The two-state-vector formalism and the entangled histories formalism are attempts to better understand quantum correlations in time. Both formalisms share some similarities, but they are not identical, having subtle differences in their interpretation and manipulation of quantum temporal structures. However, the main objective of this paper is to prove that, with appropriately defined scalar products, both formalisms can be made isomorphic. We show, for instance, that they treat operators and states on equal footing, leading to the same statistics for all measurements. In particular, we discuss the topic of quantum correlations in time and show how they can be generated and analyzed in a consistent way using these formalisms. Furthermore, we elaborate on an unconventional behavior of quantum histories of evolving multipartite systems which do not exhibit global nonlocal correlations in time but nevertheless can lead to entangled reduced histories characterizing evolution of an arbitrarily chosen subsystem.

The one-dimensional Fermi-Hubbard model is used as test bed for strong global parameter quenches. With the aid of iterated equations of motion in combination with a suitable scalar product for operators, we describe the dynamics and the long-term behavior in particular of the system after interaction quenches. This becomes possible because the employed approximation allows for oscillatory dynamics avoiding spurious divergences. The infinite-time behavior is captured by an analytical approach based on stationary phases; no numerical averages over long times need to be computed. We study the most relevant frequencies in the dynamics after the quench and find that the local interaction $U$ as well as the bandwidth $W$ dominate. In contrast to former studies, a crossover instead of a sharp dynamical transition depending on the strength of the quench is identified. For weak quenches, the bandwidth is more important while for strong quenches the local interaction $U$ dominates.

Separation of the spin and orbital angular momenta of the electromagnetic field has been discussed frequently in recent years. The spin and orbital angular momenta cannot be made simultaneously gauge invariant and Lorentz covariant and are not conserved separately. After analyzing the source of the problem, we find that the electromagnetic 4-potential depends on the local reference frame instead of the global reference frame. The transformation of the local reference frame is the intrinsic degree of freedom of the electromagnetic field. Therefore, considering only the Lorentz transformation of the global reference frame and neglecting the Lorentz transformation of the local reference frame may lead to the noncovariance of the electromagnetic 4-potential. Accordingly, we redescribe these difficulties of the electromagnetic field from the perspective of quantum field theory. By using the behavior of the electromagnetic 4-potential that satisfies the Coulomb gauge in Lorentz coordinate transformation, we can construct the electromagnetic vector in the tetrad bundle. The various physical quantities that are induced by this electromagnetic vector satisfy Lorentz covariance in the tetrad bundle. This electromagnetic vector, which is projected onto space-time, is an electromagnetic 4-potential that satisfies the Coulomb gauge; thus, the electromagnetic vector is gauge invariant.

Hybrid exceptional points (HEPs) are non-Hermitian degeneracies that exhibit different dispersion relations (e.g., linear or square-root) along different directions in the parameter space. Here, we show that a two-state system consisting of coupled ferromagnetic waveguides applied with a bias magnetic field can support the presence of a HEP. A configuration is designed to dynamically encircle the HEP, showing a nonchiral behavior due to the unique topological structure of energy surfaces around the HEP. This is in contrast to the chiral dynamics found when an exceptional point is encircled dynamically. We performed numerical simulations and microwave experiments to demonstrate the topological transition enabled by the HEP in the proposed system.

Photons traveling in a background electromagnetic field may bend via the vacuum polarization effect with the background field. The bending in a Coulomb field by a heavy nucleus is small even at a large atomic number, rendering it difficult to detect experimentally. As an amplifying mechanism of the effect we consider the bending of light traveling in a chamber of Coulomb gas. The Gaussian nature of the bending in the gas increases the total bending angle in proportion to the square root of the photon travel distance. The enhancement can be orders of magnitude over the bending by a single nucleus at a small impact parameter, which may help experimental observation of the Coulombic bending.

An ab initio analysis of strong-field three-electron ionization in a restricted-dimensionality model reveals the dynamics of the ionization process and the dominant channels for double (DI) and triple ionization (TI). Simulations using wave functions that respect the Pauli principle show that the most likely channel is a sequence of single ionization (SI) and DI, while direct TI has a much lower probability. The dominant DI process has the highest probability for a singlet of up- and down-spin electrons. The results demonstrate the significance of the Pauli principle for the selection of dominant pathways in ionization and possibly other many-electron processes in strong fields.

