The absolute values of the cross section for formation of a $2p$ atom pair in the photoexcitation of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ are measured against the incident photon energy in the range of doubly excited states by means of the coincidence detection of two Lyman-$\alpha$ photons. The cross-section curves are explained only by the contribution of the doubly excited $Q_2{}^1\mathrm{\Pi }_u\left(1\right)$ state. The isotope effect on the oscillator strengths of $2p+2p$ pair formation for ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ from the $Q_2{}^1\mathrm{\Pi }_u\left(1\right)$ state is almost the same as that on the oscillator strengths of $2s+2p$ pair formation from the $Q_2{}^1\mathrm{\Pi }_u\left(1\right)$ state obtained by our group [T. Odagiri et al., Phys. Rev. A 84, 053401 (2011)]. This channel independence indicates that both isotope effects are dominated by the early dynamics of the $Q_2{}^1\mathrm{\Pi }_u\left(1\right)$ state, before reaching the branching point into $2p+2p$ pair formation and $2s+2p$ pair formation.

This paper investigates the fast Hamiltonian control of $n=2$ density operators by continuously varying the flag (i.e., the eigenspaces) as one moves away from the completely mixed state. In general, the critical points and zeros of the purity derivative can only be solved analytically in the limit of minimal purity. We derive differential equations that maintain these features as the purity increases. In particular, there is a thread of points in the Bloch ball that locally maximizes the purity derivative and a corresponding thread that minimizes it. Additionally, we show there is a closed surface of points inside of which the purity derivative is positive and outside of which is negative. We argue that this approach may be useful in studying higher-dimensional systems.

A hydrogen atom, initially prepared in the $2s$ and/or $2p(m=\pm 1)$ states, is assumed irradiated by $0.8\mathrm{keV}$ ($1.5$ nm) photons in pulses of $1–250$ fs duration and intensities in the range ${10}^{20}$ to ${10}^{23}\mathrm{W}/{\mathrm{cm}}^2$. Solving the corresponding time-dependent Schrödinger equation from first principles, we show that the ionization and excitation dynamics of the laser-atom system is strongly influenced by interactions beyond the electric dipole approximation. A beyond-dipole two-photon Raman-like transition between the $2s$ and $2p(m=\pm 1)$ states is found to completely dominate the underlying laser-matter interaction. It turns out that the large difference in the ionization rates of the $2s$ and $2p(m=\pm 1)$ states is important in this context, effectively leading to a symmetry breaking in the corresponding (beyond-dipole) bound-bound dynamics with the result that a net population transfer between the states occurs throughout the laser-matter interaction period. Varying the x-ray exposure time as well as the laser intensity, we probe the phenomenon as the bound wave packet oscillates between having $2s$ and $2p(m=\pm 1)$ character, eventually giving rise to a Rabi-like oscillation pattern in the populations.

We investigate how confining a transverse spatial dimension influences the few- and many-body properties of nonrelativistic bosons with pointlike interactions. Our main focus is on the dimensional crossover from three to two dimensions, which is of relevance for ultracold-atom experiments. Using functional-renormalization-group equations and $T$-matrix calculations we study how the phase transition temperature changes as a function of the spatial extent of the transverse dimension and relate the three- and two-dimensional $s$-wave scattering lengths. The analysis reveals how the properties of the lower-dimensional system are inherited from the higher-dimensional one during renormalization-group evolution. We limit the discussion to confinements in a potential well with periodic boundary conditions and argue why this qualitatively captures the physics of other compactifications such as transverse harmonic confinement as in cold-atom experiments.

A general method is introduced for verifying multitime quantum correlations through the characteristic function of the time-dependent $P$ functional that generalizes the Glauber-Sudarshan $P$ function. Quantum correlation criteria are derived which identify quantum effects for an arbitrary number of points in time. The Magnus expansion is used to visualize the impact of the required time ordering, which becomes crucial in situations when the interaction problem is explicitly time dependent. We show that the latter affects the multi-time-characteristic function and, therefore, the temporal evolution of the nonclassicality. As an example, we apply our technique to an optical parametric process with a frequency mismatch. The resulting two-time-characteristic function yields full insight into the two-time quantum correlation properties of such a system.

