Using the space-time analogy, we compare the performance of quantum temporal imaging with its classical counterpart. We consider a temporal imaging scheme, based on the sum-frequency generation time lens, but our results can be applied to other temporal imaging schemes such as, for instance, four-wave mixing. Extending the theory presented in our previous publications, in this paper we take into account the finite time aperture of the imaging system, characterized by its pupil function. Using the quantum theory, we obtain a unitary transformation of the quantum field from the input to the output of the imaging scheme and identify the contribution of the vacuum fluctuations missing in the classical theory. This contribution plays a key role in the quantum temporal imaging of nonclassical temporal waveforms, characterized by nonclassical fluctuations of the electromagnetic field. As an example, we consider quantum temporal imaging of broadband squeezed light and formulate the criteria for conservation of its squeezing properties at the output of the system.

We provide theoretical and experimental evidence of statistical outliers in random laser emission that are not accounted for by the, now established, power-law tailed (Lévy) distribution. Such outliers manifest themselves as single, large isolated spikes over an otherwise smooth background. A statistical test convincingly shows that their probability is larger than the one extrapolated from lower-intensity events. To compare with experimental data, we introduced the anomaly parameter that allows for an identification of such rare events from experimental spectral measurements and that agrees as well with the simulations of our Monte Carlo model. A possible interpretation in terms of Black Swans or Dragon Kings, large events having a different generation mechanism from their peers, is discussed.

We analyze how a star network topology shapes the dynamics of coupled ${\mathrm{CO}}_2$ lasers with an intracavity electro-optic modulator that exhibits bistability. Such a network supports spreading and stationary activation patterns. In particular, we observe an activation spreading where the activated periphery turns on the center element, an activated center which drifts the periphery into the active region, and an activation of the whole system from the passive into the active region. Pinned activation, namely activation localized only in the center or the peripheral elements, is also found. Similar dynamical behavior has been observed recently in complex networks of coupled bistable chemical reactions. The current work aims at revealing those phenomena in laser arrays, giving emphasis to the essential role of the coupling structure in fashioning the overall dynamics.

One of the most captivating properties of diffraction-free optical fields is their ability to reconstruct upon propagation in the presence of an obstacle, both classically and in the quantum regime. Here we demonstrate that the local entanglement, or nonseparability, between the spatial and polarization degrees of freedom also experience self-healing. We measured and quantified the degree of nonseparability between the two degrees of freedom when propagating behind various obstructions, which were generated digitally. Experimental results show that even though the degree of nonseparability reduces after the obstruction, it recovers to its maximum value within the classical self-healing distance. To confirm our findings, we performed a Clauser-Horne-Shimony-Holt Bell-like inequality measurement, proving the self-reconstruction of nonseparability. These results indicate that local entanglement between internal degrees of freedom of a photon, can be recovered by suitable choice of the enveloping wave function.

Single-qubit thermometry presents the simplest tool to measure the temperature of thermal baths with reduced invasivity. At thermal equilibrium, the temperature uncertainty is linked to the heat capacity of the qubit, however, the best precision is achieved outside the equilibrium condition. Here, we discuss a way to generalize this relation in a nonequilibrium regime, taking into account purely quantum effects such as coherence. We support our findings with an experimental photonic simulation.

