Recently, an exponentially decaying waveform (the time dependence of detection probability) of a Mössbauer γ-ray photon was transformed into a regular sequence of short pulses in a sinusoidally vibrating recoilless resonant absorber [F. Vagizov, V. Antonov, Y. V. Radeonychev, R. N. Shakhmuratov, and O. Kocharovskaya, Coherent control of the waveforms of recoilless γ-photons, Nature (London) 508, 80 (2014)]. In the present paper, we show that the peak amplitude of the pulses can be considerably increased via joint adjustment of optical depth of the absorber and the initial phase of its vibration. This is due to reduction of the photoelectric absorption and maximizing the constructive temporal interference of spectral content of the single-photon wave packet in an optically deep absorber. The ultimate capabilities for transforming a waveform of a 14.4-keV photon from a ${}^{57}\mathrm{Co}$ radioactive source into a regular train of pulses in a harmonically vibrating ${}^{57}\mathrm{Fe}$ recoilless resonant absorber are discussed. We show that the shortest pulse duration, produced by this technique, is limited by the highest available vibration frequency of a piezoelectric transducer and at present can be as short as 7.7 ps. The maximum achievable detection probability of the transformed photon at the experimentally feasible conditions is more than two times higher than peak detection probability of the photon emitted by the source and nearly 5.5 times higher than obtained in the above reference.

We report a mechanism to generate dissipative steady-state entanglement in coupled solid-state qubits driven by strong periodic fields. We demonstrate that steady entanglement can be induced and tuned by changing the amplitude of the driving field. A rich dynamic behavior with creation, death, and revival of entanglement can be observed near multiphoton resonances. Coupled superconducting qubits are good candidates for the observation of these effects.

We study the excitation to Rydberg states in the interaction of the hydrogen atom with a short, strong laser pulse. Utilizing solutions of the time-dependent Schrödinger equation we find that the parity of the populated angular momentum states agrees with the selection rules for multiphoton resonant absorption at low intensities, if the pulse length is not too short. In contrast, the parity effect cannot be observed for ultrashort pulses as well as for long pulses at high intensities. We further identify signatures of the population in the excited states via the line emissions from the populated $np$ states after the end of the pulse exhibiting the parity effect.

We present two methods for computing the Rayleigh and Raman scattering cross sections for photon scattering on atomic hydrogen or hydrogenlike systems. Both methods are applicable for incident photon energies above the ionization threshold. The first method implements the well-known Gaussian quadrature approach to deal with principal value integration and relies on evaluation of the exact eigenfunctions of hydrogen. The second, more computationally efficient approach uses a finite-$L^2$ basis expansion of the target and applies complex exterior scaling methods to accurately account for the contribution of the intermediate continuum states. This method is much more general in that it does not rely on analytic solutions to the Hamiltonian, or evaluation of any special functions, and is expected to be applicable to more complex systems where exact wave functions are cumbersome to evaluate. Both methods are in complete agreement with previous work based on analytical representations of the Green's function or the dipole matrix elements. Rayleigh, Raman, and photoionization cross sections for scattering on the first few excited states of atomic hydrogen are presented and compared with previous results where available.

We investigate a single-photon diode realized by a one-dimensional waveguide chirally coupled to $N$ spatially separated $\mathrm{\Lambda }$-type three-level emitters. An external laser is introduced to drive the emitters. The single photons moving in opposite directions show different transmission probabilities, which underpins the diode behavior. In the case of a single emitter, the single-photon diode works well at specific frequency points, which are tunable by the external laser. In the case of multiple emitters, the optimal-working-frequency point of the diode can be turned to a broad band.

Quantum states of light with multiple spatial modes are fundamental for quantum imaging and parallel quantum information processing. Thus their characterization, which can be achieved through measurements of the coherence area, is an important area of research. We present a comparative study between two different measurement techniques for the coherence area of bright entangled twin beams of light generated with a four-wave mixing process in a hot rubidium vapor cell. The first one provides a direct characterization of the size of the coherence area and is based on correlation measurements between spatial intensity fluctuations of the twin beams with an electron-multiplying charge-coupled-device camera. The second one provides an indirect measure and is based on a noise analysis of different spatial regions of the twin beams in the time domain with a single photodiode. We show that the indirect technique, which can be implemented with a significantly less complicated measurement scheme, gives an estimate of the size of the coherence area consistent with the direct measurement technique performed in the spatial domain.

A propagation of optical beams in active waveguides with quadratic nonlinearity is considered. It is shown that gain in one harmonic can compensate losses in the other harmonic. As a result of this process, stationary beams can be formed in the system. Exact solutions for stationary modes of a single waveguide are obtained, and their stability is analyzed. A possibility of the Hopf bifurcation that results in emergence of stable periodic regimes in a monomer is demonstrated. Stationary solutions are also found for a dimer with identical waveguides and a dimer with parity-time symmetry. The stability analysis demonstrates that stable beams exist in a wide range of the system parameters.

