Antihydrogen atoms are routinely formed at the Antiproton Decelerator at CERN in a wide range of Rydberg states. To perform precision measurements, experiments rely on ground-state antimatter atoms which are currently obtained only after spontaneous decay. In order to enhance the number of atoms in ground state, we propose and assess the efficiency of different methods to stimulate their decay. First, we investigate the use of THz radiation to simultaneously couple all $n$-manifolds down to a low-lying one with sufficiently fast spontaneous emission toward ground state. We further study a deexcitation scheme relying on state mixing via microwave and/or THz light and a coupled (visible) deexcitation laser. We obtain close to unity ground-state fractions within a few tens of $\mu \mathrm{s}$ for a population initiated in the $n=30$ manifold. Finally, we study how the production of antihydrogen atoms via stimulated radiative recombination can favorably change the initial distribution of states and improve the overall number of ground-state atoms when combined with stimulated deexcitation.

We investigate the dynamics of atomic twin beams produced from a phase-fluctuating source, specifically a one-dimensional Bose gas in the quasicondensate regime, motivated by the experiment reported in Bücker et al. [Nat. Phys. 7, 608 (2011)]. A short-time analytic model is constructed, which is a modified version of the undepleted pump approximation widely used in quantum and atom optics, except that here we take into account the initial phase fluctuations of the pump source as opposed to assuming long-range phase coherence. We use this model to make quantitative and qualitative predictions of how phase fluctuations of the source impact the two-particle correlations of scattered atom pairs. The model is benchmarked against detailed numerical simulations using stochastic phase-space methods, and is shown to validate the intuitive notion that the broadening of momentum-space correlation functions between atoms scattered from a quasicondensate is driven by the broadened momentum width of the source compared to a true phase-coherent condensate. Finally, we combine these theoretical tools and results to investigate the effect phase fluctuations of the twin-beam source can have on a proposed demonstration of a violation of a Bell inequality, which intrinsically relies on phase-sensitive pair correlations.

Driven-dissipative Kerr lattices with two-photon driving are experimentally relevant systems known to exhibit a symmetry-breaking phase transition, which belongs to the universality class of the thermal Ising model for the parameter regime studied here. In this work, we perform finite-size scaling of this system as it is quenched to the transition, and the dynamical critical exponent is found to be compatible with $z\approx 2.18$, corresponding with metropolis dynamics in classical simulations. Furthermore, we show that the Liouvillian gap scales with the same exponent, similar to scaling of the Hamiltonian gap at quantum phase transitions in closed systems.

On dielectric periodic structures with a reflection symmetry in a periodic direction, there can be antisymmetric standing waves (ASWs) that are symmetry-protected bound states in the continuum (BICs). The BICs have found many applications, mainly because they give rise to resonant modes of extremely large quality factors ($Q$ factors). The ASWs are robust to symmetric perturbations of the structure, but they become resonant modes if the perturbation is nonsymmetric. The $Q$ factor of a resonant mode on a perturbed structure is typically $O(1/{\delta }^2)$, where $\delta$ is the amplitude of the perturbation, but special perturbations can produce resonant modes with larger $Q$ factors. For two-dimensional (2D) structures with a one-dimensional (1D) periodicity, we derive conditions on the perturbation profile such that the $Q$ factors are $O(1/{\delta }^4)$ or $O(1/{\delta }^6)$. For the unperturbed structure, an ASW is surrounded by resonant modes with a nonzero Bloch wave vector. For 2D structures with a 1D periodicity, the $Q$ factors of nearby resonant modes are typically $O(1/{\beta }^2)$, where $\beta$ is the Bloch wave number. We show that the $Q$ factors can be $O(1/{\beta }^6)$ if the ASW satisfies a simple condition.

We investigate operational probabilistic theories where the pure states of every system are the vertices of a simplex. A special case of such theories is that of classical theories, i.e., simplicial theories whose pure states are jointly perfectly discriminable. The usual classical theory satisfies also local discriminability. However, simplicial theories—including the classical ones—can violate local discriminability, thus admitting entangled states. First, we prove sufficient conditions for the presence of entangled states in arbitrary probabilistic theories. Then we prove that simplicial theories are necessarily causal, and this represents a no-go theorem for conceiving noncausal classical theories. We then provide necessary and sufficient conditions for simplicial theories to exhibit entanglement and classify their system-composition rules. We conclude by proving that, in simplicial theories, an operational formulation of the superposition principle cannot be satisfied, and that—under the hypothesis of $n$-local discriminability—no mixed state admits a purification. Our results hold also in the general case where the sets of states fail to be convex.