The microscopic Maxwell-Lorentz equations show an intrinsic symmetry if magnetic charge and current densities are included. Starting from the symmetrized set of field equations a propagator approach is used to describe the transverse (photon) electrodynamics in space-time. Two dyadic propagators are needed: (i) a transverse electric propagator with a near-field part and (ii) a magnetic propagator with a midfield part. The first quantized photon wave mechanical theory, based on the analytical parts of the two transverse Riemann-Silberstein vectors, is extended to include magnetic monopole dynamics. The dynamical equations for the photon helicity eigenvectors and the local energy conservation in the photon field are discussed. The Dirac string concept and the fiber bundle approach are avoided using the double-potential formalism of Cabibbo and Ferrari. The transverse ($T$) parts of the electric (${\mathbf{A}}_T^e$) and magnetic (${\mathbf{A}}_T^m$) vector potentials relate in a simple manner to our propagator theory if this is formulated in terms of the Huygens scalar propagator and the transverse electric and magnetic current densities. A transformation of the transverse vector potentials, which is nonlocal in space and time, allows one to express the transverse parts of the electric and magnetic fields as curls of combinations of the original (${\mathbf{A}}_T^e,{\mathbf{A}}_T^m$) and the transformed (${\mathcal{A}}_T^e,{\mathcal{A}}_T^m$) transverse vector potentials. The momentum of the particle-photon system is discussed, and it is shown that the electromagnetic parts of the canonical electric (charge $e$) and magnetic (charge $g$) particle momenta are given by $e({\mathit{A}}_T^e+{\mathcal{A}}_T^m)$ and $g({\mathit{A}}_T^m-{\mathcal{A}}_T^e)$, respectively. The angular momentum of the particle-photon system is analyzed and contact is made to the well-known nonretarded Saha-Wilson orbital angular momentum for an ($e,g$) pair. The near field of a magnetic monopole is studied based on the overlooked fact that the magnetic near field contains both longitudinal ($L$) (with $\mathbf{\nabla }·{\mathbf{B}}_L=0$) and transverse vector field components. The sum of the two parts always is Einstein retarded, with proper account of the limitation caused by the lack of complete spatial photon localization. Finally, the relativistic electron-photon Hamiltonian in an external (prescribed) magnetic monopole field is discussed, paying particular attention to a determination of the transformed transverse magnetic vector potential, ${\mathcal{A}}_T^m$, and its positive-frequency part.

A discrete-time staggered quantum walk was recently introduced as a generalization that allows the unification of other versions, such as the coined and Szegedy's walk. However, it also produces forms of quantum walks not covered by previous versions. To explore their properties, we study here the staggered walk on a hexagonal lattice. Such a walk is defined using a set of overlapping tessellations that cover the graph edges, and each tessellation is a partition of the node set into cliques. The hexagonal lattice requires at least three tessellations. Each tessellation is associated with a local unitary operator and the product of the local operators defines the evolution operator of the staggered walk on the graph. After defining the evolution operator on the hexagonal lattice, we analyze the quantum walk dynamics with the focus on the position standard deviation and localization. We also obtain analytic results for the time complexity of spatial search algorithms with one marked node using cyclic boundary conditions.

We demonstrate an optical quantum nondemolition (QND) interaction gate with a bandwidth of about 100 MHz. Employing this gate, we are able to perform QND measurements in real time on randomly fluctuating signals. Our QND gate relies on linear optics and offline-prepared squeezed states. In contrast to previous demonstrations on narrow sideband modes, our gate is compatible with quantum states temporally localized in a wave-packet mode including non-Gaussian quantum states. This is the cornerstone of realizing quantum error correction and universal gate operations.

The multiconfiguration Dirac-Hartree-Fock method is applied to study the electron affinities of At and its homologous elements Cl, Br, and I. Our method of calculation is validated through the comparison with the available experimental electron affinities of Cl, Br, I, and other theoretical values. The agreement between our predicted electron affinities and the available experimental values for Cl, Br, and I is within 0.2%, which is an improvement of more than a factor of 10 over previous theoretical studies. Applying the same method to At, the electron affinity of At is predicted to be 2.3729(46) eV.

We investigated the charge distribution and dissociation dynamics of glycine (${\mathrm{NH}}_2{\mathrm{CH}}_2\mathrm{COOH}$) molecules irradiated with 310-eV x rays from the Advanced Light Source synchrotron. With simultaneous measurements of the fragment ion yield, dissociation angle, and kinetic energy, we were able to reconstruct a three-dimensional image of the x-ray-initiated molecular dissociation. Using coincidence and correlated analysis and applying a systematic comparison of properties of ion species, we partially disentangled the fragmentation pathways and identified the most probable fragmentation channels that lead to the observed fragment ions. In addition, we showed anisotropic angular distributions of dissociation subsequent to core-level photoionization and Auger decay and found an association between the initial bond-breaking sites and the kinetic energies of the final fragment ions.