An exact calculation of the local electric field $\mathbf{E}\left(\mathbf{r}\right)$ is described for the case of a time-dependent point electric dipole $\mathbf{p}{e}^{-i\omega t}$ in the top layer of an ${\epsilon }_2, {\epsilon }_1, {\epsilon }_2$ three parallel slabs composite structure, where the ${\epsilon }_1$ layer has a finite thickness $2d$ but the ${\epsilon }_2$ layers are infinitely thick. For this purpose we first calculate all the eigenstates of the full Maxwell equations for the case where $\mu =1$ everywhere in the system. The eigenvalues appear as special, nonphysical values of ${\epsilon }_1$ when ${\epsilon }_2$ is given. These eigenstates are then used to develop an exact expansion for the physical values of $\mathbf{E}\left(\mathbf{r}\right)$ in the system characterized by physical values of ${\epsilon }_1\left(\omega \right)$ and ${\epsilon }_2\left(\omega \right)$. Results are compared with those of a previous calculation of the local field of a time-dependent point charge in the quasistatic regime. Numerical results are shown for the local electric field in practically important configurations where attaining an optical image with subwavelength resolution has practical significance.

Superconducting circuits provide a new platform for study of nonstationary cavity QED phenomena. An example of such a phenomenon is the dynamical Lamb effect, which is the parametric excitation of an atom due to nonadiabatic modulation of its Lamb shift. This effect was initially introduced for a natural atom in a varying cavity, while we suggest its realization in a superconducting qubit-cavity system with dynamically tunable coupling. In the present paper, we study the interplay between the dynamical Lamb effect and the energy dissipation, which is unavoidable in realistic systems. We find that despite naive expectations, this interplay can lead to unexpected dynamical regimes. One of the most striking results is that photon generation from vacuum can be strongly enhanced due to qubit relaxation, which opens another channel for such a process. We also show that dissipation in the cavity can increase the qubit excited-state population. Our results can be used for experimental observation and investigation of the dynamical Lamb effect and accompanying quantum effects.

Materials with vanishing permittivity (epsilon-near-zero or ENZ materials) show unconventional optical behavior. Here we show that plasmonic dipole antennas on an ultrathin ENZ substrate have properties significantly different from antennas on a traditional substrate. Specifically, the presence of a 23-nm-thick ENZ material strongly modifies the linear response of plasmonic antennas and, as a result, the resonant wavelength is independent of the linear dimensions of the dipole antenna.

Time and space are dual variables which bring a lot of analogies during theoretical study. In this paper, we extend the concept of a spatial optical waveguide to the temporal domain. Here we show that it is possible to confine the optical pulse within a time interval by introducing the temporal index boundaries. The confined pulse will propagate at a speed of the index change in the waveguide, and it will be behind the original optical pulse which propagates without the temporal index variations. In this way, we may offer an approach to broaden the bandwidth of the slow light and to tune the light speed based on the existing slow light devices. The temporal waveguide has modes, which are the temporal waveforms maintaining their shapes during the propagation. In a single-mode temporal waveguide, the pulse retains its shape as the only mode of the waveguide just like an optical soliton. In a multimode temporal waveguide, multimode interference effect exists, which can duplicate a single pulse into multiple copies and be potentially implemented for all-optical signal processing.

