The temporal dynamics of stimulated emission is studied, with particular emphasis on stimulated emission induced by x-ray pulses interacting with nuclei. In typical nuclear forward scattering experiments, the short incident x-ray pulse is accompanied by a huge number of off-resonant background photons. This prompts the question of whether stimulated emission can be observed in the delayed nuclear scattering signal which is emitted after the incident pulse has passed. We find that the stimulated photons essentially overlap with the stimulating pulse. To overcome this problem, we identify the reduction of the delayed scattered light intensity as an alternative signature for the stimulated emission. We further study a phase-sensitive variant of stimulated emission in the low-excitation regime, which provides convenient control parameters to facilitate the detection. Finally, we analyze the possibility of observing stimulated emission in nuclei driven by free electron lasers or synchrotron radiation sources.

High-order harmonic generation (HHG) in a bulk crystal was first observed in 2011 [S. Ghimire, A. D. DiChiara, E. Sistrunk, P. Agostini, L. F. DiMauro, and D. A. Reis, Nat. Phys. 7, 138 (2011)]. Only odd-order harmonics were observed as expected on the basis of the selection rules in solids, which were derived when only the interband currents were taken into consideration. Here we study HHG in solids when the intraband currents are taken into consideration as well. We show that the dynamical selection rules are broken in solids and the possibility of generation of even-order harmonics cannot be excluded on the basis of the dynamical symmetry analysis. However, a simple analysis of the expression we obtained for the amplitude of the emitted high-order harmonics shows, without the need to carry out numerical calculations, that the even-order harmonics are suppressed due to the localization of the field-free one-electron density probability on the atoms in the solids.

Polarization-controlled focusing of surface plasmon polaritons (SPPs) is experimentally and theoretically investigated in the terahertz (THz) frequency range. The spiral phase distribution and the phase singularities, which, respectively, lead to the transverse shift of the focal spot for the circularly polarized THz radiation and the split of the focal spot for the linearly polarized THz radiation, are experimentally observed. Furthermore, an alternative theoretical model based on the Huygens-Fresnel principle and Fourier transformation of THz SPPs is proposed to analyze the transverse shift of the focal spot, which provides a simple way to understand the polarization-related SPP phenomena. This research is instructive for the design of polarization-based plasmonic devices.

We determine the Landau-Zener transition probability in a dissipative environment including both longitudinal as well as transversal quantum-mechanical noise originating from a single noise source. For this, we use the numerically exact quasiadiabatic path integral, as well as the approximative nonequilibrium Bloch equations. We find that transversal quantum noise in general influences the Landau-Zener probability much more strongly than longitudinal quantum noise does at a given temperature and system-bath coupling strength. In other words, transversal noise contributions become important even when the coupling strength of transversal noise is smaller than that of longitudinal noise. We furthermore reveal that transversal noise renormalizes the tunnel coupling independent of temperature. Finally, we show that the effect of mixed longitudinal and transversal noise originating from a single bath cannot be obtained from an incoherent sum of purely longitudinal and purely transversal noise.

We analyze tunneling of a single particle, whose initial state is given by a superposition of spatially separated wave-packet modes. It is shown that “pile up” of different components in the scatterer may change the tunneling probabilities, making such states a convenient tool for probing the barrier's scattering times. Interference effects arising in resonance tunneling are studied in detail. The analysis allows us to gain further insight into the origin of interference effects in scattering of several identical particles.

We study a semi-device-independent (SDI) randomness expansion protocol with a trusted source or detector. It is not difficult to characterize the source or the detector when their dimensions are limited in the SDI scenario. By adding the reasonable assumption, our protocol offers higher min-entropy and, thus, can be used to certify more randomness than the standard SDI protocol of H.-W. Li et al. [Phys. Rev. A 84, 034301 (2011)]. Moreover, randomness expansion can be realized with a lower detection efficiency (about 85.4%) in the trusted source case than the standard one (about 93.4%). Meanwhile, this advantage is robust against parameter deviations of the source. But on the other hand, the sharp measurement cannot improve the performance much in this case. So, there is no need to characterize the measurement accurately. Furthermore, an analytical formula describing the relationship between the dimension witness value and the min-entropy of the output string is derived directly.