We demonstrate that nonlinear entanglement witnesses can be made particularly useful for entanglement detection in hyperentangled or multilevel states. We test this idea experimentally on the platform of linear optics using a hyperentangled state of two photons. Instead of several simultaneous copies of two-photon entangled states, one can directly measure the witness on a single copy of a hyperentangled state. Our results indicate that hyperentanglement can be used for quick entanglement detection and it provides a practical test bed for experiments with nonlinear entanglement witnesses.

We study the interaction of a copropagating finite-energy Airy pulse and a soliton in a Kerr medium under third-order dispersion. It is observed that a strong radiation appears when a self-accelerating Airy pulse collides with a delayed soliton in time domain. We confirm both analytically and numerically that this radiation can only be initiated in the environment of third-order dispersion (TOD). The radiation frequency is red or blue shifted depending on the numeric sign of TOD parameter. We adopt a simple Si-based waveguide to validate the normalized parameter that we use for the analysis. We develop a theory to explain the collision-mediated radiation and our theoretical results are found to be in good agreement with the direct numerical modeling of the generalized nonlinear Schrödinger equation.

We describe the formation of bulk and edge arcs in the dispersion relation of two-dimensional coupled-resonator arrays that are topologically trivial in the Hermitian limit. Each resonator provides two asymmetrically coupled internal modes, as realized in noncircular open geometries, which enables the system to exhibit non-Hermitian physics. Neighboring resonators are coupled chirally to induce non-Hermitian symmetries. The bulk dispersion displays Fermi arcs connecting spectral singularities known as exceptional points and can be tuned to display purely real and imaginary branches. At an interface between resonators of different shape, one-dimensional edge states form that spectrally align along complex arcs connecting different parts of the bulk bands. We also describe conditions under which the edge-state arcs are freestanding. These features can be controlled via anisotropy in the resonator couplings.

We predict and observe Bessel-beam optical self-trapping in a strongly self-focusing medium. Nonlinearity leads to solutions with a signature pulsing along the propagation direction, a breather that we observe in a photorefractive potassium-lithium-tantalate-niobate crystal. The nonlinear construct is formed as an interference pattern locks together with a diffracting wave.

Quantum random walk finds application in efficient quantum algorithms as well as in quantum network theory. Here we study the mixing time of a discrete quantum walk over a square lattice in the presence of dynamic percolation and decoherence. We consider bit-flip and phase damping noise, and evaluate the instantaneous mixing time for both the cases. Using numerical analysis we show that, in the case of phase damping noise, the probability distribution of walker's position is sufficiently close to the uniform distribution after infinite time. However, during the action of bit-flip noise, even after infinite time the total variational distance between the two probability distributions is large enough.

Classifying locally equivalent graph states, and stabilizer states more broadly, is a significant problem in the theories of quantum information and multipartite entanglement. A special focus is given to those graph states for which equivalence through local unitaries implies equivalence through local Clifford unitaries (LU $\iff$ LC). Identification of locally equivalent states in this class is facilitated by a convenient transformation rule on the underlying graphs and an efficient algorithm. Here we investigate the question of local equivalence of the graph states behind the all-photonic quantum repeater. We show that complete bipartite graph (biclique) states, imperfect repeater graph states, and small “crazy graph” states satisfy LU $\iff$ LC. We continue by discussing biclique states more generally and placing them in the context of counterexamples to the LU-LC conjecture. To this end, we offer some alternative proofs and clarifications on existing results.

Although the three-body Rydberg antiblockade regime (RABR) can produce Rabi oscillation between the Rydberg collective excited state and the collective ground state, it is still hard to use the RABR to construct the three-qubit quantum logic gate in one step since the effective Hamiltonian is always accompanied by undesired Stark shifts. In order to overcome this difficulty, an additional laser is introduced to eliminate the Stark shifts in the ground-state subspace. And the initial RABR condition is modified to eliminate the remaining undesired Stark shifts in the collective-excitation subspace. The modified RABR is then generalized to the $n (n>3)$-qubit case. Based on the proposed regime, one-step schemes to construct three- and $n$-qubit quantum controlled-PHASE gates are proposed without the requirement of atomic addressability. The asymmetric Rydberg-Rydberg interaction, which is more practical for Rydberg atoms, is also discussed and proven to be feasible for the modified RABR and quantum controlled-PHASE gate in theory. A full-Hamiltonian-based master equation is used to evaluate the performance and some experimental parameters are also considered.

