Rates of above-threshold detachment (ATD) of ${\mathrm{H}}^{-}$ ($s$-electron) and ${\mathrm{F}}^{-}$ ($p$-electron) ions, calculated using the Sturmian-Floquet quasienergy approach, are investigated as a function of intensity for the case that the laser field intensity passes across ponderomotively shifted detachment thresholds. Pronounced enhancements of both detached electron angular distributions and fixed-angle ATD spectra in the low- and mid-energy plateau region are found at near-threshold laser intensities. These enhancements are sensitive to the initial state symmetry (i.e., the $s$ or $p$ symmetry) of the valence electron and occur in a narrow angular interval that is far from the polarization axis at electron energies near the onset of the plateau and moves toward the polarization axis with increasing electron energy. Although enhancements in detached electron energy distributions have not been observed for energies near the cutoff of the ATD plateau, the corresponding electron angular distributions are found to change shape dramatically as a function of intensity near the closing of ponderomotively shifted detachment thresholds. A comparison of our quantum-mechanically calculated angular distributions and those predicted classically in the high-energy region of the ATD plateau is presented and discussed.

We consider a homogeneous Bose gas of particles with an attractive interaction in an elongated three-dimensional box with periodic boundary conditions. Mean-field theory predicts for this system a spontaneous breaking of the translational symmetry at a certain value of the interaction strength. We show that at this point a second-order quantum phase transition occurs. We investigate the system in the vicinity of the critical point using Bogoliubov theory and a continuous description, that allows us to analyze quantum fluctuations in the system even when the Bogoliubov approach breaks down.

We investigate magnetic properties and statistical effects in one-dimensional (1D) strongly repulsive two-component fermions and in a 1D mixture of strongly repulsive polarized fermions and bosons. Universality in the characteristics of phase transitions, magnetization, and susceptibility in the presence of an external magnetic field $H$ are analyzed from the exact thermodynamic Bethe ansatz solution. We show explicitly that polarized fermions with a repulsive interaction have antiferromagnetic behavior at zero temperature. A universality class of linear field-dependent magnetization persists for weak and finite strong interaction. The system is fully polarized when the external field exceeds the critical value $H_c^F\approx \frac{8}{\gamma }{E}_F$, where $E_F$ is the Fermi energy and $\gamma$ is the dimensionless interaction strength. In contrast, the mixture of polarized fermions and bosons in an external field exhibits square-root field-dependent magnetization in the vicinities of $H=0$ and the critical value $H=H_c^M\approx \frac{16}{\gamma }{E}_F$. We find that a pure boson phase occurs in the absence of the external field, fully polarized fermions and bosons coexist for $0<H<H_c^M$, and a fully polarized fermion phase occurs for $H⩾H_c^M$. This phase diagram for the Bose-Fermi mixture is reminiscent of weakly attractive fermions with population imbalance, where the interacting fermions with opposite spins form singlet pairs.

Subwavelength aperture arrays in thin metal films can enable enhanced transmission of light waves. The phenomenon relies on resonant excitation and interference of the plasmon-polariton waves on the metal surface. We show a mechanism that could provide great resonant and nonresonant transmission enhancements of the light waves passed through the apertures not by the surface waves, but by the constructive interference of diffracted waves (beams generated by the apertures) at the detector placed in the far-field zone. In contrast to other models, the mechanism depends on neither the nature of the beams (continuous waves and pulses) nor material and shape of the multiple-beam source (arrays of one- and two-dimensional subwavelength apertures, fibers, dipoles, and atoms). The Wood anomalies in transmission spectra of gratings, a long standing problem in optics, follow naturally from the interference properties of our model. The point is the prediction of the Wood anomaly in a classical Young-type two-source system. The mechanism could be interpreted as a nonquantum analog of the super-radiance emission of a subwavelength ensemble of atoms (the light power and energy scales as the number of light-sources squared, regardless of periodicity) predicted by the well-known Dicke quantum model.