The displacement effect studied in a recent paper [Ivanov et al., Phys. Rev. A 90, 043401 (2014)] in atomic ionization by a short XUV pulse is investigated in more detail. It is shown that achieving a significant displacement critically depends on the assumption of a plateau in the envelope function of the electric field, and that the ramp-on is fine-tuned in such a way that a drift velocity generated during the ramp-on phase can increase this displacement further. Seemingly minor variations in the electric fields defined in slightly different ways cause significant changes in the final results, in particular regarding the angular-momentum distribution of the ejected electron. In light of such a strong sensitivity seen in the predictions made with idealized pulse shapes and the likely difficulties of preparing such pulses experimentally, an experimental realization of the displacement effect will likely be a major challenge.

We propose and demonstrate a type of magneto-optical trap (MOT) for alkaline-earth-metal-like (AEML) atoms where the narrow intercombination ${}^1{S}_0\to {}^3{P}_1$ transition and the broad ${}^1{S}_0\to {}^1{P}_1$ transition are spatially arranged into a core-shell configuration. Our scheme resolves the main limitations of previously adopted MOT schemes for AEML atoms, leading to significant increases in both the loading rate and the steady-state atom number. We apply this scheme to our experimental setup with ${}^{174}\mathrm{Yb}$ atoms and observe more than two orders of magnitude improvement in the loading rate and tenfold improvement in the steady-state atom number compared to a conventional intercombination MOT scheme. This technique could be readily extended to other AEML atoms to increase the statistical sensitivity of many different types of precision experiments.

A gain-based four-level atomic medium for the stability in superluminal light propagation using control field and inverse Doppler shift as coherence generators is studied. In regimes of weak and strong control field, a broadband and multiple controllable transparency windows are, respectively, identified with significantly enhanced group indices. The observed Doppler effect for the class of high atomic velocity of the medium is counterintuitive in comparison to the effect of the class of low atomic velocity. The intensity of each of the two pump fields is kept less than the optimum limit reported in [M. D. Stenner and D. J. Gauthier, Phys. Rev. A 67, 063801 (2003)] for stability in the superluminal light pulse. Consequently, superluminal stable domains with the generated coherence are explored.

We numerically investigate harmonic generation in Ar gas under high ionization conditions and demonstrate that a lighthouse effect is present. We examine the structure of the driving field during propagation in temporal, spectral, and spatial domains, and conclude that the complete depletion of neutral Ar on axis gives rise to additional wavelets at off-axis regions. We show that these wavelets propagate with increasing divergence as the radial distances from the axis increase, generating the rotation of the wave front, thus fulfilling a necessary condition for the lighthouse effect. We obtain attosecond bursts of light emitted with different divergences in successive optical half-cycles so that in the far field these bursts arrive at different distances from the beam axis.

Precision control of quantum systems is the driving force for both quantum technology and the probing of physics at the quantum and nanoscale levels. We propose an implementation-independent method for in situ quantum control that leverages recent advances in the direct estimation of quantum gate fidelity. Our algorithm takes account of the stochasticity of the problem, is suitable for closed-loop control, and requires only a constant number of fidelity-estimating experiments per iteration independent of the dimension of the control space. It is efficient and robust to both statistical and technical noise.

Grover's search algorithm drives a quantum system from an initial state $|s\rangle$ to a desired final state $|t\rangle$ by using selective phase inversions of these two states. Earlier, we studied a generalization of Grover's algorithm that relaxes the assumption of the efficient implementation of $I_s$, the selective phase inversion of the initial state, also known as a diffusion operator. This assumption is known to become a serious handicap in cases of physical interest. Our general search algorithm works with almost any diffusion operator $D_s$ with the only restriction of having $|s\rangle$ as one of its eigenstates. The price that we pay for using any operator is an increase in the number of oracle queries by a factor of $O\left(B\right)$, where $B$ is a characteristic of the eigenspectrum of $D_s$ and can be large in some situations. Here we show that by using a quantum Fourier transform, we can regain the optimal query complexity of Grover's algorithm without losing the freedom of using any diffusion operator for quantum searching. However, the total number of operators required by the algorithm is still $O\left(B\right)$ times more than that of Grover's algorithm. So our algorithm offers an advantage only if the oracle operator is computationally more expensive than the diffusion operator, which is true in most search problems.

The nonradiative charge-transfer processes of ${\mathrm{Li}}^{+}+\mathrm{Na}\left(3s\right)$ collisions have been investigated by using the fully quantum-mechanical molecular-orbital close-coupling method and the two-center atomic-orbital close-coupling method for the energy range of ${10}^{-4}–2\mathrm{keV}/\mathrm{u}$ and $0.2–10\mathrm{keV}/\mathrm{u}$, respectively. The radiative charge-transfer, radiative decay, and radiative-association processes have been studied by employing the fully quantum, optical-potential, and semiclassical methods for the energy range of $2\times {10}^{-10}–110\mathrm{eV}/\mathrm{u}$. The nonradiative charge-transfer processes dominate the collisions for energies above 0.2 eV/u while radiative decay processes dominate in the lower-energy region. Especially, we found that the radiative-association process is more important than the radiative charge-transfer process when $E<2\times {10}^{-2}\mathrm{eV}/\mathrm{u}$. The rate coefficients of nonradiative and radiative processes are also given for the temperature range of $3\times {10}^4-2\times {10}^9\mathrm{K}$ and ${10}^{-6}-{10}^3\mathrm{K}$, respectively.

