We demonstrate the Purcell spontaneous-emission enhancement effect for Raman scattering from gases in a $\sim$10-$\mu$m-long Fabry-Pérot microcavity with a mode volume of $\sim$100 $\mu$m${}^3$ and a peak finesse of 50 000. The cavity increases the pump laser intensity by resonant recirculation while simultaneously enhancing the rate of Raman scattering into a separate cavity mode by modification of the density of states of the intracavity electromagnetic vacuum field. The result is an overall enhancement of $\sim$10${}^7$ compared to free-space emission into the same solid angle as the cavity, enabling trace gas sensing requiring only minute sample quantities.

In this paper we investigate the ground- and excited-state properties of a two-mode Dicke model, which is realized in an ensemble of many four-level atoms interacting simultaneously with two quantized cavity fields and two pairs of Raman lasers. We reveal rich phase diagrams, including a first-order quantum phase transition from an electric superradiant phase to a magnetic superradiant phase. In particular, when the two collective atom-photon coupling strengths are equal, we find a hidden continuous $\text{U}\left(1\right)$ symmetry for the two-mode Dicke model. If these identical coupling strengths are sufficiently strong, a quantum phase called the electromagnetic superradiant phase is predicted. In such a phase, the predicted continuous $\text{U}\left(1\right)$ symmetry breaks down and the gapless excitation spectrum (the Nambu-Goldstone mode), which could be measured experimentally by combining Bragg spectroscopy and cavity-enhanced Bragg scattering, is thus achieved. Our results pave the way to explore the physics for a model with continuous symmetry in quantum optics without using the rotating-wave approximation.

We investigate the single-photon transport properties of a one-dimensional coupled cavity array (CCA) containing a single qubit in its central site by coupling the CCA to two transmission lines supporting propagating bosonic modes with linear dispersion. We find that even in the nominally weak light-matter coupling regime, the transmission through a long array exhibits two ultra-narrow resonances corresponding to long-lived, self-protected polaritonic states localized around the site containing the qubit. The lifetime of these states is found to increase exponentially with the number of array sites, in sharp distinction to the polaritonic Bloch modes of the cavity array.

As shown in the experiment of She et al. [Phys. Rev. Lett. 101, 243601 (2008)], a weak laser beam sent through a vertically hanging fiber exerts a transverse force and produces a lateral displacement of the fiber's lower end. The experiment is of obvious theoretical interest in connection with the electromagnetic theory of media. Suggested explanations given for this effect in the past include the famous Abraham-Minkowski issue concerning the “correct” photon momentum in matter. In our opinion such an explanation is most likely not right. Instead, we propose a very simple description of the effect implying that the sideways deflection is caused by the radiation force on the obliquely cut lower end face of the fiber. From a calculation based upon geometrical optics, we find quite good agreement with the observations. We present also, as an alternative approach, a calculation involving wave optics instead of geometrical optics, and find comparable results.

In previous work, we proposed a method for leveraging efficient classical simulation algorithms to aid in the analysis of large-scale fault-tolerant circuits implemented on hypothetical quantum information processors. Here, we extend those results by numerically studying the efficacy of this proposal as a tool for understanding the performance of an error-correction gadget implemented with fault models derived from physical simulations. Our approach is to approximate the arbitrary error maps that arise from realistic physical models with errors that are amenable to a particular classical simulation algorithm in an “honest” way; that is, such that we do not underestimate the faults introduced by our physical models. In all cases, our approximations provide an “honest representation” of the performance of the circuit composed of the original errors. This numerical evidence supports the use of our method as a way to understand the feasibility of an implementation of quantum information processing given a characterization of the underlying physical processes in experimentally accessible examples.

We investigate orientation effects in the fragmentation of He${}_2$ and ${\mathrm{Ne}}_2$ induced by $S^{14+}$ projectiles at an impact energy of 11.37 MeV/u. Multiple ionization shows a strong dependence on the orientation of the dimer axis with respect to the projectile beam axis. We attribute these effects to the impact-parameter-dependent ionization probability $P\left(b\right)$ for the atomic scattering process S${}^{14+}+\mathrm{He}$ and S${}^{14+}+\mathrm{Ne}$ and compare our data with a Monte Carlo simulation.

We theoretically investigate high-order harmonic generation in UV-assisted midinfrared plasmonic fields. It is found that the quantum paths contributing to the harmonics can be dramatically enhanced by adding a femtosecond UV pulse, which permits high efficient harmonic radiation under relatively low driving intensity condition and thus potentially avoids thermal damage of nanosystems. We present discussions on the influence of UV pulse parameters, including the delay, pulse duration, wavelength, and intensity, to demonstrate the prediction. Our results also show that the UV pulse, acting as a temporal trigger, in combination with the spatial inhomogeneity effect of the midinfrared plasmonic field, is robust for broadband supercontinuum generation, and this process is general and not limited to a certain driving pulse duration, inhomogeneous distribution form, and carrier-envelope phase of the midinfrared electric field.