We investigate the effects of sub-Poissonian photon statistics and photon antibunching in the light generation by a photonic-crystal one-atom laser. The physical system consists of a two-level light emitter strongly coupled to a high-quality microcavity engineered within a photonic crystal and coherently driven by a strong external laser field. This study reveals that the electromagnetic environment provided by the photonic crystal facilitates light generation characterized by pronounced sub-Poissonian photon statistics and photon antibunching, and strongly enhanced relative to that from a one-atom laser in a conventional optical cavity. The characteristics of the cavity photon statistics are fundamentally distinct from those of a corresponding microcavity in ordinary vacuum. For large discontinuities in the photon density of states between Mollow spectral components of atomic resonance fluorescence, in the good cavity regime, the photon statistics is sub-Poissonian, in contrast to the case of a conventional cavity where sub-Poissonian photon statistics is present only for a bad cavity. These results suggest the possibility of using a photonic-crystal one-atom laser as an efficient source of nonclassical light.

The Schmidt measure was introduced as a tool to quantify the degree of multipartite entanglement, which is a natural generalization of the Schmidt rank for a bipartite system and is particularly suitable for characterizing the multiparticle entanglement of graph states and other quantum states with underlying discrete structures. In this paper, by a counterexample we show that the Schmidt measure is not fully additive for pure states.

The continuous spontaneous localization (CSL) theory of dynamical wave-function collapse is an experimentally testable alternative to nonrelativistic quantum mechanics. In it, collapse occurs because particles interact with a classical random field. However, particles gain energy from this field, i.e., particle energy is not conserved. Recently, it has been shown how to construct a theory dubbed ‘‘completely quantized collapse’’ (CQC) which is predictively equivalent to CSL. In CQC, a quantized random field is introduced, and CSL’s classical random field becomes its eigenvalue. In CQC, energy is conserved, which allows one to understand that energy is conserved in CSL, as the particle’s energy gain is compensated by the random field’s energy loss. Since the random field has energy, it should have gravitational consequences. For that, one needs to know the random field’s energy density. In this paper, it is shown how to construct a symmetric, conserved, energy-momentum-stress-density tensor associated with the quantized random field, even though this field obeys no dynamical equation and has no Lagrangian. Then, three examples are given involving the random field’s energy density. One considers interacting particles, the second treats a “cosmological” particle creation model, the third involves the gravity of the random field.

We propose a scheme to generate two-photon, two-atom, or atom-photon entangled states with a coupled system of two cavities. In our scheme, two cavity photons are exchanged by the direct intercavity coupling, while atoms in the cavities simply generate and probe them. By virtue of the high efficiency of atomic state measurement, this method enables the realization of efficient heralded entanglement generation which is robust against photon loss, which greatly facilitates applications in quantum-information processing.

Quasimolecular dipole transition moments have been considered analytically within the framework of the two-state approximation. In the process, the conditions that lead to the occurrence of roots (zeros) for the dipole moments have been carefully studied. The underlying reason for these roots is the orthogonality of the quasimolecular adiabatic states. In the second part of the theory specific attention is given to spectral manifestations connected with dipole moment peculiarities. Among the main enhanced results, it was found that the existence of a root in the vicinity of the extremum of the potential energy curve produces a dip in the spectral profile rather than a satellite as usually supposed. Our work allows an obvious advance in the theory of radiative collisions and the study of the influence of the interaction in quasimolecules.

Detailed calculations have been carried out for the direct electron-impact excitation and resonant excitation cross sections from the ground state to the individual magnetic sublevels of ${\left(2p_{3∕2}^{-1}3s_{1∕2}\right)}_2$ of highly charged ions of neonlike barium by using a fully relativistic distorted-wave method. The contributions of resonant excitations to the linear polarization of the magnetic quadrupole $\left(M2\right)$ line $\left[{\left(2p_{3∕2}^{-1}3s_{1∕2}\right)}_2\text{−}2p^6{}^{1}S_0\right]$ have been investigated systematically. It is found that the $4l5l^{\prime }$, $4l6l^{\prime }$, and $5l5l^{\prime }$ resonances have significant depolarizing or enhancing effects on the linear polarization. These results agree very well with the experimental measurements by Takács et al. [Phys. Rev. A 54, 1342 (1996)].