Cyclic systems of dichotomous random variables have played a prominent role in contextuality research, describing such experimental paradigms as the Klyachko-Can-Binicioğlu-Shumovsky, Einstein-Podolsky-Rosen-Bell, and Leggett-Garg ones in physics, as well as conjoint binary choices in human decision making. Here, we understand contextuality within the framework of the Contextuality-by-Default (CbD) theory, based on the notion of probabilistic couplings satisfying certain constraints. CbD allows us to drop the commonly made assumption that systems of random variables are consistently connected (i.e., it allows for all possible forms of “disturbance” or “signaling” in them). Consistently connected systems constitute a special case in which CbD essentially reduces to the conventional understanding of contextuality. We present a theoretical analysis of the degree of contextuality in cyclic systems (if they are contextual) and the degree of noncontextuality in them (if they are not). By contrast, all previously proposed measures of contextuality are confined to consistently connected systems, and most of them cannot be extended to measures of noncontextuality. Our measures of (non)contextuality are defined by the $L_1$-distance between a point representing a cyclic system and the surface of the polytope representing all possible noncontextual cyclic systems with the same single-variable marginals. We completely characterize this polytope, as well as the polytope of all possible probabilistic couplings for cyclic systems with given single-variable marginals. We establish that, in relation to the maximally tight Bell-type CbD inequality for (generally, inconsistently connected) cyclic systems, the measure of contextuality is proportional to the absolute value of the difference between its two sides. For noncontextual cyclic systems, the measure of noncontextuality is shown to be proportional to the smaller of the same difference and the $L_1$-distance to the surface of the box circumscribing the noncontextuality polytope. These simple relations, however, do not generally hold beyond the class of cyclic systems, and noncontextuality of a system does not follow from noncontextuality of its cyclic subsystems.

We present a photonic integrated circuit architecture for a quantum programmable gate array (QPGA) capable of preparing arbitrary quantum states and operators. The architecture consists of a lattice of phase-modulated Mach-Zehnder interferometers, which perform rotations on path-encoded photonic qubits, and embedded quantum emitters, which use a two-photon scattering process to implement a deterministic controlled-${\sigma }_z$ operation between adjacent qubits. By appropriately setting phase shifts within the lattice, the device can be programmed to implement any quantum circuit without hardware modifications. We provide algorithms for exactly preparing arbitrary quantum states and operators on the device, and we show that gradient-based optimization can train a simulated QPGA to automatically implement highly compact approximations to important quantum circuits with near-unity fidelity.

Controllable multipartite entanglement is a crucial element in quantum information processing. Here we present a scheme that generates switchable bipartite and genuine tripartite entanglement between microwave and optical photons via an optoelectromechanical interface, where microwave and optical cavities are coupled to a mechanical mode with controllable coupling constants. We show that by tuning an effective gauge phase between the coupling constants to the “sweet spots,” bipartite entanglement can be generated and switched between designated output photons. The bipartite entanglement is robust against the mechanical noise and the signal loss to the mechanical mode when the couplings are chosen to satisfy the impedance-matching condition. When the gauge phase is tuned away from the “sweet spots,” genuine tripartite entanglement can be generated and verified with homodyne measurement on the quadratures of the output fields. Our result can lead to the implementation of controllable and robust multipartite entanglement in hybrid quantum systems operated in distinctively different frequencies.

We study quantum information masking based on isometric linear operators that distribute the information encoded in pure states to the correlations in bipartite states. It is shown that an isometric linear operator cannot mask any nonzero measure set of pure states. We present a geometric characterization of the maskable sets and show that any maskable set must be on a spherical circle in certain Euclidean spaces. Detailed examples and potential applications such as secret sharing and quantum cryptography are analyzed.