Most applications of memory effects in disordered optical media, such as the tilt-tilt and shift-shift spatial correlations, have focused on imaging through and inside biological tissues. Here we put forward a metamaterial platform not only to enhance but also to tune memory effects in random media. Specifically, we investigate the shift-shift and tilt-tilt spatial correlations in metamaterials composed of coated spheres and cylinders by means of the radiative transfer equation. Based on the single-scattering phase function, we calculate the translation correlations in anisotropically scattering media with spherical or cylindrical geometries and find a simple relation between them. We show that the Fokker-Planck model can be used with the small-angle approximation to obtain the shift-tilt memory effect with ballistic light contribution. By considering a two-dimensional scattering system, composed of thick dielectric cylinders coated with subwavelength layers of thermally tunable magneto-optical semiconductors, we suggest the possibility of tailoring and controlling the shift-shift and tilt-tilt memory effects in light scattering. In particular, we show that the generalized memory effect can be enhanced by increasing the temperature of the system, and it can be decreased by applying an external magnetic field. Altogether our findings unveil the potential applications that metamaterial systems may have to control externally memory effects in disordered media.

Interferometry is a widely used technique for precision measurements in both classical and quantum contexts. One way to increase the precision of phase measurements, for example, in a Mach-Zehnder interferometer (MZI), is to use high-intensity lasers. In this paper we study the phase sensitivity of a MZI in two detection setups (difference intensity detection and single-mode intensity detection) and for three input scenarios (coherent, double coherent, and coherent plus squeezed vacuum). For the coherent and double coherent input, both detection setups can reach the quantum Cramér-Rao bound, although at different values of the optimal phase shift. The double coherent input scenario has the unique advantage of changing the optimal phase shift by varying the input power ratio.

A unified bound on the quantum speed limit is obtained for open quantum systems with a mixed initial state by utilizing the function of relative purity proposed by F. Campaioli et al. [Phys. Rev. Lett. 120, 060409 (2018)]. As applications, it is found that the quantum speed limit bound for the damped Jaynes-Cummings model is determined by the competition among the non-Markovianity, the population of the initial excited state, and the initial-state coherence, which shows that the population of the initial excited state and the coherence of the initial state can make the quantum speed limit bound tighter even though the non-Markovian effects can accelerate the evolution of open quantum system. For the dephasing model, a simple factorization law with the initial-state coherence shows that the quantum speed limit is governed by only the competition between the non-Markovianity and the coherence of the initial state.

A quantum analog of friction (understood as a completely positive, Markovian, translation-invariant, phenomenological model of dissipation) is known to be at odds with detailed balance in the thermodynamic limit. We show that this is not the case for quantum systems with internal (e.g., spin) states nonadiabatically coupled to translational dynamics. For such systems, a quantum master equation is derived which phenomenologically accounts for the frictional effect of a uniform zero-temperature environment. A simple analytical example is provided. Conjectures regarding the finite-temperature case are also formulated. The results are important for efficient simulations of complex molecular dynamics and quantum reservoir engineering applications.

The Bell inequalities for azimuthal quantum number, i.e., orbital angular momentum, have been extensively reported to show quantum correlations between the spatial degree of freedom of entangled photon pairs. Here we investigate instead the Bell inequalities for radial quantum number. We demonstrate, both theoretically and experimentally, the violation of a suitable Clauser-Horne-Shimony-Holt Bell inequality for the radial index $p$ of the Laguerre-Gaussian mode, thus manifesting the entanglement in the radial structure of two-photon wave functions.

We present a protocol to prepare rapidly a stationary three-atom singlet state with high fidelity, which takes advantage of the dissipative factor originating from the spontaneous emission of short-lived excited states and Rydberg blockade. The short lifetime of the excited states is an active ingredient in the state preparation, and the numerical simulation results of the master equation show that the fidelity of the created state can achieve 0.98 when the evolution time of the system is as short as 300 $\mu \mathrm{s}$ under certain reasonable conditions. Moreover, the protocol is only weakly dependent on the Rydberg interaction strength and needs no state initialization, optical cavity, feedback controls, and fine controlling of the system parameters, which can greatly reduce the experimental difficulty.

We present self-testing protocols to certify the presence of tensor products of Pauli measurements on maximally entangled states of local dimension $2^n$ for $n\in \mathbb{N}$. This provides self-tests of sets of informationally complete measurements in arbitrarily high dimension. We then show that this can be used for the device-independent certification of the entanglement of all bipartite entangled states by exploiting a connection to measurement-device-independent entanglement witnesses and quantum networks. This work extends a more compact parallel work on the same subject [Bowles et al., Phys. Rev. Lett. 121, 180503 (2018)] and provides all the required technical proofs.

We performed scattering experiments using a rubidium (Rb) atomic beam on paraffin films and measured the angular and velocity distributions of scattered atoms. The paraffin films were prepared in various ways and characterized by atomic force microscopy and x-ray diffraction. The films exhibited various roughnesses and crystal structures. The paraffin films preserved the spin polarization of the scattered atoms. The measured angular distributions of all prepared films were consistent with Knudsen's cosine law. The velocity distributions were well fitted by Maxwell's distribution, characterized by a temperature much closer to the film temperature than to the atomic-beam temperature. We therefore concluded that the translational motion of the scattered atoms was thermalized with the paraffin films via single-scattering events.