We have theoretically studied the population transfer in a five-level ${\mathrm{Li}}_2$ dimer by $(2+2)$-photon stimulated hyper-Raman nonadiabatic passage (STIHRNAP) from the initial ground level $v_g=0$, $J_g=0$ to the final rovibrational levels $v_f=1$ (2), $J_f=0,2$, of the ground electronic state $X{}^1\Sigma _g^{+}$ via the resonant intermediate levels $v_i=1$, $J_i=0,2$, of the excited electronic state $2{}^1\Sigma _g^{+}$. Linearly chirped pump and Stokes laser pulses with different chirp rates and without any initial detuning are applied simultaneously. Both the pulses are taken to have the same temporal shape, pulse width, and linear parallel polarizations. The density matrix method has been used to compute the populations of the levels. We have investigated in detail the population transfer for laser wavelengths (at time $t=0$) in the range of $992–1028\mathrm{nm}$ and (peak) intensities in the range of $1.5\times {10}^{10}–3.0\times {10}^{11}\mathrm{W}∕{\mathrm{cm}}^2$. The required pulse widths are of the order of $35–275\mathrm{ps}$ for maximum population inversion. We have controlled rotational branching in population transfer to the final rotational levels $J_f=0$ ($Q$ branch) and $J_f=2$ ($S$ branch) of the fundamental $(v_f=1)$ and first overtone $(v_f=2)$ transitions by judicious choice of laser parameters. We have applied a $(2+2)$-photon STIHRNAP process to a model multilevel molecular system $\left({\mathrm{Li}}_2\right)$ and achieved almost complete population transfer from the initial ground to the final target rovibrational levels with chirped ir laser pulses.

Guided-wave atom interferometers measure interference effects using atoms held in a confining potential. In one common implementation, the confinement is primarily two dimensional, and the atoms move along the nearly free dimension after being manipulated by an off-resonant standing wave laser beam. In this configuration, residual confinement along the nominally free axis can introduce a phase gradient to the atoms that limits the arm separation of the interferometer. We experimentally investigate this effect in detail, and show that it can be alleviated by having the atoms undergo a more symmetric motion in the guide. This can be achieved by either using additional laser pulses or by allowing the atoms to freely oscillate in the potential. With these techniques, we demonstrate interferometer measurement times up to $72\mathrm{ms}$ and arm separations up to $0.42\mathrm{mm}$ with a well controlled phase, or times of $0.91\mathrm{s}$ and separations of $1.7\mathrm{mm}$ with an uncontrolled phase.

We demonstrate that strongly nonlocal nonlinear media can support two-dimensional solitons in the form of rotating crescent vortices, which are generated by a superposition of two ordinary concentric vortices with equal powers and topological charges differing by 1 (similar objects were recently found in the local Gross-Pitaevskii equation with rotation and an anharmonic trapping potential). These solitons demonstrate additional swinging motion under the action of a kick. Nested crescent solitons, which feature synchronous rotation, are generated by the superposition of several pairs of vortices. Power imbalance between the underlying vortices gives rise to unsteady crescent solitons.

We demonstrate two orders of magnitude enhancement of light absorption in certain thin-film photonic crystal (PC) architectures due to strong resonances arising from parallel interface refraction (PIR). This anomalous type of refraction is acutely negative and usually out of the plane of incidence. Over a wide range of frequencies, light impinging on idealized two-dimensional (2D) thin-film photonic crystals, over a cone of at least 20° in off-normal directions, couples to Bloch modes propagating nearly parallel to the thin-film–to–air interface. For realistic three-dimensional PC films of cubic symmetry, synthesized by photoelectrochemical etching, the PIR effect persists over a spectral range of at least 15% relative to the center frequency and within a cone of 50° of incident angles, normal to the film. This leads to anomalously long optical path lengths and long dwell times before the light beam exits the thin film. This near continuum of high-quality-factor optical resonances, associated with “transverse optical slow modes” in a spectral range of high electromagnetic density of states, can be much more effective for trapping and absorbing light than that of the previously reported longitudinal slow-group-velocity effects. The parallel interface refraction effect is general and can be found in specific spectral ranges of both 2D and 3D photonic crystals with cubic or other appropriate symmetries. In the presence of weak optical absorption within the PC backbone, energy conversion enhancement is interpreted using a simple temporal mode-coupling model. It is shown that absorption is optimized when the structural quality factor (in the absence of absorption) of the transverse optical slow modes is comparable to $\omega {\tau }_{\mathrm{abs}}$, where $\omega$ is the optical frequency and ${\tau }_{\mathrm{abs}}$ is the absorption time scale of the film material. Quantitative numerical results for light harvesting efficiency are obtained by finite-difference time-domain simulations of the electromagnetic wave field. It is shown that small amounts of fabrication-related structural disorder can introduce additional resonances and broaden existing resonances, thereby improving the overall harvesting of broadband wide-acceptance-angle light.

