We demonstrate a technique for broadband conical pulse generation based on a continuum seeded parametric amplification process in quadratic nonlinear media. We show that by using an appropriately focused broadband weak seed we are able to selectively generate and amplify a single phase-locked conical wave packet. We present experimental spatiotemporal spectra obtained with two different crystals in type I phase-matching configurations, showing the generation and amplification of single X-type and O-type wave packets, respectively, and we compare the results with the stationary wave modes that can be supported in the parametric amplification process. On the basis of numerical calculations, we also show that the broadband $\text{signal}+\text{idler}$ phase-matched field generated by the amplifier may be characterized by a well-defined phase velocity, whose control is guaranteed by the tunability of the amplifier spectral gain curves.

Spectral properties of entangled photons generated via type-I frequency-nondegenerate spontaneous parametric down-conversion are studied theoretically and experimentally. For monochromatic pumping, the joint spectral intensity function for the photon pair is found to be symmetric even though each photon has a different central wavelength. The single-photon (first-order) interference shows the familiar triangular interferogram, usually associated with the cw-pumped frequency-degenerate type-II spontaneous parametric down-conversion. For broadband (ultrafast) pumping, on the other hand, the joint spectral intensity function is shown to be asymmetric, similar to the case of ultrafast-pumped frequency-degenerate type-II spontaneous parametric down-conversion. The spectral studies reported here are expected to play an important role in developing ultrabright two-color entangled photon sources.

We propose a cooling mechanism for ultracold fermionic atom ensembles, which capitalizes on the energy dependence of inelastic collisions near a Feshbach resonance. We first discuss a single magnetic resonance and find that the final temperature and cooling rate is limited by the resonance width. A concrete example for a $p$-wave resonance of ${}^{40}\text{K}$ is given. We then improve upon this setup by using both a sharp optical or radio-frequency-induced resonance and a broad magnetic resonance and show that one can improve upon temperatures reached with current technologies.

In this work, we report an experimental cross-section determination of elastic electron collisions with tetrahydrofuran in the intermediate-energy range. More specifically, absolute differential cross sections are measured and reported in the (50–1000) eV range. The measurements were performed using a crossed electron beam–molecular beam geometry. The angular distributions of the scattered electrons were converted to absolute cross sections using the relative flow technique. A procedure based on the method of initial rate is applied to determine the flow rate of vaporized liquid samples. Moreover, integral and momentum transfer cross sections are derived from the measured differential cross sections. Comparison is made between our measured data with the existing experimental results and with our theoretical results calculated using the independent atom model at the static-exchange-polarization plus absorption level of approximation.

The dissociation dynamics of anionic and excited neutral fragments of gaseous ${\text{CCl}}_4$ and ${\text{CCl}}_4$ adsorbed on Si(100) $\sim 90\text{K}$ following $\text{Cl}2p$ core-level excitations were investigated on combining measurements of photon-induced anionic dissociation, x-ray absorption, and uv-visible dispersed fluorescence. The transitions of core electrons to high Rydberg states or doubly excited states near $\text{Cl}2p$ ionization thresholds of gaseous ${\text{CCl}}_4$ remarkably enhance the production of excited neutral fragments (${\text{C}}^{\ast }$ and ${\text{CCl}}^{\ast }$); this enhancement is attributed to the contribution from the shake-modified resonant Auger decay and/or postcollision interaction (PCI). The ${\text{Cl}}^{-}$ anion is significantly reinforced in the vicinity of the $\text{Cl}2p_{1/2,3/2}$ ionization threshold of gaseous ${\text{CCl}}_4$, originating from PCI-mediated photoelectron recapture. The $\text{Cl}2p\to 7a_1^{\ast }$ excitation for ${\text{CCl}}_4/\text{Si}\left(100\right)$ at $\sim 90\text{K}$ enhances the ${\text{Cl}}^{-}$ desorption yield at a submonolayer level. This resonant enhancement of ${\text{Cl}}^{-}$ yield at the $7a_1^{\ast }$ resonance in the $\text{Cl}2p$ edge at a submonolayer level occurs through the formation of high-lying molecular-ion states of ${\text{CCl}}_4$ adsorbed on a Si surface.

Conversion of microwave radiation into the optical range has been predicted to reach unity quantum efficiency in whispering-gallery resonators made from an optically nonlinear crystal and supporting microwave and optical modes simultaneously. In this work, we theoretically explore and experimentally demonstrate a resonator geometry that can provide the required phase matching for such a conversion at any desired frequency in the sub-THz range. We show that such a ring-shaped resonator not only allows for the phase matching but also maximizes the overlap of the interacting fields. As a result, unity-efficient conversion is expected in a resonator with feasible parameters.

