec{p}$) gauge. By a “hybrid” gauge transformation, we understand a transformation in which the scalar and vector potentials are modified, but the wave function remains unaltered. The gauge invariance of the leading term is well known, while we here show that gauge invariance under perturbations holds only if one takes into account an additional correction to the transition current, which persists only in the velocity gauge. We find a general expression for this current, and apply the formalism to radiative and relativistic corrections to the dynamic Stark effect, which is described by the sum of two polarizability matrix elements.

The realization of high-fidelity quantum gates in a multiqubit system, with a typical target set at $99.9\%$, is a critical requirement for the implementation of fault-tolerant quantum computation. To reach this level of fidelity, one needs to carefully analyze the noises and imperfections in the experimental system and optimize the gate operations to mitigate their effects. Here, we consider one of the leading experimental systems for the fault-tolerant quantum computation, ions in an anharmonic linear Paul trap, and optimize entangling quantum gates using segmented laser pulses with the assistance of all the collective transverse phonon modes of the ion crystal. We present detailed analyses of the effects of various kinds of intrinsic experimental noises as well as errors from imperfect experimental controls. Through explicit calculations, we find the requirements on these relevant noise levels and control precisions to achieve the targeted high fidelity of $99.9\%$ for the entangling quantum gates in a multi-ion crystal.

We introduce several classes of quantum combinatorial designs, namely quantum Latin squares, cubes, hypercubes, and a notion of orthogonality between them. A further introduced notion, quantum orthogonal arrays, generalizes all previous classes of designs. We show that mutually orthogonal quantum Latin arrangements can be entangled in the same way in which quantum states are entangled. Furthermore, we show that such designs naturally define a remarkable class of genuinely multipartite highly entangled states called $k$-uniform, i.e., multipartite pure states such that every reduction to $k$ parties is maximally mixed. We derive infinitely many classes of mutually orthogonal quantum Latin arrangements and quantum orthogonal arrays having an arbitrary large number of columns. The corresponding multipartite $k$-uniform states exhibit a high persistency of entanglement, which makes them ideal candidates to develop multipartite quantum information protocols.

We introduce a general method for the construction of quasiprobability representations for arbitrary notions of quantum coherence. Our technique yields a non-negative probability distribution for the decomposition of any classical state. Conversely, quantum phenomena are certified in terms of signed distributions, i.e., quasiprobabilities, and a residual component unaccessible via classical states. Our unifying method combines well-established concepts, such as phase-space distributions in quantum optics, with resources of quantumness relevant for quantum technologies. We apply our approach to analyze various forms of quantum coherence in different physical systems. Moreover, our framework renders it possible to uncover complex quantum correlations between systems, for example, via quasiprobability representations of multipartite entanglement.

The impact of lossy multilayer structures on nonequilibrium atom-surface interactions is discussed. Specifically, the focus lies on a fully non-Markovian and nonequilibrium description of quantum friction, the fluctuation-induced drag force acting on an atom moving at constant velocity and height above the multilayer structures. Compared to unstructured bulk material, the drag force for multilayer systems is considerably enhanced and its behavior as a function of both the atom's velocity and distance from the surface exhibits different regimes. These features are linked to the appearance of coupled interface polaritons within the superlattice structures. Our results are not only useful for an experimental investigation of quantum friction but also highlight a way to tailor the interaction by simply modifying the structural composition of the multilayer systems.

We report the production of ultracold heteronuclear ${\mathrm{Cs}}^{*}\mathrm{Yb}$ molecules through one-photon photoassociation applied to an ultracold atomic mixture of Cs and Yb confined in an optical dipole trap. We use trap-loss spectroscopy to detect molecular states below the $\mathrm{Cs}\left({}^{2}P_{1/2}\right)+\mathrm{Yb}\left({}^{1}S_0\right)$ asymptote. For ${}^{133}\mathrm{Cs}{}^{174}\mathrm{Yb}$, we observe 13 rovibrational states with binding energies up to $\sim 500\mathrm{GHz}$. For each rovibrational state we observe two resonances associated with the Cs hyperfine structure and show that the hyperfine splitting in the diatomic molecule decreases for more deeply bound states. In addition, we produce ultracold fermionic ${}^{133}\mathrm{Cs}{}^{173}\mathrm{Yb}$ and bosonic ${}^{133}\mathrm{Cs}{}^{172}\mathrm{Yb}$ and ${}^{133}\mathrm{Cs}{}^{170}\mathrm{Yb}$ molecules. From mass scaling, we determine the number of vibrational levels supported by the 2(1/2) excited-state potential to be 154 or 155.