A scheme for generating W state by using single-photon interference and time-bin encoding in polarization noise channels is proposed. We show that polarization noise cannot affect the fidelity of the intended entangled state generation, and that the successful probability does not change as the number of atoms in a single quantum node increases. Moreover, the proposed scheme can be generalized to prepare entanglement among $N$ quantum nodes.

A bipartite state is called classical (with regard to correlations) if it is left undisturbed by a certain local von Neumann measurement, and is called separable if it can be represented as a convex combination of product states. Due to the perfect distinguishability of orthogonal vectors, a classical state can essentially be identified with a convenient bivariate probability distribution, and moreover, it is separable, but not vice versa. The notion of separability plays a key role in quantum information theory because entanglement is defined via separability. However, the definition of separability is ad hoc and formal. In this paper, we present an intrinsic characterization of separable states via classical states from the measurement perspective: Separable states are precisely those states that are reductions of classical states in higher dimensions with the natural partitions. Consequently, entangled states are precisely those states that cannot be represented as such reductions of classical states. This observation highlights the hidden mutually exclusive and complementary relations between classicality and entanglement.

We study time evolution of entanglement between two qubits, which are part of a larger system, after starting from a random initial product state. We show that, due to randomness in the initial product state, entanglement is present only between directly coupled qubits and only for short times. Time dependence of the entanglement appears essentially independent of the specific Hamiltonian used for time evolution and is well reproduced by a parameter-free two-body random matrix model.

We compare different quantum master equations for the time evolution of the reduced density matrix. The widely applied secular approximation (rotating wave approximation) applied in combination with the Born-Markov approximation generates a Lindblad-type master equation ensuring for completely positive and stable evolution and is typically well applicable for optical baths. For phonon baths however, the secular approximation is expected to be invalid. The usual Markovian master equation does not generally preserve positivity of the density matrix. As a solution we propose a coarse-graining approach with a dynamically adapted coarse-graining time scale. For some simple examples we demonstrate that this preserves the accuracy of the integro-differential Born equation. For large times we analytically show that the secular approximation master equation is recovered. The method can in principle be extended to systems with a dynamically changing system Hamiltonian, which is of special interest for adiabatic quantum computation. We give some numerical examples for the spin-boson model of cases where a spin system thermalizes rapidly, and other examples where thermalization is not reached.

We address the limit of the Gaussian operations and classical communication in the problem of quantum state discrimination. We show that the optimal Gaussian strategy for the discrimination of the binary phase shift keyed (BPSK) coherent signal is a simple homodyne detection. We also propose practical near-optimal quantum receivers that beat the BPSK homodyne limit in all areas of the signal power. Our scheme is simple and does not require realtime electrical feedback.

We present two efficient quantum key distribution schemes over two different collective-noise channels. The accepted hypothesis of collective noise is that photons travel inside a time window small compared to the variation of noise. Noiseless subspaces are made up of two Bell states and the spatial degree of freedom is introduced to form two nonorthogonal bases. Although these protocols resort to entangled states for encoding the key bit, the receiver is only required to perform single-particle product measurements and there is no basis mismatch. Moreover, the detection is passive as the receiver does not switch his measurements between two conjugate measurement bases to get the key.