In this paper, we present a theoretical investigation for the ground state of an impurity immersed in a Fermi sea. The molecular regime is considered where a two-body bound state between the impurity and one of the fermions is formed. Both interaction and exchange of the bound fermion take place between the dimer and the Fermi sea. We develop a formalism based on a two channel model allowing us to expand systematically the ground state energy of this immersed dimer with the scattering length $a$. Working up to order $a^3$, associated to the creation of two particle-hole pairs, reveals the first signature of the composite nature of the bosonic dimer. Finally, a complementary variational study provides an accurate estimate of the dimer energy even at large scattering length.

We report on experimental and theoretical study of the nonlinear Faraday effect under conditions of electromagnetically induced transparency at the $5S_{1/2}\to 5P_{3/2}\to 5D_{5/2}$ two-photon transition in rubidium vapor. These transitions realize the inverted $Y$ model which combines the $\Lambda$ and ladder systems. Strong nonlinearity allowing for large rotation angles of a probe beam tuned to the $S\to P$ transition was obtained by creation of quantum superpositions of magnetic sublevels (Zeeman coherences) in the rubidium ground state ($\Lambda$ scheme). Additionally, electromagnetically induced transparency was accomplished in a ladder scheme by acting with a second strong coupling laser on the $P\to D$ transition. Under conditions of a two-photon resonance the rotation was significantly reduced, which is interpreted as a competition between the coherent processes. The effect was observed in sub-Gauss magnetic fields and could be used for efficient coherent control of generation of the ground-state coherences, e.g., for controlling the polarization state of the probe light.

We observed angular multiplexing storage of multiple light pulses in a ${\text{Pr}}^{3+}:{\text{Y}}_2{\text{SiO}}_5$ crystal based on electromagnetically induced transparency. Addressable routing of stored light pulses with an angular resolution of $\sim 1°$ and without cross-talk between two neighboring channels, which serves as an addressable optical buffer memory, was demonstrated experimentally in a 3-mm ${\text{Pr}}^{3+}:{\text{Y}}_2{\text{SiO}}_5$ crystal. Angular multiplexing storage of light pulses opens a way to manipulate light pulses selectively and in parallel, and therefore, is of important potential applications in quantum information processing and addressable all-optical delay lines and buffers.

We present theoretical and experimental studies on quantum path interferences in high-order harmonic generation. Simulations of the single-atom response allow us to calculate the different quantum paths contributions; their relative phases and the resulting interferences can be finely controlled through the laser intensity that provides an efficient means for controlling the electron trajectories with an accuracy on the ten attoseconds time scale. Simulations of the macroscopic response demonstrate the need of spatial and spectral filtering of the harmonic beam in order to observe the interferences between the two shortest quantum paths. Our numerical results are in very good agreement with experimental data. These investigations represent a step toward the full characterization and control of the atomic harmonic dipole.

We propose a scheme for the entanglement generation of two cavity modes using a four-level Raman-driven quantum-beat laser (QBL). The atomic coherence is generated by a Raman-type two-photon process. Two different sufficient conditions for entanglement generation have been investigated. The effect of cavity losses and strength of the driving fields on entanglement generation is studied for different initial states of the resonant cavity modes. The results are in agreement with the earlier scheme for the entanglement generation using a directly driven quantum-beat laser. We observe entanglement generation even for high cavity losses and with low strength of the driving fields. Further, there is no dipole-forbidden transition in the Raman-driven QBL system. Thus the proposed scheme for the entanglement generation is experimentally more viable as compared to the previous directly driven QBL system.

Electron emission due to grazing scattering of fast protons from LiF and KCl surfaces is studied under axial channeling conditions. The differential electron emission probability is calculated within a distorted-wave formalism, taking into account realistic channeled trajectories. For electronic spectra along low-index crystallographic directions, we find that channeling effects on the electron emission yield depend strongly on the penetration rate of projectiles into the solid.

We describe a proof-of-principle experiment demonstrating a Fourier technique for measuring the shape of biphoton wave packets. The technique is based on the use of synchronously driven fast modulators and slow (integrating) detectors.

Any continuous-variable distillation protocol that does not change the effective loss of the transmission channel cannot be used to achieve maximum Einstein-Podolsky-Rosen style correlations after distillation. We analyze the continuous-variable entanglement distillation protocol described by [Browne et al., Phys. Rev. A 67, 062320 (2003)] and show that it does not change the effective loss of the transmission of entanglement. We extend this scheme to one which does change the effective transmission coefficient of the channel. Our scheme has the added benefit that the states generated exhibit isotropic noise which is best suited to applications of continuous-variable entanglement such as continuous-variable teleportation.