The high harmonic generation of heteronuclear hydrogen molecular ions in few-cycle infrared strong laser fields is studied by numerically simulating the time-dependent Schrödinger equation. By reversing the carrier envelope phase of the few-cycle laser pulse, the electron dynamics is reversed as well. However, due to the different nuclear masses, two nuclei move with different repulsive speeds, which makes the rescattering processes not identical when the carrier envelope phase shifts by $\pi$. The different rescattering processes lead to distinct harmonic spectra in the cutoff region. The mass-dependent harmonic spectrum offers a view angle to diagnose the isotope dynamics in molecules with unprecedent high time and spatial resolutions.

We propose and numerically demonstrate a method for selecting the electrons with specific transverse momentum at the exit of tunneling. The strategy is based on the manipulation of the consequent acceleration and recollision by adding a circularly polarized control field (CPCF) to an intense laser field. The CPCF offers a gate which only allows electrons with specific initial momentum to recollide with the parent ion. The scheme is illustrated by the example of tricircular laser fields. To intuitively reveal the mechanism of the gate, the Hamiltionian of the system and the electron dynamics are discussed in a rotating frame, where the CPCF becomes a static electric field and an additional Coriolis term emerges due to the choice of the noninertial frame. This momentum gate offers a powerful tool to study the nature of the tunnel electron wave packet.

Ultracold paramagnetic and polar diatomic molecules are among the promising systems for quantum simulation of lattice-spin models. Unfortunately, their experimental observation is still challenging. Based on our recent ab initio calculations, we analyze the feasibility of all-optical schemes for the formation of ultracold ${}^{87}\mathrm{Rb}{}^{84}\mathrm{Sr}$ bosonic molecules. A first possibility is photoassociation followed by spontaneous emission. The photoassociation rate coefficients toward electronic states converging to the ${}^{87}\mathrm{Rb}\left(5s{}^{2}S_{1/2}\right)+{}^{84}\mathrm{Sr}(5s5p{}^{3}P_{0,1,2}$) asymptotes are particularly small for vibrational levels close to the asymptote. The creation of molecules would be more interesting by using deeply bound levels which preferentially relax to the $v^{\prime \prime }=0$ level of the ground state. On the other hand, the photoassociation rate coefficients toward electronic states correlated to the $\mathrm{Rb}\left(5p{}^{2}P_{1/2,3/2}\right)+\mathrm{Sr}(5s^2{}^{1}S_0$) are significant for levels close to the asymptote. The spontaneous emission thus creates weakly bound molecules in a single vibrational level. A second option relies on stimulated Raman adiabatic passage implemented in a tight optical trap. It efficiently creates weakly bound ground-state molecules in a well-defined level, thus providing a promising alternative to magnetic Feshbach resonances for further population transfer toward the absolute ground state of the RbSr molecule.

We develop a speckle-tracking method for x-ray phase-contrast imaging, based on the concept of geometric flow. This flow is a conserved current associated with deformation of illuminating x-ray speckles induced by passage through a sample. The method provides a rapid, efficient, and accurate algorithm for quantitative phase imaging. It is highly photon efficient and able to image dynamic objects, since a single radiograph of the sample is sufficient for the phase recovery. We experimentally quantify the resolution and contrast of the approach with both two-dimensional and three-dimensional phase-imaging applications using x-ray synchrotron radiation. Finally, we discuss adaptations of the method to imaging with compact x-ray sources that have a large source size and significant spectral bandwidth.

A quasicritical coupling state was demonstrated both in theory and experiment with a few-mode tapered-fiber (TF) coupled whispering-gallery-mode (WGM) cavity. In such a configuration, the coupling mode to activate the WGM and the drop mode are in the same TF and of almost the same decay rate with the variation of the gap between the TF and WGM cavity. Consequently, when the gap decreases to zero, the transmission of the coupling mode decreases monotonously to zero at the resonance wavelength, while the transmission of the drop mode increases monotonously to unit, which means that neither the coupling efficiency in the coupling mode nor the transmission of the drop mode will decrease in the traditional overcoupling region. Thus a steadily high coupling efficiency can be achieved accordingly, which we defined as quasicritical coupling. Such a coupling state will be helpful to develop WGM-cavity-based filters and benefit the fabrication of ultranarrow-linewidth lasers.

