We study directional emission through dynamical tunneling in a two-dimensional deformed microcavity. The tunneling emissions are characterized by free-space emergence with a finite distance $\delta$ from the cavity boundary. As the cavity deformation increases, it is shown numerically that one of two main outputs becomes dominant with a decreasing $\delta$ value. This asymmetrical enhancement of tunneling outputs can be understood by the influence of asymmetric short-time ray dynamics near the critical line on the dynamical tunneling process. In addition, the change of $\delta$ with deformation can be explained by the interference of tangential wave components brought by the dynamical tunneling.

We introduced a weak prepulse 0–2 ns before the main pulse in the formation of a terahertz (THz) wave in air plasma by laser pulses composed of fundamental and second-harmonic waves. The prepulse suppressed the THz wave generated by the main pulse. We analyzed the suppression factor as a function of the time interval between the prepulse and the main pulse and found that the THz wave from the main pulse decreases immediately after the prepulse, recovers partially within several hundred picoseconds, and then recovers slowly. This suppression is attributed to the dense plasma, which absorbs and screens the generated THz wave, since the relaxation time is close to the lifetime of the fluorescence from the plasma. The results support the idea that the major mechanism of THz wave generation is explained by the plasma current model.

We experimentally study the strongly coupled three-level atom-cavity system at both cavity and coupling frequency detuning cases. Side peak splitting and anti-crossing-like phenomena are observed under different experimental conditions. Intracavity dispersion properties are used to explain qualitatively the complicated cavity resonance structures in the composite system of inhomogeneously broadened three-level atoms inside an optical ring cavity with relatively strong driving intensities.

We provide an approach to significantly break the diffraction limit in coherent anti-Stokes Raman scattering (CARS) microscopy via an additional probe-beam-induced photon depletion (APIPD). The additional probe beam, whose profile is doughnut shaped and whose wavelength is different from the Gaussian probe beam, depletes the phonons to yield an unwanted anti-Stokes signal within a certain bandwidth at the rim of the diffraction-limited spot. When the Gaussian probe beam that follows immediately arrives, no anti-Stokes signal is generated in this region, resembling stimulated emission depletion (STED) microscopy, and the spot-generating useful anti-Stokes signals by this beam are substantially suppressed to a much smaller dimension. Scanning the spot renders three-dimensional, label-free, and chemically selective CARS images with subdiffraction resolution. Also, resolution-enhanced images of the molecule, specified by its broadband even-total CARS spectral signals not only by one anti-Stokes signal for its special chemical bond, can be obtained by employing a supercontinuum source.

The role of the interaction with the nearest electronic state ${}^3{\Sigma }_{0^{-}}^{+}$ on the hyperfine structure and magnetic properties of the $a(1)[{}^3{\Sigma }_1^{+}]$ state of PbO is assessed. The accounting for this contribution leads to the difference between $g$ factors of the $J=1$ $\Omega$-doublet levels $\Delta g=37\times {10}^{-4}$, which is in good agreement with the experimental data $\Delta g=30(8)\times {10}^{-4}$. The contribution of this interaction rapidly grows with $J$. For $J=30$, the difference of $g$ factors of $\Omega$-doublet states reaches 100%; for hyperfine constants, it reaches 18%. These differences also depend on the electric field, and, for $E=11$ V/cm for ${}^{207}\mathrm{Pb}$O, the difference in $g$ factors turns to zero. The latter is important for suppressing systematic effects in the electron electric dipole moment search experiment.

Several laser techniques have been suggested and demonstrated recently for preparing polarizable molecules in rapidly spinning states with a disk-like angular distribution. We consider motion of these spinning disks in inhomogeneous fields and show that the molecular trajectories may be precisely controlled by the tilt of the plane of the laser-induced rotation. The feasibility of the scheme is illustrated by optical deflection of linear molecules twirled by two delayed cross-polarized laser pulses. These results open new ways for many applications involving molecular focusing, guiding, and trapping and may be suitable for separating molecular mixtures by optical and static fields.