We investigate the physical mechanism behind the counterintuitive phenomenon, the distribution of continuous-variable entanglement between two distant modes by sending a third separable auxiliary mode between them. For this purpose, we propose a more simple and more efficient protocol resulting in distributed entanglement with more than an order of the magnitude higher logarithmic negativity than in the previously proposed protocol. This protocol shows that the distributed entanglement originates from the entanglement of one mode and the auxiliary mode used for distribution, which is first destroyed by local correlated noises and restored subsequently by the interference of the auxiliary mode with the second distant separable correlated mode.

We analyze the density-functional theory (DFT) description of weak interactions by employing diffusion and reptation quantum Monte Carlo (QMC) calculations, for a set of benzene-molecule complexes. While the binding energies depend significantly on the exchange-correlation approximation employed for DFT calculations, QMC calculations show that the electron density is accurately described within DFT, including the quantitative features in the reduced density gradient. We elucidate how the enhancement of the exchange-energy density at a large reduced density gradient plays a critical role in obtaining accurate DFT description of weakly interacting systems.

We study the influence of optical pumping on Doppler-broadened alkali-metal-atom $D$ line spectra by solving the time-dependent density matrix for an open two-level system. The time-dependent absorption is averaged over the distribution of interaction times obtained from the three-dimensional beam geometry and atomic velocity distribution and over the longitudinal velocity distribution. The optical pumping is significant at much lower intensities than saturation and depends strongly on the intensity, beam radius, vapor cell length, and spontaneous-decay branching ratio. The result is in agreement with our earlier steady-state solution for a wide range of parameters and predicts two interesting line-shape features.

Photoionization cross sections for ${\text{Ce}}^{q+}$ $\left(1\le q\le 9\right)$ ions were measured in the 105–180 eV energy range of the $4d$ inner-shell giant resonance by merging a mass-to-charge-ratio-selected ion beam with a beam of monochromatized synchrotron radiation. The Cowan atomic structure code was used as an aid to interpret the experimental data. Four Rydberg series for $4d\to nf$ $\left(n\ge 4\right)$ and $4d\to np$ $\left(n\ge 6\right)$ autoionizing excitations were assigned using the quantum-defect theory in the ${\text{Ce}}^{3+}$ photoionization cross section. The experimental data show the collapse of the $nf$ wave functions $\left(n\ge 4\right)$ with increasing ionization stage as outer-shell electrons are stripped from the parent ion. The $nf$ orbital collapse occurs partially for ${\text{Ce}}^{2+}$ and ${\text{Ce}}^{3+}$ ions and completely for ${\text{Ce}}^{4+}$, where these wave functions penetrate the core region of the ion. A strong contribution to the total oscillator strength was observed in multiple photoionization channels for ${\text{Ce}}^{+}$, ${\text{Ce}}^{2+}$, and ${\text{Ce}}^{3+}$, whereas most of the $4d$ excitations of the higher charge states decay by ejection of one electron.

We investigate the ionization and high-order harmonic generation (HHG) of diatomic molecules using the strong-field approximation (SFA) in both the length gauge (LG) and the velocity gauge. We focus on the effect of two-center interference that has an important role in both molecular ionization and HHG. Our simulations reveal the significant disagreement between the two gauges for the description of this effect. For the behavior of molecular ionization rate with respect to the internuclear distance, which is closely related to this effect in ionization, the two gauges give qualitatively conflicting answers in some cases. For the striking hollow structure in the molecular HHG spectrum, where this effect dominates, the predictions of the two gauges also differ significantly. Further comparisons suggest the preference of the LG SFA in describing this interference effect in intense-field–molecule interaction.

We describe the efficient implementation of an explicit method to solve systems of stiff differential equations either on a grid or within a spectral approach. This method is based on an ansatz that approximates the solution. This ansatz depends on stiffness parameters that are shown to be related to the eigenfrequencies of the system. The accuracy and the performance of the method are tested in three different cases. First, we treat a highly stiff single differential equation, where explicit schemes converge rather slowly. Then, we solve the stationary Schrödinger equation associated to the quantum reflection of an ultracold atom by a surface. Finally, we consider the interaction of atomic hydrogen with a strong low-frequency laser pulse whose duration is of the order of 25 fs. We focus on the calculation of the above-threshold ionization electron spectrum, a problem which, under such realistic physical conditions, is computationally very demanding.