We present a numerical method to approximate the long-time asymptotic solution ${\rho }_{\infty }\left(t\right)$ to the Lindblad master equation for an open quantum system under the influence of an external drive. The proposed scheme uses perturbation theory to rank individual drive terms according to their dynamical relevance and adaptively determines an effective Hamiltonian. In the constructed rotating frame, ${\rho }_{\infty }$ is approximated by a time-independent, nonequilibrium steady state. This steady state can be computed with much better numerical efficiency than asymptotic long-time evolution of the system in the laboratory frame. We illustrate the use of this method by simulating recent transmission measurements of the heavy-fluxonium device, for which ordinary time-dependent simulations are severely challenging due to the presence of metastable states with lifetimes of the order of milliseconds.

Following the recent report of energy and angle interferences in below-threshold photoexcitation Auger processes due to postcollision interactions [Wang and Robicheaux, Phys. Rev. A 98, 013421 (2018)], similar interferences are also found in double-continuum wave packets from a two-step time-delayed photoionization of a two-electron atom. Properties of the interferences are studied in detail with respect to laser pulse time widths, laser frequency chirping, and ionization time delay. The effects of these physical quantities on the resulting interferences are discussed.

The dynamics of a weakly interacting Bose gas at low temperatures is close to integrable due to the approximate quadratic nature of the many-body Hamiltonian. While the short-time physics after an abrupt ramp of the interaction constant is dominated by integrable dynamics, integrability is broken at longer times by higher-order interaction terms in the Bogoliubov Hamiltonian, in particular, by Beliaev-Landau scatterings involving three quasiparticles. The two-stage relaxation process is highlighted in the evolution of local observables such as the density-density correlation function: an integrable dephasing mechanism leads the system to a prethermal stage, followed by true thermalization conveyed by quasiparticle collisions. Our results bring the crossover from prethermalization to thermalization within reach of current experiments with ultracold atomic gases.

We demonstrate that persistent currents can be induced in a quantum system in contact with a structured reservoir, without the need of any applied gauge field. The working principle of the mechanism leading to their presence is based on the extension to the many-body scenario of nonreciprocal Lindblad dynamics, recently put forward by Metelmann and Clerk, Phys. Rev. X 5, 021025 (2015): Nonreciprocity can be generated by suitably balancing coherent interactions with their corresponding dissipative version, induced by the coupling to a common structured environment, so as to make the total interactions directional. Specifically, we consider an interacting spin- (or boson-) model in a ring-shaped one-dimensional lattice coupled to an external bath. By employing a combination of cluster mean-field, exact diagonalization, and matrix-product-operator techniques, we show that solely dissipative effects suffice to engineer steady states with a persistent current that survives in the limit of large systems. We also verify the robustness of such current in the presence of additional dissipative or Hamiltonian perturbation terms.

We study the quantum counterpart of the theorem on energy equipartition for classical systems. We consider a free quantum Brownian particle modeled in terms of the Caldeira-Leggett framework: a system plus thermostat consisting of an infinite number of harmonic oscillators. By virtue of the theorem on the averaged kinetic energy $E_k$ of the quantum particle, it is expressed as $E_k=\langle {\mathcal{E}}_k\rangle$, where ${\mathcal{E}}_k$ is the thermal kinetic energy of the thermostat per 1 degree of freedom and $\langle \cdots \rangle$ denotes averaging over the frequencies $\omega$ of thermostat oscillators which contribute to $E_k$ according to the probability distribution $\mathbb{P}\left(\omega \right)$. We explore the impact of various dissipation mechanisms, via the Drude, Gaussian, algebraic, and Debye spectral density functions, on the characteristic features of $\mathbb{P}\left(\omega \right)$. The role of the system-thermostat coupling strength and the memory time on the most probable thermostat oscillator frequency as well as the kinetic energy $E_k$ of the Brownian particle is analyzed.