The classical field simulation technique is used to study a trapped quasi-two-dimensional Bose gas. We calculate the central curvature of the system density and fluctuations of the condensate mode in the degenerate regime. These results provide new understanding of the system behavior in the region of the superfluid transition.

Precision saturation spectroscopy of the ${}^{88}\text{Sr}$ ${{}^{1}S}_0\text{−}{{}^{3}P}_1$ is performed in a vapor cell filled with various rare gases including He, Ne, Ar, and Xe. By continuously calibrating the absolute frequency of the probe laser, buffer-gas-induced collision shifts of $\sim \text{kHz}$ are detected with gas pressure of 1–20 mTorr. Helium gave the largest fractional shift of $1.6\times {10}^{-9}{\text{Torr}}^{-1}$. Comparing with a simple impact calculation and a Doppler-limited experiment of Holtgrave and Wolf [J. C. Holtgrave and P. J. Wolf, Phys. Rev. A 72, 012711 (2005)], our results show larger broadening and smaller shifting coefficient, indicating effective atomic loss due to velocity-changing collisions. The applicability of the result to the ${{}^{1}S}_0\text{−}{{}^{3}P}_0$ optical lattice clock transition is also discussed.

We have examined high-order harmonic generation from transiently aligned room-temperature ${\text{N}}_2$ molecules in a hollow-core waveguide. We identify alignment-dependent propagation effects which can mask the single-molecule response. In particular, we observe an intensity-dependent reversal in the harmonic efficiency from parallel to perpendicular alignment. The reversal can be attributed to the alignment dependence of the tunneling ionization rate and the concomitant modification of the phase-matching conditions due to the changes in the free-electron density in the fiber.

We discuss a quantum-metrology protocol designed to estimate a physical parameter in a Bose-Einstein condensate of $N$ atoms, and we show that the measurement uncertainty can decrease faster than $1/N$. The $1/N$ scaling is usually thought to be the best possible in any measurement scheme. From the perspective of quantum information theory, we outline the main idea that leads to a measurement uncertainty that scales better than $1/N$. We examine in detail some potential problems and challenges that arise in implementing such a measurement protocol using a Bose-Einstein condensate. We discuss how some of these issues can be dealt with by using lower-dimensional condensates trapped in nonharmonic potentials.

Experiments violating Bell inequalities have formed our belief that the world at its smallest is genuinely nonlocal. While many nonlocality experiments use the first quantized picture, the physics of fields of indistinguishable particles is captured most conveniently by second quantization. This implies the possibility of nonlocal correlations, such as entanglement, between modes of the field. In this paper, we propose an experimental scheme that tests the theoretically predicted entanglement between modes in space occupied by massive bosons. A successful test of this scheme would not only indicate that mode entanglement is as genuine as particle entanglement, despite the particle number superselection rule, but would also prove that this superselection rule can be overcome.

We study the suppression of noise-induced phase decoherence in a single atomic qubit by employing pulse sequences. The atomic qubit is composed of a single neutral atom in a far-detuned optical dipole trap and the phase decoherence may originate from the laser intensity and beam pointing fluctuations, as well as magnetic field fluctuations. We show that suitable pulse sequences may prolong the qubit coherence time substantially as compared with the conventional spin-echo pulse.

We experimentally demonstrate that decoherent errors for a qubit can be reduced using dynamic decoupling control. The quantum system in our experiment is a praseodymium ground-state hyperfine transition in ${\text{Pr}}^{3+}:{\text{Y}}_2{\text{SiO}}_5$. These experiments were undertaken using an ensemble, where care was taken to reduce incoherent errors resulting from inhomogeneity across the sample. The volume of the Bloch sphere after transformation by the applied pulse sequence was used as a measure of fidelity of the qubit. The strength of the qubit-environment coupling can be tuned by the application of a magnetic field, allowing for regimes of varying decoherence rates to be investigated. We show that the Bloch sphere volume, with the application of dynamic decoupling pulse sequences, decays at a slower rate than observed under free evolution.