We study the interplay between the Hofstadter butterfly, strong interactions, and the Zeeman field within the mean-field Bogoliubov–de Gennes theory in real space, and explore the ground states of the attractive single-band Hofstadter-Hubbard Hamiltonian on a square lattice, including the exotic possibility of imbalanced vector potentials. We find that the cooperation between the vector potential and superfluid order breaks the spatial symmetry of the system, and stripe-ordered Fulde-Ferrell-Larkin-Ovchinnikov–like superfluid and supersolid phases occur that can be distinguished and characterized according to their coexisting pair-density-, charge-density-, and spin-density-wave orders. We also discuss confined systems and comment on the likelihood of observing such stripe-ordered phases by loading neutral atomic Fermi gases on laser-induced optical lattices under laser-generated artificial gauge fields.

Coherent manipulation of single-photon wave packets is essentially important for optical quantum communication and quantum information processing. In this paper, we realize controllable splitting and modulation of single-photon-level pulses by using a tripod-type atomic medium. The adoption of two control beams enable us to store one signal pulse into superposition of two distinct atomic collective excitations. By controlling the time delay between the two control pulses, we observe splitting of a stored wave packet into two temporally distinct modes. By controlling the frequency detuning of the control beams, we observe both temporal and frequency-domain interference of the retrieval signal pulses, which provides a method for pulse modulation and multisplitting of the signal photons.

We develop a nonlinear model of a laser exploiting two-dimensional (2D) distributed feedback (2D DFL). This feedback mechanism can be realized in a 2D Bragg resonator formed by a dielectric structure with the thickness having double-periodical sinusoidal or chessboard modulation. A 2D Bragg resonator is shown to possess high selectivity over both the longitudinal and the transverse coordinates and to have fundamental modes in the center of the forbidden band gap. Within the semiclassical approach, we study the nonlinear dynamics of 2D DFL and demonstrate spatial synchronization of radiation from an extended active medium. Specific features of 2D DFL operation with active medium amplifying modes of TM and TE (quantum-well lasers) polarization are also discussed.

The influence of the initial preparation on dephasing in open quantum dynamics is studied using an exactly solvable model of a two-level system (qubit) interacting with a bosonic bath. It is found that, for some classes of nonselective preparation measurements, qubit-bath correlations lead to a significant enhancement of coherence in the qubit at the initial stage of evolution. The time behavior of the qubit purity and entropy in the regime of enhancement of coherence is considered for different temperatures and coupling strengths.

We address a broad class of optimization problems of finding quantum measurements, which includes the problems of finding an optimal measurement in the Bayes criterion and a measurement maximizing the average correct probability with a fixed rate of inconclusive results. Our approach can deal with any problem in which each of the objective and constraint functions is formulated by the sum of the traces of the multiplication of a Hermitian operator and a detection operator. We first derive dual problems and necessary and sufficient conditions for an optimal measurement. We also consider the minimax version of these problems and provide necessary and sufficient conditions for a minimax solution. Finally, for optimization problem having a certain symmetry, there exists an optimal solution with the same symmetry. Examples are shown to illustrate how our results can be used.

To the active-basis-choice decoy-state quantum-key-distribution systems with detector efficiency mismatch, we present a modified attack strategy, which is based on the faked states attack, with quantum nondemolition measurement ability to restress the threat of detector efficiency mismatch. Considering that perfect quantum nondemolition measurement ability doesn't exist in real life, we also propose a practical attack strategy using photon number resolving detectors. Theoretical analysis and numerical simulation results show that, without changing the channel, our attack strategies are serious threats to decoy-state quantum-key-distribution systems. The eavesdropper may get some information about the secret key without causing any alarms. Besides, the lower bound of detector efficiency mismatch to run our modified faked states attack successfully with perfect quantum nondemolition measurement ability is also given out, which provides the producers of quantum-key-distribution systems with a reference and can be treated as the approximate secure bound of detector efficiency mismatch in decoy-state quantum-key-distribution systems.