We study theoretically the interaction between two photons in a nonlinear cavity. The photons are absorbed into the cavity by an effective tuning of its input-output coupling via external control of a coupling to a second, strongly output-coupled cavity mode. Such “dynamically coupled” cavities, which can be implemented using bulk ${\chi }^{\left(2\right)}$ and ${\chi }^{\left(3\right)}$ nonlinearities, enable incoming photon wave packets to be absorbed into the cavity with high fidelity when the duration of the control is similar to that of the wave packets. Further, this configuration can be used to avoid limitations in the photon-photon interaction time set by the delay-bandwidth product of passive cavities and enables the elimination of wave-packet distortions caused by dispersive cavity transmission and reflection. We consider three kinds of nonlinearities, two arising from ${\chi }^{\left(2\right)}$ and ${\chi }^{\left(3\right)}$ materials and one due to an interaction with a two-level emitter. To analyze the input and output of few-photon wave packets, we use a Schrödinger-picture formalism in which traveling-wave fields are discretized into infinitesimal time bins. We suggest that dynamically coupled cavities provide a very useful tool for improving the performance of quantum devices relying on cavity-enhanced light-matter interactions such as single-photon sources and atomlike quantum memories with photon interfaces. As an example, we present simulation results showing that high-fidelity two-qubit entangling gates may be constructed using any of the considered nonlinear interactions.

We create weakly bound bosonic ${}^{23}{\mathrm{Na}}^{39}\mathrm{K}$ molecules in a mixture of ultracold ${}^{23}\mathrm{Na}$ and ${}^{39}\mathrm{K}$. The creation is done in the vicinity of a so far undetected Feshbach resonance at about 196 G which we identify in this work by atom-loss spectroscopy. We investigate the involved molecular state by performing destructive radio-frequency binding-energy measurements. For the constructive molecule creation we use radio-frequency pulses with which we assemble up to 6000 molecules. We analyze the molecule creation efficiency as a function of the radio-frequency pulse duration and the atom number ratio between ${}^{23}\mathrm{Na}$ and ${}^{39}\mathrm{K}$. We find an overall optimal efficiency of $6\%$ referring to the ${}^{39}\mathrm{K}$ atom number. The measured lifetime of the molecules in the bath of trapped atoms is about 0.3 ms.

High-order harmonic generation (HHG) is a manifestation of the strongly nonlinear response of matter to intense laser fields and has, as the basis for coherent XUV sources, a variety of applications. Recently, HHG from atoms in a phase and polarization structured laser was demonstrated and interpreted based on the transverse electric-field component of the driving pulse. Here, we point out that as dictated by Maxwell equations, such fields have a longitudinal component which in general has a fundamental influence on the charge dynamics. For instance, its interplay with the transversal field component enables endowing the emitted radiation locally with circular polarization and a defined polarity. It is shown that the time-dependent Stokes parameters defining the polarization state of HHG can be tuned by varying the waist of the driving field, which in turn changes the ratio between the longitudinal and transverse electric-field components of the driving laser. In addition, employing a multipole expansion of the produced harmonics exposes the specific multipolar character and the relation to the spatial structure of the driving field polarization states. The scheme proposed here allows a full polarization control of the emitted harmonics by only one driving laser. A tighter focusing of the driving pulse renders possible the emission of harmonics with both even and odd spatial symmetry. The underlying mechanism is due to the fundamental interplay between the transverse and longitudinal components of the laser's electromagnetic vector potential. The ratio between those components is controllable by just focusing the laser spot, pointing to an accessible tool for polarization and polarity control of the high-order harmonics.

We theoretically examine the rotational and vibrational dynamics of ${\mathrm{O}}_2{}^{+}$ molecular ions exposed to intense, short laser pulses for conditions realized in contemporary pump-probe experiments. We solve the time-dependent Schrödinger equation within the Born-Oppenheimer approximation for an initial distribution of randomly aligned molecular ions. For fixed peak intensities, our numerical results show that total, angle-integrated ${\mathrm{O}}_2{}^{+}\to \mathrm{O}\left({}^{3}P\right)+\mathrm{O}{}^{+}\left({}^{4}S^0\right)$ dissociation yields do not monotonically increase with increasing infrared-probe pulse duration. We find this pulse-duration-dependent stabilization to be consistent with the transient trapping of nuclear probability density in a light-induced (bond-hardening) potential-energy surface and robust against rotational excitation. We analyze this stabilization effect and its underlying bond-hardening mechanism (i) in the time domain, by following the evolution of partial nuclear probability densities associated with the dipole-coupled ${\mathrm{O}}_2{}^{+}\left(a{}^4\mathrm{\Pi }_u\right)$ and ${\mathrm{O}}_2{}^{+}\left(f{}^4\mathrm{\Pi }_g\right)$ cationic states, and (ii) in the frequency domain, by examining rovibrational quantum-beat spectra for the evolution of the partial nuclear probability densities associated with these states. Our analysis reveals the characteristic timescale for the bond-hardening mechanism in ${\mathrm{O}}_2{}^{+}$ and explains the onset of bond stabilization for sufficiently long pulse durations.