The photoionization of a two-shell endohedral $A@{\text{C}}_{N1}@{\text{C}}_{N2}$ is considered. Formulas are presented for cross sections and angular anisotropy parameters, both dipole and nondipole. The effect of the fullerenes shell upon photoelectron from atom $A$ is taken into account substituting the action of the fullerene by two zero-thickness “bubble potential.” The fullerenes shells polarization is included assuming that the radius of the outer shell $R_2$ is much bigger than the inner $R_1$ and both much exceed the atomic radius $r$. This permits to express the effect via ${\text{C}}_{N1,N2}$ polarizabilities, which are connected to their photoionization cross sections. The interaction between shells ${\text{C}}_{N1}$ and ${\text{C}}_{N2}$ is taken into account in the random phase approximation (RPA). The effect of photoelectron scattering by both “bubble potentials” is included in the lowest order and in the RPA frame. As concrete examples, two endohedrals $\text{Ar}@{\text{C}}_{60}@{\text{C}}_{240}$ and $\text{Xe}@{\text{C}}_{60}@{\text{C}}_{240}$ are considered. In the $\text{Ar}@{\text{C}}_{60}@{\text{C}}_{240}$ case we consider $3p$ and $3s$, while in the case $\text{Xe}@{\text{C}}_{60}@{\text{C}}_{240}$ $5p$, $5s$ and $4d$ subshells are considered. A whole variety of peculiarities are found that deserve experimental verification.

We revisit recent developments in the theoretical foundations of time-dependent density-functional theory (TDDFT). TDDFT is then applied to the calculation of total cross sections for ionization processes in the $\overline{p}\text{-He}$ collision system. The Kohn-Sham potential is approximated as the sum of the Hartree-exchange potential and a correlation potential that was proposed in the context of laser-induced ionization. Furthermore, some approaches to the problem of calculating the ionization probabilities from the density are discussed. Small projectile energies $\left(\le 5\text{keV}\right)$ are considered as well as those in the range of 5 to 1000 keV. Results are compared to former calculations and to experimental data. We find that the correlation potential yields no obvious improvement of the results over the exchange-only approximation where the correlation potential is neglected. Furthermore, we find the problem of calculating the desired observables crucial, introducing errors of at least the same order of magnitude as the correlation potential. For the case of small energies, we find that trajectory effects play an important role: the ionization cross sections are enlarged significantly if curved instead of straight-line trajectories are used for the projectile motion.

We propose a scheme to realize persistent mass and spin currents in cold atoms using optically generated gauge potentials. Two degenerate dark states can be created by coupling “tripod” atoms to three Laguerre-Gaussian laser beams carrying relative orbital angular momentum. Atoms in these dark states feel opposite effective magnetic fields of the flux-tube type and circulate in opposite directions around the axis of the tube, generating persistent mass currents in the system. Persistent spin currents can also be generated, with or without spin polarization.

It is investigated experimentally that stimulated Raman scattering (SRS) can be enhanced by stimulated Brillouin scattering (SBS). Two physical mechanisms of these phenomena were analyzed. These phenomena show that not only the competition between SBS and SRS exists, the pumping effect of stimulated Brillouin scattering on back-stimulated Raman scattering is also a commonly existing rule regardless of the experimental conditions.

The analytical and numerical study of the propagation of elliptically polarized light in the medium composed of fixed $(j_1=\frac{1}{2})-(j_2=\frac{1}{2})$ atoms is presented. Properties of the ringing produced by light pulses are analyzed. The stabilization of the total propagating field area is presented. The redistribution of the energy between two phase shifted laser fields with orthogonal linear polarizations is investigated.

We present a method for generating a regular train of ultrashort optical pulses in a prepared two-level medium. The train develops from incident monochromatic probe radiation traveling in a medium of atoms, which are in a quantum mechanical superposition of dressed internal states. In the frame of linear theory for the probe radiation, the energy of individual pulses is an exponentially growing function of atom density and of interaction cross section. Pulse repetition rate is determined by the pump field’s generalized Rabi frequency and can be around 1 THz and greater. We also show that the terms, extra to the dipole approximation, endow the gas by a new property: nonsaturating dependence of refractive index on dressing monochromatic field intensity. Contribution of these nonsaturating terms can be compatible with the main dipole approximation term contribution in the wavelength region of about ten micrometers (the range of ${\text{CO}}_2$ laser) or larger.

It is shown that for a $N$-boson system the parity of $N$ can be responsible for a qualitative difference in the system response to variation in a parameter. The nonlinear boson model is considered, which describes tunneling of boson pairs between two distinct modes $X_{1,2}$ of the same energy and applies to a Bose-Einstein condensate in an optical lattice. By varying the lattice depth, one induces the parity-dependent quantum switching, i.e., $X_1\to X_2$ for even $N$ and $X_1\to X_1$ for odd $N$, for arbitrarily large $N$. A simple scheme is proposed for the observation of the parity effect on the mesoscopic scale by using the bounce switching regime, which is insensitive to the initial state preparation (as long as only one of the two $X_l$ modes is significantly populated), stable under small perturbations, and requires an experimentally accessible coherence time.