We study the lowest energy states for fixed total momentum, i.e., yrast states, of $N$ bosons moving on a ring. As in the paper of Syrwid and Sacha [Phys. Rev. A 92, 032110 (2015)], we compare mean-field solitons with the yrast states, being the many-body Lieb-Liniger eigenstates. We show that even in the limit of vanishing interaction the yrast states possess features typical for solitons, like phase jumps and density notches. These properties are simply effects of the bosonic symmetrization and are encoded in the Dicke states hidden in the yrast states.

We demonstrate laser cooling and magneto-optical trapping of europium. The atoms are optically pumped to a metastable state and then loaded from an atomic-beam source via conventional Zeeman slowing and magneto-optical trapping techniques using a $J=13/2\leftrightarrow J=15/2$ quasicyclic transition. The trapped populations contained up to $1\times {10}^7$ atoms, and a two-body loss rate is estimated as $1\times {10}^{-10}{\mathrm{cm}}^3/\mathrm{s}$ from the nonexponential loss of atoms at high densities. We also observed leakage out of the quasicyclic transition to the two metastable states with $J=9/2$ and $11/2$, which is adequate to pump the laser-cooled atoms back to the $J=7/2$ ground state.

Light scattering in dense media is a fundamental problem of many-body physics, which is also relevant for the development of optical devices. In this work we investigate experimentally light propagation in a dense sample of randomly positioned resonant scatterers confined in a layer of subwavelength thickness. We locally illuminate the atomic cloud and monitor spatially resolved fluorescence away from the excitation region. We show that light spreading is well described by a diffusion process, involving many scattering events in the dense regime. For light detuned from resonance we find evidence that the atomic layer behaves as a graded-index planar waveguide. These features are reproduced by a simple geometrical model and numerical simulations of coupled dipoles.

According to the world view of macrorealism, the properties of a given system exist prior to and independent of measurement, which is incompatible with quantum mechanics. Leggett and Garg put forward a practical criterion capable of identifying violations of macrorealism, and so far experiments performed on microscopic and mesoscopic systems have always agreed with quantum mechanics. However, a macrorealist can always assign the cause of such violations to the perturbation that measurements effect on such small systems, and hence a definitive test would require using noninvasive measurements, preferably on macroscopic objects, where such measurements seem more plausible. However, the generation of truly macroscopic quantum superposition states capable of violating macrorealism remains a big challenge. In this work we propose a setup that makes use of measurements on the polarization of light, a property that has been extensively manipulated both in classical and quantum contexts, hence establishing the perfect link between the microscopic and macroscopic worlds. In particular, we use Leggett-Garg inequalities and the criterion of no signaling in time to study the macrorealistic character of light polarization for different kinds of measurements, in particular with different degrees of coarse graining. Our proposal is noninvasive for coherent input states by construction. We show for states with well-defined photon number in two orthogonal polarization modes, that there always exists a way of making the measurement sufficiently coarse grained so that a violation of macrorealism becomes arbitrarily small, while sufficiently sharp measurements can always lead to a significant violation.

Zero-field nuclear magnetic resonance (NMR) has been recently developed as a complementary tool for nondestructive structural investigations of matter. However, precise and efficient control in such systems is still at the beginning. Based on the controllability of spin systems, we theoretically study the issue of quantum control in zero-field NMR by using a numerical optimization method based on the gradient ascent pulse engineering (GRAPE) algorithm. A set of quantum gates, including single-qubit and two-qubit gates, are achieved as examples. In particular, we show that homonuclear spins can be individually manipulated in zero-field NMR by the GRAPE method, which extends the ability to manipulate spin systems at zero magnetic field. Quantum control realized by the numerical GRAPE method features the properties of high fidelity, robustness, and hardware-friendly characteristics. High-fidelity quantum control in zero-field NMR systems might provide promising applications in material science, chemical analysis, quantum information processing, and fundamental physics.

We aim to perform direct optical spectroscopy of the ${}^{229}\mathrm{Th}$ nuclear isomer to measure its energy and lifetime, and to demonstrate optical coupling to the nucleus. To this end, we develop ${}^{229}\mathrm{Th}$-doped ${\mathrm{CaF}}_2$ crystals, which are transparent at the anticipated isomer wavelength. Such crystals are illuminated by tunable VUV undulator radiation for direct excitation of the isomer. We scan a $\pm 5\sigma$ region around the assumed isomer energy of 7.8(5) eV and vary the excitation time in sequential scans between 30 and 600 seconds. Suffering from an unforeseen strong photoluminescence of the crystal, the experiment is sensitive only to short radiative isomer lifetimes. In our data analysis, we assume that radiative decay is the dominant de-excitation channel. Further, we suppose that the luminescence background has a super-Poissonian character and may be represented as a sum of a constant term and several decaying exponents whose time constants do not depend on the wavelength of the excitation. With these assumptions, we can exclude an isomer with lifetime between 0.2 and 1.1 s and energy between 7.5 and 10 eV at the 95% confidence level.