Spin-orbit-interaction-activated interchannel coupling (SOIAIC) has been investigated theoretically in the time delay domain for $3d$ photoemission in the isoelectronic sequence ${\mathrm{I}}^{-}$, Xe, and ${\mathrm{Cs}}^{+}$ using the relativistic-random-phase approximation with relaxation. The results show that SOIAIC becomes more important with increasing nuclear charge, and that time delay is affected more strongly than cross sections or photoelectron angular distribution $\beta$ parameters.

We introduce an atomic gravimetric sequence using Raman-type composite light pulses that excites a superposition of two momentum states with the same internal level. The scheme allows the suppression of common noise, making it less sensitive to external fluctuations of electromagnetic fields. The Raman beams are generated with a fiber modulator and are capable of momentum transfer in opposite directions. We obtain analytical expressions for the interference fringes in terms of three perturbative parameters that characterize the imperfections due to undesired frequencies introduced by the modulation process. We find special values of the Rabi frequency that improve the fringes' visibility.

Topological quantum pumping, also known as topological charge pumping, represents an important quantum phenomenon that shows the fundamental connection to the topological properties of dynamical systems. Here we introduce a pumping process in a spin-dependent double-well optical lattice with glide symmetry. In the dynamic process, the glide symmetry protects the band-touching points, and topological properties of the system are characterized by the non-Abelian Berry curvature. By engineering a suitable form of coupling between different spin components, the model not only demonstrates topological phase transition but also shows hybridization between the spatial and temporal domain with topological features captured by the Wilson line along the synthetic directions. Our work provides a model based on ultracold atoms towards the implementation of versatile topological matters and topological phenomena in condensed matter systems.

On periodic structures sandwiched between two homogeneous media, a bound state in the continuum (BIC) is a guided Bloch mode with a frequency within the radiation continuum. BICs are useful, since they give rise to high quality-factor ($Q$ factor) resonances that enhance local fields for diffraction problems with given incident waves. For any BIC on a periodic structure, there is always a surrounding family of resonant modes with $Q$ factors approaching infinity. We analyze field enhancement around BICs using analytic and numerical methods. Based on a perturbation method, we show that field enhancement is proportional to the square root of the $Q$ factor, and it depends on the adjoint resonant mode and its coupling efficiency with incident waves. Numerical results are presented to show different asymptotic relations between the field enhancement and the Bloch wave vector for different BICs. Our study provides a useful guideline for applications relying on resonant enhancement of local fields.

We employ the $R$ matrix with time-dependence method to study attosecond angular streaking of ${\mathrm{F}}^{-}$. Using this negative ion, free of long-range Coulomb interactions, we elucidate the role of short-range electron correlation effects in an attoclock scheme. Through solution of the multielectron time-dependent Schrödinger equation, we aim to bridge the gap between experiments using multielectron targets, and one-electron theoretical approaches. We observe significant negative offset angles in the photoelectron momentum distributions, despite the short-range nature of the binding potential. We show that the offset angle is sensitive to the atomic structure description of the residual F atom. We also investigate the response of co- and counter-rotating electrons, and observe an angular separation in their emission.

Optical lattices are typically created via the ac Stark shift and are limited by diffraction to periodicities $\ge \lambda /2$, where $\lambda$ is the wavelength of light used to create them. Lattices with smaller periodicities may be useful for many-body physics with cold atoms and can be generated by stroboscopic application of a phase-shifted lattice with subwavelength features. Here we demonstrate a $\lambda /4$-spaced lattice by stroboscopically applying optical Kronig-Penney-like potentials which are generated using spatially dependent dark states. We directly probe the periodicity of the $\lambda /4$-spaced lattice by measuring the average probability density of the atoms loaded into the ground band of the lattice. We measure lifetimes of atoms in this lattice and discuss the mechanisms that limit the applicability of this stroboscopic approach.