The link of two concepts, indistinguishability and entanglement, with the energy-time uncertainty principle is demonstrated in a system composed of two strongly coupled bosonic modes. Working in the limit of a short interaction time, we find that the inclusion of the antiresonant terms to the coupling Hamiltonian leads the system to relax to a state which is not the ground state of the system. This effect occurs passively by just presence of the antiresonant terms and is explained in terms of the time-energy uncertainty principle for the simple reason that at a very short interaction time, the uncertainty in the energy is of order of the energy of a single excitation, thereby leading to a distribution of the population among the zero, singly and doubly excited states. The population distribution, correlations, and entanglement are shown to substantially dependent on whether the modes decay independently or collectively to an exterior reservoir. In particular, when the modes decay independently with equal rates, entanglement with the complete distinguishability of the modes is observed. The modes can be made mutually coherent if they decay with unequal rates. However, the visibility in the single-photon interference cannot exceed $50\%$. When the modes experience collective damping, they are indistinguishable even if decay with equal rates and the visibility can, in principle, be as large as unity. We find that this feature derives from the decay of the system to a pure entangled state rather than the expected mixed state. When the modes decay with equal rates, the steady-state values of the density matrix elements are found dependent on their initial values.

A metasurface that absorbs waves from all directions of incidence can be achieved if the surface impedance is made to vary as a function of incidence angle in a specific manner. Here we show that a periodic array of planar nanoparticles coupled to a metal film can act as an absorbing metasurface with an angle-dependent impedance. Through a semi-analytical calculation based on coupled-mode theory, we find the perfect absorbing condition is equivalent to balancing the Ohmic and radiative losses of the nanoparticles at normal incidence. Absorption over a wide range of incidence angles can then be obtained by tailoring the scattered far-field pattern of the individual planar nanoparticles such that their radiative losses remain constant. The theory provides a means of understanding the behavior of perfect absorbing structures that have been observed experimentally or numerically, reconciling previously published theories and enabling the optimization of absorbing surfaces.

We study the direction- and wavelength-dependent polarization anisotropy in light scattering at the air–photonic crystal interface as a function of angle of incidence for TE and TM polarized light. This is done using optical reflectivity measurements at high-symmetry points in the Brillouin zone of a three-dimensional photonic crystal with fcc symmetry. Polarized reflectivity measurements indicate the presence of stop gap branching for TE polarization, which is absent for TM polarization until the Brewster angle at the $K$ point. In contrast, stop gap branching is present for both TE and TM polarizations at the $W$ point due to the intricate mixing of crystal planes. This characteristic behavior signifies the inevitable role of energy exchange in the stop gap branching. The measured polarization anisotropy shows a prominent shift in the Brewster angle for the on-resonance wavelength as compared to the off-resonance along both $K$ and $W$ points, and that is in accordance with theory. Our results have implications in polarization-induced light scattering in subwavelength photonic structures such as plasmonic crystals, and metamaterials.

The quantum Talbot effects with orbital angular momentum (OAM) photons have been investigated theoretically. Two schemes, holographic ghost-image and twin-photon configuration, have been considered. They exhibit unique properties, which are different from the cases of previous classical and quantum Talbot effects. The Talbot effects in the present schemes originate from lateral translation of the periodic object instead of transverse coherences of optical fields or photon wave functions. The Talbot pattern in the present case can be manipulated by the OAM number of the incident photons, which is equivalent to the Talbot effect of an effective periodic object illuminated by photons without the OAM. This means that complex diffraction patterns of an unusual periodic object can be obtained by designing the Talbot effect with OAM photons, which provides a promising idea in optical designs for the diffraction patterns.

Recently, Lucamarini et al. proposed the device-independent entanglement-based Bennett 1992 protocol [Phys. Rev. A 86, 032325 (2012)]. By exploiting the results in device-independent quantum key distribution with the Clauser-Horne-Shimony-Holt inequality, they claimed the security of their protocol. We will show, however, the way of encoding and the process for the latent eavesdropper to obtain the illegal information are different in these two protocols. Thus the security of their protocol cannot rely on Bell's theorem only.