A mixture of two species of pseudospin-$\frac{1}{2}$ Bose gases exhibits interesting interplay between spin and orbital degrees of freedom. Expectation values of various quantities of the collective spins of the two species play crucial roles in the Gross-Pitaevskii-like equations governing the four orbital wave functions in which Bose-Einstein condensation occurs. Consequently, the elementary excitations of these orbital wave functions reflect properties of the collective spins. When the coupling between the two collective spins is isotropic, the energy gap of the gapped orbital excitation peaks. There is a quantum phase transition in the ground state of the effective Hamiltonian of the two collective spins, which have previously been found to be maximally entangled.

We construct an analog computer based on light interference to encode the hyperbolic function $f(\zeta )\equiv 1/\zeta$ into a sequence of skewed curlicue functions. The resulting interferogram when scaled appropriately allows us to find the prime number decompositions of integers. We implement this idea exploiting polychromatic optical interference in a multipath interferometer and factor seven-digit numbers. We give an estimate for the largest number that can be factored by this scheme.

Analytic formulas describing high-order harmonic generation (HHG) by atoms in a short laser pulse are obtained quantum mechanically in the tunneling limit. These results provide analytic expressions of the three-step HHG scenario, as well as of the returning electron wave packet, in a few-cycle pulse. Our results agree well with those of numerical solutions of the time-dependent Schrödinger equation for the H atom, while for Xe they predict many-electron atomic dynamics features in few-cycle HHG spectra and significant dependence of these features on the carrier-envelope phase of a laser pulse.

By measuring the density fluctuations in a highly elongated weakly interacting Bose gas, we observe and quantify the transition from the ideal gas to a quasicondensate regime throughout the dimensional crossover from a purely one-dimensional (1D) to an almost three-dimensional (3D) gas. We show that that the entire transition region and the dimensional crossover are described surprisingly well by the modified Yang-Yang model. Furthermore, we find that at low temperatures the linear density at the quasicondensate transition scales according to an interaction-driven scenario of a longitudinally uniform 1D Bose gas, whereas at high temperatures it scales according to the degeneracy-driven critical scenario of transverse condensation of a 3D ideal gas.

Phase precision in optimal two-channel quantum interferometry is studied in the limit of large photon number $N\gg 1$, for losses occurring in either one or both channels. For losses in one channel an optimal state undergoes an intriguing sequence of local bifurcations as the number of photons (or losses) increase. The optimal state has a continuous form in the Fock state basis for large $N$. The loss parameter limits any precision improvement over classical light to at most a constant factor independent of $N$. We determine a crossover value of photon number $N_c$ beyond which supraclassical precision is progressively lost.

We propose one and a half criteria for determining how many measurements are needed to quantify entanglement reliably. We base these criteria on Bayesian analysis of measurement results and apply our methods to four-qubit entanglement, but generalizations to more qubits are straightforward.

We consider a set of fully connected spin models that display first- or second-order transitions and for which we compute the ground-state entanglement in the thermodynamical limit. We analyze several entanglement measures (concurrence, Rényi entropy, and negativity) and show that, in general, discontinuous transitions lead to a jump of these quantities at the transition point. Interestingly, we also find examples where this is not the case.

We present a general framework that reveals substructures of genuine multipartite entanglement. Via simple inequalities it is possible to discriminate different sets of multipartite qubit states. These inequalities are beneficial regarding experimental examinations as only local measurements are required. Furthermore, the number of observables scales favorably with system size. In exemplary cases we demonstrate the noise resistance and discuss implementations.

Secure two-party cryptography is possible if the adversary’s quantum storage device suffers imperfections. For example, security can be achieved if the adversary can store strictly less then half of the qubits transmitted during the protocol. This special case is known as the bounded-storage model, and it has long been an open question whether security can still be achieved if the adversary’s storage were any larger. Here, we answer this question positively and demonstrate a two-party protocol which is secure as long as the adversary cannot store even a small fraction of the transmitted pulses. We also show that security can be extended to a larger class of noisy quantum memories.