We report on a multicenter three-distorted-wave (MCTDW) approach to describe the electron-impact-ionization dynamics of molecules. In MCTDW, both the scattered projectile and the ejected electrons are described by distorted waves. The continuum wave functions of the incident and the two outgoing electrons are solved in the multicenter potential of neutral molecule and molecular ion, respectively. The fully differential cross section is then obtained for a given molecular orientation. Here, we present the spherically averaged triple-differential cross sections of water molecule by 81 eV electron impact. The results show good agreement with experiment in three-dimensional kinematics concerning both the angular dependence and the relative magnitude of the cross sections over a large range of analyzed angle and energy conditions.

We present an all-optical method for achieving molecular Bose-Einstein condensates of ${}^6\mathrm{Li}$. We demonstrate this with mixtures in the lowest two (1-2), and second lowest two (2-3) hyperfine states. For the 1-2 mixture, we can achieve condensate fractions of 36%, with $9\times {10}^4$ atoms at $0.05\mu \mathrm{K}$ temperature. For the 2-3 mixture, we have 28% condensed with $3.2\times {10}^4$ atoms at $0.05\mu \mathrm{K}$ temperature. We use mostly standard methods, but make a number of refinements in the magnetic bias coils compared with earlier work. Our method imposes minimal constraints on subsequent experiments by allowing plenty of optical access while requiring only one high-vacuum chamber. We use an optical system designed around minimizing the number of active elements, and we can accomplish slowing and sub-Doppler cooling with a single tapered amplifier.

A nuclear-spin-exchange interaction exists between two ultracold fermionic alkali-earth-metal-like atoms in the electronic ${}^1{S}_0$ state ($g$ state) and ${}^3{P}_0$ state ($e$ state) and is an essential ingredient for the quantum simulation of the Kondo effect. We study the control of this spin-exchange interaction for two atoms simultaneously confined in a quasi-one-dimensional (quasi-1D) tube, where the $g$ atom is freely moving in the axial direction while the $e$ atom is further localized by an additional axial trap and behaves as a quasi-0D impurity. In this system, the two atoms experience effective 1D spin-exchange interactions in both even and odd partial-wave channels, whose intensities can be controlled by the characteristic lengths of the confinements via confinement-induced resonances (CIRs). In a previous work, we studied this problem with a simplified pure 1D model [Y. Cheng et al., Phys. Rev. A 96, 063605 (2017)]. In the present work, we go beyond that pure 1D approximation. We model the transverse and axial confinements by harmonic traps with finite characteristic lengths $a_{\perp }$ and $a_z$, respectively, and exactly solve the quasi-1D–quasi-0D scattering problem between these two atoms. Using the solutions, we derive the effective 1D spin-exchange interaction and investigate the locations and widths of the even-odd wave CIRs for our system. It is found that when the ratio $a_z/a_{\perp }$ is larger, the CIRs can be induced by weaker confinements, which are easier to be realized experimentally. The comparison between our results and a recent experiment by L. Riegger et al. [Phys. Rev. Lett. 120, 143601 (2018)] shows that the two experimentally observed resonance branches of the spin-exchange effect are due to an even-wave CIR and an odd-wave CIR, respectively. Our results are advantageous for the control and description of either the effective spin-exchange interaction or other types of interactions between ultracold atoms in quasi-(1+0)-dimensional systems.

We study hybrid atom-molecule quantum walks in one-dimensional optical lattices with two interacting bosonic atoms which may be converted into a molecule. The hybrid atom-molecule energy bands include a continuum band and two isolated bands, which respectively correspond to scattering states and dressed bound states (DBSs). Because of the atom-molecule coupling, the DBSs may appear even in the absence of atom-atom interaction. From an initial state of two atoms occupying the same site, in addition to independent quantum walks which correspond to scattering states, correlated quantum walks appear as a signature of DBSs. Even if the atom-atom interaction and the atom-molecule coupling are much stronger than the tunneling strengths, independent quantum walks may still appear under certain resonant conditions. The correlated quantum walks show two light cones with different propagation velocities, which can be analytically explained by the effective tunneling strengths of the two different DBSs. Furthermore, the effective nearest-neighbor tunneling of DBSs can be suppressed to zero, which can be explained by the destructive interference between the atomic pair and the molecule.

We analytically and numerically investigate the propagation of ultrashort tightly focused laser pulses in vacuum, with particular emphasis on Hermite-Gaussian and Laguerre-Gaussian modes. We revisit the Lax series approach for forward-propagating linearly polarized laser pulses, to obtain Maxwell-consistent and symmetry-preserving analytical solutions for the propagation of all field components beyond the paraxial approximation in four-dimensional geometry (space and time). We demonstrate that our solution conserves the energy, which is set by the paraxial-level term of the series. The full solution of the wave equation towards which our series converges is calculated in the Fourier space. Three-dimensional numerical simulations of ultrashort tightly focused pulses validate our analytical development.