Our paper [Phys. Rev. A 86, 032325 (2012)], setting forth a previously unknown connection between the Bennett 1992 protocol and a Bell inequality, has recently received a Comment [Phys. Rev. A 93, 066303 (2016)] about a possible flaw in its potential device-independent (DI) implementation. We point the authors of the Comment to prior works showing that what they assume to be specific to our protocol's DI implementation is actually a standard assumption in DI quantum key distribution. Therefore there is no need to revise any of the conclusions drawn in Lucamarini et al. [Phys. Rev. A 86, 032325 (2012)].

We present improvements in both theoretical understanding and experimental implementation of the cross resonance (CR) gate that have led to shorter two-qubit gate times and interleaved randomized benchmarking fidelities exceeding 99%. The CR gate is an all-microwave two-qubit gate that does not require tunability and is therefore well suited to quantum computing architectures based on two-dimensional superconducting qubits. The performance of the gate has previously been hindered by long gate times and fidelities averaging 94–96%. We have developed a calibration procedure that accurately measures the full CR Hamiltonian. The resulting measurements agree with theoretical analysis of the gate and also elucidate the error terms that have previously limited gate fidelity. The increase in fidelity that we have achieved was accomplished by introducing a second microwave drive tone on the target qubit to cancel unwanted components of the CR Hamiltonian.

Collapse in dipolar Bose-Einstein condensates may be arrested by quantum fluctuations. Due to the anisotropy of the dipole-dipole interactions, the dipole-driven collapse induced by soft excitations is compensated by the repulsive Lee-Huang-Yang contribution resulting from quantum fluctuations of hard excitations, in a similar mechanism as that recently proposed for Bose-Bose mixtures. The arrested collapse results in self-bound filamentlike droplets, providing an explanation for the intriguing results of recent dysprosium experiments. Arrested instability and droplet formation are general features directly linked to the nature of the dipole-dipole interactions, and should hence play an important role in all future experiments with strongly dipolar gases.

We study the inverse problem of constructing an appropriate Hamiltonian from a physically reasonable set of orthogonal wave functions for a quantum spin system. Usually, we are given a local Hamiltonian and our goal is to characterize the relevant wave functions and energies (the spectrum) of the system. Here, we take the opposite approach; starting from a reasonable collection of orthogonal wave functions, we try to characterize the associated parent Hamiltonians, to see how the wave functions and the energy values affect the structure of the parent Hamiltonian. Specifically, we obtain (quasi) local Hamiltonians by a complete set of (multilayer) product states and a local mapping of the energy values to the wave functions. On the other hand, a complete set of tree wave functions (having a tree structure) results to nonlocal Hamiltonians and operators which flip simultaneously all the spins in a single branch of the tree graph. We observe that even for a given set of basis states, the energy spectrum can significantly change the nature of interactions in the Hamiltonian. These effects can be exploited in a quantum engineering problem optimizing an objective functional of the Hamiltonian.

Quantum walks (QWs) describe particles evolving coherently on a graph. The internal degree of freedom corresponds to a Hilbert space, called a coin system. We consider QWs on Cayley graphs of some group $G$. In the literature, investigations concerning infinite $G$ have been focused on graphs corresponding to $G={\mathbb{Z}}^d$ with a coin system of dimension 2, whereas for a one-dimensional coin (so-called scalar QWs) only the case of finite $G$ has been studied. Here we prove that the evolution of a scalar QW with $G$ infinite Abelian is trivial, providing a thorough classification of this kind of walks. Then we consider the infinite dihedral group $D_{\infty }$, that is, the unique non-Abelian group $G$ containing a subgroup $H\cong \mathbb{Z}$ with two cosets. We characterize the class of QWs on the Cayley graphs of $D_{\infty }$, and, via a coarse-graining technique, we show that it coincides with the class of spinorial walks on $\mathbb{Z}$ which satisfies parity symmetry. This class of QWs includes the Weyl and the Dirac QWs. Remarkably, there exist also spinorial walks that are not coarse graining of a scalar QW, such as the Hadamard walk.