The resonance energy spectrum of a system interacting with the Yukawa potential and embedded in an external homogeneous and static electric field is investigated. The positions and widths of levels have been computed by using the complex coordinate rotation method. Interesting avoided-crossing and crossing effects appear.

We systematically study the influence of phase matching on interference minima in high-harmonic spectra. We concentrate on structures in atoms due to interference of different angular-momentum channels during recombination. For this purpose, we use the Cooper minimum (CM) in argon at $~$47 eV as a marker in the harmonic spectrum. We measure two-dimensional harmonic spectra in argon as a function of wavelength and angular divergence. While we identify a clear CM in the spectrum when the target gas jet is placed after the laser focus, we find that the appearance of the CM varies with angular divergence and can even be completely washed out when the gas jet is placed closer to the focus. We also show that the argon CM appears at different wavelengths in harmonic and photoabsorption spectra measured under conditions independent of any wavelength calibration. We model the experiment with a simulation based on coupled solutions of the time-dependent Schrödinger equation and the Maxwell wave equation, thereby including both the single-atom response and macroscopic effects of propagation and phase matching. The single-atom calculations confirm that the ground state of argon can be represented by its field-free $p$ symmetry, despite the strong laser field used in high-harmonic generation. Because of this, the CM structure in the harmonic spectrum can be described as the interference of continuum $s$ and $d$ channels, whose relative phase jumps by $\pi$ at the CM energy, resulting in a minimum shifted from the photoionization result. We also show that the full calculations reproduce the dependence of the CM on the macroscopic conditions. We calculate simple phase-matching factors as a function of harmonic order and explain our experimental and theoretical observation in terms of the effect of phase matching on the shape of the harmonic spectrum. Our results emphasize that phase matching must be taken into account to fully understand spectral features related to harmonic spectroscopy. Furthermore, we show that in some cases phase matching can be actively used to enhance the visibility of interference minima in high-harmonic spectra.

We study time delay for the process of photoionization of a hydrogen atom in a strong electromagnetic field. We compute this quantity by solving the time-dependent Schrödinger equation. We show that even a moderately strong field can have quite a considerable effect on the time delay. Analysis of the wave-packet motion performed by means of the Gabor transform shows that a simple semiclassical model can explain this phenomenon.

The theory of resonant Auger decay of atoms in a high-intensity coherent x-ray pulse is presented. The theory includes the coupling between the ground state and the resonance due to an intense x-ray pulse, taking into account the decay of the resonance and the direct photoionization of the ground state, both populating the final ionic states coherently. The theory also considers the impact of the direct photoionization of the resonance state itself which typically populates highly excited ionic states. The combined action of the resonant decay and of the direct ionization of the ground state in the field induces a non-Hermitian time-dependent coupling between the ground and the “dressed” resonance stats. The impact of these competing processes on the total electron yield and on the 2$s$${}^2$2$p$${}^4({}^{1}D)$3$p$ ${}^{2}P$ spectator and $2s^{1}2p^6$ ${}^{2}S$ participator Auger decay spectra of the Ne $1s\to 3p$ resonance is investigated. The role of the direct photoionization of the ground state and of the resonance increases dramatically with the field intensity. This results in strong interference effects with distinct patterns in the electron spectra, which differ for the participator and spectator final states.

A mechanism is proposed to explain the puzzling damping of collective excitations, which was recently observed in the experiments of strongly interacting Fermi gases below the superfluid critical temperature on the fermionic (BCS) side of Feshbach resonance. Sound velocity, superfluid density, and damping rate are calculated with effective field theory. We find that a dominant damping process is due to the interaction between superfluid phonons and thermally excited fermionic quasiparticles, in contrast to the previously proposed pair-breaking mechanism. Results from our effective model are compared quantitatively with recent experimental findings, showing a good agreement.