We theoretically study the generation of coherence in generalized two-level atoms with hyperfine structure by utilizing the detuning-induced stimulated Raman adiabatic passage (D-STIRAP). As expected, the degree of attainable coherence between the ground and excited states cannot be as large as that for the ideal two-level atoms without hyperfine structure. However, we find that the substantial degree of coherence can still be produced with small modulations, and the modulation period is essentially determined by the hyperfine splittings in the ground and excited states. The D-STIRAP scheme in generalized two-level atoms is sufficiently robust against the various parameters such as intensities of lasers, initial detuning, time delay between the laser pulses, and Doppler broadening. As a specific example, we show realistic results for the $D_1$ transition of Na.

We analyze a resonant mechanism for spontaneous laser-assisted electron bremsstrahlung (BrS) involving the resonant transition (via either laser-assisted electron-ion recombination or electron-atom attachment) into a laser-dressed intermediate quasibound state (corresponding, respectively, to either a field-free neutral atom or a negative-ion bound state) accompanied by ionization or detachment of this state by the laser field. This mechanism leads to resonant enhancement (by orders of magnitude) of the BrS spectral density for emitted photon energies corresponding to those for laser-assisted recombination or attachment. We present an accurate parametrization of the resonant BrS amplitude in terms of the amplitudes for nonresonant BrS, for recombination or attachment to the intermediate state, and for ionization or detachment of this state. The high accuracy of our general analytic parametrization of the resonant BrS cross section is shown by comparison with exact numerical results for laser-assisted BrS spectra obtained within time-dependent effective range theory. Numerical estimates of resonant BrS in electron scattering from a Coulomb potential are also presented.

We compute the ground-state phase diagram of the two-dimensional (2D) Bose-Hubbard model with anisotropic hopping using quantum Monte Carlo simulations, connecting the one-dimensional (1D) to the 2D system. We find that the tip of the lobe lies on a curve controlled by the 1D limit over the full anisotropy range, while the universality class is always the same as in the isotropic 2D system. This behavior can be derived analytically from the lowest renormalization-group equations and has a shape typical for the underlying Kosterlitz-Thouless transition in one dimension. We also compute the phase boundary of the Mott lobe at unit density for strong anisotropy and compare it to the 1D system. Our calculations shed light on recent cold gas experiments monitoring the dynamics of an expanding cloud.

The propagation characteristics of Airy beams is investigated and fully described under the traveling-wave approach analogous to that used for nondiffracting Bessel beams. This is possible when noticing that Airy functions are, in fact, Bessel functions of fractional order $\frac{1}{3}$. We show how physical principles impose restrictions such that the nondiffracting Airy beams cannot be of infinite extent as has been argued and introduce quantitative expressions for the maximum transverse and longitudinal extent of Airy beams. We show that under the appropriate physical conditions it is possible to obtain higher-order Airy beams.

Two-photon states entangled in continuous variables such as wave vector or frequency represent a powerful resource for quantum-information protocols in higher-dimensional Hilbert spaces. At the same time, there is a problem of addressing separately the corresponding Schmidt modes. We propose a method of engineering two-photon spectral amplitude in such a way that it contains several nonoverlapping Schmidt modes, each of which can be filtered losslessly. The method is based on spontaneous parametric down-conversion (SPDC) pumped by radiation with a comblike spectrum. There are many ways of producing such a spectrum; here we consider the simplest one, namely, passing the pump beam through a Fabry-Perot interferometer. For the two-photon spectral amplitude (TPSA) to consist of nonoverlapping Schmidt modes, the crystal dispersion dependence, the length of the crystal, the Fabry-Perot free spectral range, and its finesse should satisfy certain conditions. We experimentally demonstrate the control of TPSA through these parameters. We also discuss a possibility to realize a similar situation using cavity-based SPDC.

The ground state of interacting spin chains in external magnetic fields can undergo a quantum phase transition (QPT) characterized by dramatic changes at a critical value of the magnetic field. In this paper, we use Bell-type inequalities to study the multipartite correlations (including multipartite entanglement and multipartite nonlocality in an $n$-spin subsystem) in the QPT of an infinite $XY$ chain. An efficient numerical optimization procedure is proposed to figure out the violation measure $M_n$ of the inequalities. For $n\le 7$, the magnetic-field ($\lambda$) dependence of $M_n$ is studied. We find the derivative of $M_n$ is divergent exactly at the QPT point ${\lambda }_c=1$ for any $n$. In addition, with the increase of $n$, $M_n$ converges quickly for $\lambda <{\lambda }_c$ and converges very slowly for $\lambda >{\lambda }_c$, which can be regarded as another signal for the QPT. Furthermore, in the vicinity of ${\lambda }_c$, high-order Bell-type inequalities will be violated as long as $n$ is large enough. This indicates that high-level multipartite correlation will be present when the system is in the vicinity of the QPT point. Nevertheless, genuine $n$-partite entanglement or genuine $n$-partite nonlocality is not observed in the QPT.