The Néel state in fermionic mixtures of two pseudospin species in an optical lattice is analyzed at low temperatures. Experimentally it remains a challenge to demonstrate antiferromagnetic correlations in ultracold fermionic quantum gases. We find that, while in balanced systems the Néel order parameter can point in any spatial direction, in imbalanced mixtures antiferromagnetism is strictly perpendicular to the quantization axis (i.e., the $z$ axis). Since, experimentally, one always has to assume some minimal imbalance this should have important consequences for ongoing experiments.

We investigate theoretically the evolution of the two-point density correlation function of a low-dimensional ultracold Bose gas after release from a tight transverse confinement. In the course of expansion thermal and quantum fluctuations present in the trapped systems transform into density fluctuations. For the case of free ballistic expansion relevant to current experiments, we present simple analytical relations between the spectrum of “density ripples” and the correlation functions of the original confined systems. We analyze several physical regimes, including weakly and strongly interacting one-dimensional (1D) Bose gases and two-dimensional (2D) Bose gases below the Berezinskii-Kosterlitz-Thouless (BKT) transition. For weakly interacting 1D Bose gases, we obtain an explicit analytical expression for the spectrum of density ripples which can be used for thermometry. For 2D Bose gases below the BKT transition, we show that for sufficiently long expansion times the spectrum of the density ripples has a self-similar shape controlled only by the exponent of the first-order correlation function. This exponent can be extracted by analyzing the evolution of the spectrum of density ripples as a function of the expansion time.

An ultracold gas of coupled two-component atoms in an optical field is studied. Due to the internal two-level structure of the atoms, three competing energy terms exist: atomic kinetic, atomic internal, and atom-atom interaction energies. A novel outcome of this interplay, not present in the regular Bose-Hubbard model, is that in the single band and tight-binding approximations four different phases appear: two superfluid and two Mott phases. When passing through the critical point between the two superfluid or the two Mott phases, a swapping of the internal atomic populations takes place. By means of the strong-coupling expansion, we find the full phase diagram for the four different phases.

An optical spectroscopic method based on the principle of electromagnetically induced transparency (EIT) is proposed as quite a generic probing tool that provides valuable insights into the nature of Fermi paring in ultracold Fermi gases of two hyperfine states. This technique has the capability of allowing spectroscopic response to be determined in a nearly nondestructive manner and the whole spectrum may be obtained by scanning the probe-laser frequency faster than the lifetime of the sample without re-preparing the atomic sample repeatedly. A quasiparticle picture is constructed to facilitate a simple physical explanation of the pairing signature in the EIT spectra.

Dynamic coupling of cavities to a quantum network is of major interest to distributed quantum information processing schemes based on cavity quantum electrodynamics. This can be achieved by actively tuning a mediating atom-cavity system. In particular, we consider the dynamic coupling between two coupled cavities, each interacting with a two-level atom, realized by tuning one of the atoms. One atom-field system can be controlled to become maximally and minimally coupled with its counterpart, allowing high fidelity excitation confinement, $Q$ switching, and reversible state transport. As an application, we first show that simple tuning can lead to emission of near-Gaussian single-photon pulses that is significantly different from the usual exponential decay in a passive cavity-based system. The influences of cavity loss and atomic spontaneous emission are studied in detailed numerical simulations, showing the practicality of these schemes within the reach of current experimental solid-state technology. We then show that when the technique is employed to an extended coupled-cavity scheme involving a multilevel atom, arbitrary temporal superposition of single photons can be engineered in a deterministic way.

We present a systematic method to investigate the self-similar propagation of two coupled optical pulses in nonlinear optical fiber amplifiers. By taking into account the intrinsic self-similarity of the two coupled nonlinear Schrödinger equations, we cast them into two simple algebraic equations. We then show that the determining of the most stable power profiles for the self-similar evolution of each pulse is equivalent to the finding of the ground-state solution of binary Bose-Einstein condensate mixtures. Based on this, we find a family of asymptotically exact self-similar solutions, including the parabolic similaritons and the piecewise parabolic similaritons. Further, we propose an approximate method to determine the intermediate evolution of initial pulses toward these similaritons. Both the analytical and numerical results show that when the parameter of cross-phase modulation nonlinearity $b$ is smaller (larger) than 1, the two coupled pulses favor the coexistence (separation) state, while when $b=1$, the collisionless shock waves could emerge.