Multiply ionized atoms of a single charge state can be captured at low energy in a compact Penning trap, allowing the measurement of the radiative lifetime of an atomic state that decays via weakly allowed transitions. Such a measurement is reported here for the radiative decay lifetime of the metastable ${}^{2}P_{1/2}$ state in fluorinelike ${\mathrm{Ar}}^{9+}$, wherein the $n=2$ shell has only one vacancy (hole) in a $2p$ orbital. Highly ionized atoms are extracted from an electron beam ion trap and guided into the ion capture apparatus. Upon capturing the ${\mathrm{Ar}}^{9+}$ ions, light from the magnetic-dipole transition is detected with a photomultiplier tube and counted with a multichannel scaler. Using this technique, the radiative lifetime of the metastable ${}^{2}P_{1/2}$ state in ${\mathrm{Ar}}^{9+}1s^{2}2s^{2}2p^5$ is measured to be $\tau =9.31\pm 0.08$ ms, consistent with a previous intra-EBIT measurement.

The spin-averaged effective Hamiltonian of orders $m{\alpha }^6$ and $m{\alpha }^6(m/M)$ for a one-electron two-center Coulombic system is derived by using the theory of nonrelativistic quantum electrodynamics (NRQED), without assuming the Born-Oppenheimer approximation. The separated singularities from the first- and second-order perturbations are shown to be canceled out analytically for both order $m{\alpha }^6$ and $m{\alpha }^6(m/M)$ corrections by regularizing the effective Hamiltonian. Our results can be used to perform high-precision spectroscopic calculations of hydrogen molecular ions.

Channel-resolved ionization of helium by laser pulses with a photon energy equal to the energy difference between the ionic ground- and first-excited states, i.e., in the core-resonant scenario, is investigated theoretically for a considered peak intensity of $\sim {10}^{14}$ W/${\mathrm{cm}}^2$. By using the essential-states formalism, we derive analytical expressions for the channel-resolved photoelectron spectra, with the pulse-envelope effects taken into account. To examine the validity of the essential-states approach, we numerically solve the time-dependent Schrödinger equation of a model helium and use the corresponding results as references. The justified analytical estimation of the channel-resolved photoelectron spectra serves as a convenient approach to explore the effects of pulse durations and envelopes. It requires a longer pulse duration to observe the spectral peak splitting in the ionic first-excited-state channel than in the ionic ground-state channel. Ionization by two sequent pulses produces more complicated multipeak structures in the channel-resolved photoelectron spectra than ionization by a single pulse. Studies for the nonzero-detuning cases are also presented, where we propose a general procedure for understanding how the photoelectron spectra are built up.

High-order harmonics generated by a bicircular laser field have helicities which alternate between $+1$ and $-1$. In order to generate circularly polarized high-harmonic pulses, which are important for applications, it is necessary to achieve asymmetry in emission of harmonics having opposite helicities. We theoretically investigated a wide range of bicircular field component intensities and found areas where both the harmonic intensity is high and the helicity asymmetry is large. We investigated the cases of $\omega –2\omega$ and $\omega –3\omega$ bicircular fields and atoms having the $s$ and $p$ ground states, exemplified by He and Ne atoms, respectively. We have shown that for He atoms, strong high harmonics having positive helicity can be generated using an $\omega –3\omega$ bicircular field with a much stronger second field component. For Ne atoms the helicity asymmetry can be large in a wider range of the driving-field component intensities and for higher harmonic orders. For the stronger second field component the harmonic intensity is higher and the helicity asymmetry parameter is larger for higher harmonic orders. The results for Ne atoms are illustrated with the parametric plots of an elliptically polarized attosecond high-harmonic field.

Multistage Zeeman deceleration is a technique used to reduce the velocity of neutral molecules with a magnetic-dipole moment. Here we present a Zeeman decelerator that consists of 100 solenoids and 100 magnetic hexapoles that is based on a short prototype design presented recently [Phys. Rev. A 95, 043415 (2017)]. The decelerator features a modular design with excellent thermal and vacuum properties and is robustly operated at a 10 Hz repetition rate. We use this decelerator to demonstrate the state-selective deceleration of atomic oxygen to final mean velocities in the range 500–125 m/s. We further characterize our decelerator with molecular oxygen, which, despite its heavier mass, is velocity tuned in the range 350–150 m/s. This corresponds to a maximum kinetic-energy reduction of 95% and 80% for atomic and molecular oxygen, respectively. The long multistage Zeeman decelerator presented here demonstrates that the concept of using alternating hexapoles and solenoids is truly phase stable. This Zeeman decelerator is ideally suited for applications in crossed-beam scattering experiments; the state-selected and velocity-controlled samples of O atoms and ${\mathrm{O}}_2$ molecules are particularly relevant for studies of inelastic and reactive processes.