The perturbed relativistic coupled-cluster (PRCC) theory is applied to calculate the electric dipole polarizabilities of alkaline-earth-metal atoms. The Dirac-Coulomb-Breit atomic Hamiltonian is used and we include the triple excitations in the relativistic coupled-cluster theory. The theoretical issues related to the triple excitation cluster operators are described in detail and we also provide details on the computational implementation. The PRCC theory results are in good agreement with the experimental and previous theoretical results. We, then, highlight the importance of considering the Breit interaction for alkaline-earth-metal atoms.

Our work is devoted to developing a general approach for nonpenetrating Rydberg states of polar molecules. We propose a method to estimate the accuracy of calculation of their wave functions and quantum defects. Basing on this method we estimate the accuracy of Born-Oppenheimer (BO) and inverse Born-Oppenheimer (IBO) approximations for these states. This estimation enables us to determine the space and energy regions where BO and IBO approximations are valid. It depends on the interplay between $l$ coupling (due to dipole potential of the core) and $l$ uncoupling (due to rotation the core). Next we consider the intermediate region where both BO and IBO are not valid. For this intermediate region we propose a modification of Fano's multichannel quantum defect theory to match BO and IBO wave functions and show that it gives more reliable results. They are demonstrated on the example of SO molecule.

We derive the relativistic formulas for the van der Waals coefficients of Yb–alkali-metal dimers that correlate to ground and excited separated-atom limits. We calculate $C_6$ and $C_8$ coefficients of particular experimental interest. We also derive a semiempirical formula that expresses the $C_8$ coefficient of heteronuclear $A+B$ dimers in terms of the $C_6$ and $C_8$ coefficients of homonuclear dimers and the static dipole and quadrupole polarizabilities of the atomic states $A$ and $B$. We report results of calculation of the $C_6$ coefficients for the Yb-Rb ${}^3{P}_1^o+5s{}^2{S}_{1/2}$ and ${}^1{S}_0+5p{}^2{P}_{1/2}^o$ dimers, and the $C_8$ coefficients for the Yb-Li ${}^1{S}_0+2s{}^2{S}_{1/2}$ and Yb-Rb ${}^1{S}_0+5s{}^2{S}_{1/2}$ dimers. Uncertainties are estimated for all predicted properties.

A quantum description of the one-electron triangular H${}_3^{2+}$ molecular ion, beyond the Born-Oppenheimer approximation, is used to study the full influence of the nuclear motion on the high-intensity photoionization and harmonic generation processes. A detailed analysis of electron and proton motions and their time-dependent acceleration allows for identification of the main electron recollision events as a function of time-dependent configuration of the protons. High-order-harmonic generation photons are shown to be produced by single-electron recollision in the second half of the pulse envelope, which also induces a redshift in the harmonics, due to the rapid few-femtosecond motions of protons. Perpendicular harmonics are produced, in general, with a linearly polarized laser pulse parallel to a bond of the triangular molecule, and, in particular, the harmonics in the cutoff region are elliptically polarized. When the laser-pulse polarization is parallel to a symmetry axis of the triangular molecular ion, creation and destruction of the chemical bond perpendicular to the polarization is predicted on a near-femtosecond time scale.

The ionization of two-active-electron systems by intense laser fields is investigated theoretically. In comparison with time-dependent Hartree-Fock and exact two-electron simulation, we show that the ionization rate is overestimated in single-active-electron approximation. A modified single-active-electron model is formulated by taking into account the dynamical core polarization. Applying our approach to Ca atoms, it is found that the polarization of the core can be considered instantaneous and the large polarizability of the cation suppresses the ionization by 50% while the photoelectron cutoff energy increases slightly. The existing tunneling ionization formulation can be corrected analytically by considering core polarization.

We consider an impurity immersed in a small Fermi gas under highly elongated harmonic confinement. The impurity interacts with the atoms of the Fermi gas through an isotropic short-range potential with three-dimensional free-space $s$-wave scattering length $a_{3\text{D}}$. We investigate the energies of the molecular branch, i.e., the energies of the state that corresponds to a gas consisting of a weakly bound diatomic molecule and “unpaired” atoms, as a function of the $s$-wave scattering length $a_{3\text{D}}$ and the ratio $\eta$ between the angular trapping frequencies in the tight and weak confinement directions. The energies obtained from our three-dimensional description that accounts for the dynamics in the weak and tight confinement directions are compared with those obtained within an effective one-dimensional framework, which accounts for the dynamics in the tight confinement direction via a renormalized one-dimensional coupling constant. Our theoretical results are related to recent experimental measurements.