In coherent anti-Stokes Raman scattering (CARS) spectroscopy experiments, usually the amplitude of the signal is measured and the phase information is lost. With a polarization- and phase-controlled pulse shaping technique, the relative phase between the resonant and nonresonant CARS signals is controlled, and spectral interferometry is performed without an interferometer. Both the real and imaginary parts of the background-free resonant CARS spectrum are measured via spectral interferometry between the resonant and nonresonant signals from the same sample. The resonant signal is amplified significantly by homodyne mixing with the nonresonant signal as a local oscillator, greatly improving the detection limit.

Nonlocality of two-mode states of light is addressed by means of Clauser-Horne-Shimony-Holt (CHSH) inequality based on displaced on/off photodetection. Effects due to nonunit quantum efficiency and nonzero dark counts are taken into account. Nonlocality of both balanced and unbalanced superpositions of few photon-number states, as well as that of multiphoton twin beams, is investigated. We find that unbalanced superpositions show larger nonlocality than balanced ones when noise affects the photodetection process. De-Gaussification by means of (inconclusive) photon subtraction is shown to enhance nonlocality of twin beams in the low-energy regime. We also show that when the measurement is described by a positive operator-valued measure, rather than a set of projectors, the maximum achievable value of the Bell parameter in the CHSH inequality is decreased, and is no longer given by the Cirel’son bound.

We propose a scheme for the measurement of a two-mode entangled field-state in a high-$Q$ cavity. The scheme utilizes the momentum distribution spectrum in the Raman-Nath regime of a three-level atom in $V$ configuration. Due to the two modes of the electromagnetic field the atom may have $x$ interactions with mode $A$, and $y$ interactions with mode $B$, causing a complex momentum distribution. The momentum distribution of the atom after interaction with the quantized cavity fields contains the information of the field photon statistics. We reconstruct the joint photon statistics of the entangled field with the help of recorded momentum spectrum. We also propose to reconstruct the Wigner function of a two-mode entangled field state by injecting two coherent states resonant to each mode into the cavity and then measuring the joint photon statistics of the displaced field.

The security of a cryptographic key that is generated by communication through a noisy quantum channel relies on the ability to distill a shorter secure key sequence from a longer insecure one. We show that—for protocols that use quantum channels of any dimension and completely characterize them by state tomography—the noise threshold for classical advantage distillation of a specific kind is substantially lower than the threshold for quantum entanglement distillation if the eavesdropper can perform powerful coherent attacks. In marked contrast, earlier investigations had shown that the thresholds are identical for incoherent attacks on the same classical distillation scheme. It remains an open question whether other schemes for classical advantage distillation have higher thresholds for coherent eavesdropping attacks.

Formation of antihydrogen atoms in a magnetized plasma of positrons and antiprotons is explicitly demonstrated in a molecular dynamics simulation. The parameters chosen are compatible with the experimental setup. We employ a special, adaptive time step symplectic integrator to perform full dynamics simulation, without using the guiding center approximation, for very long times (of the order of $\mu \mathrm{s}$). The large number of antihydrogen atoms formed allows detailed statistical analysis and distributions for the binding energy, pseudomomentum, sizes, and other quantities that characterize these atoms. We also find that a significantly smaller number of antihydrogen positive ions form during the free expansion of the plasma.

Finite-size effects not only alter the energy levels of small systems, but can also lead to additional effective interactions within these systems. Here the problem of low-energy quantum scattering by a spherically symmetric short-range potential in the presence of a general cylindrical confinement is investigated. A Green’s function formalism is developed which accounts for the full three-dimensional (3D) nature of the scattering potential by incorporating all phase shifts and their couplings. This quasi-1D geometry gives rise to scattering resonances and weakly localized states, whose binding energies and wave functions can be systematically calculated. Possible applications include, e.g., impurity scattering in ballistic quasi-1D quantum wires in mesoscopic systems and in atomic matter-wave guides. In the particular case of parabolic confinement, the present formalism can also be applied to pair collision processes such as two-body interactions. Weakly bound pairs and quasimolecules induced by the confinement and having zero or higher orbital angular momentum can be predicted, such as $p$- and $d$-wave pairings.

We show that a Bose-Einstein condensate with a vortex lattice in a rotating anisotropic harmonic potential exhibits a very low frequency scissors mode. The appearance of this mode is due to the SO(2) symmetry breaking introduced by the vortex lattice. The corresponding Goldstone mode in an isotropic trap becomes, in an anisotropic trap, a scissors mode, whose frequency is finite and tends to zero with the trap anisotropy $ϵ$, with a generic $\sqrt{ϵ}$ dependence. We present analytical formulas giving the mode frequency in the low $ϵ$ limit and we show that the mode frequency for some class of vortex lattices can tend to zero as $ϵ$ or faster. We demonstrate that the standard classical hydrodynamics approach fails to reproduce this low frequency mode, because it does not contain the discrete structure of the vortex lattice.

The nonlinear optical response of an adsorbate, whose structure is altered by the inhomogeneous electrostatic field of the substrate, has been evaluated with a particular emphasis on the sum-frequency generation process. In the limiting case of an homogeneous electrostatic field, besides the contributions associated with the induced dipole moments, we have additional contributions which only exist if the adsorbed molecule has permanent dipole moments. Also, the Franck-Condon factors of the unperturbed molecule weight the internal couplings induced by the electrostatic field. For the more general inhomogeneous electrostatic field case, while the main observations remain valid, the Franck-Condon factors are modified by the molecular structure changes induced by the electrostatic field. In addition, we have a strong redistribution of the vibronic couplings resulting from the analytical $\mathbf{Q}$ dependence of the partial charge distribution which is a signature of the field inhomogeneities.

The quantum correlation between a pair of Stokes and anti-Stokes photons, involving a Raman emission process is calculated. All realistic radiative and nonradiative decays are included in the calculation and the photon correlation between the pair for arbitrarily strong excitation fields and detunings are calculated. The correlation function shows photon antibunching, and a damped sinusoidal behavior with respect to the time delay between the measurement of the two photons. The current system can also produce four photon entanglement.

Phase matching of a spectrally broadened pump pulse and its group-velocity-mismatched third harmonic gives rise to a frequency-shifted peak, dominating the spectrum of the third harmonic, with no signal produced at the frequency $3{\omega }_0,{\omega }_0$ being the carrier frequency of the pump field. The central frequency of this peak, controlled by the phase and group-velocity mismatch of the pump and third-harmonic pulses, as well as the spectral broadening of the pump field, may considerably deviate from the frequency $3{\omega }_0$, dictated by the standard energy-conservation relation for third-harmonic generation. This non-$3\omega$ third-harmonic generation is shown to result in interesting and practically significant spectral-transformation phenomena in photonic-crystal fibers.

The principle of coherent Dicke narrowing in a thin vapor cell, in which sub-Doppler spectral line shapes are observed under a normal irradiation for a $\lambda ∕2$ thickness, is generalized to two-photon spectroscopy. Only the sum of the two wave vectors must be normal to the cell, making the two-photon scheme highly versatile. A comparison is provided between the Dicke narrowing with copropagating fields, and the residual Doppler broadening occurring with counterpropagating geometries. The experimental feasibility is discussed on the basis of a first observation of a two-photon resonance in a 300-nm-thick Cs cell. Extension to the Raman situation is finally considered.

Coherent interactions between light fields and simple quantum systems, such as $\pi$-pulses and adiabatic passage through frequency chirps and stimulated Raman interactions, generally use one- or two-photon processes. This limits the use of such interactions to energy scales close to the photon energy of the control field. However, recent calculations have shown that a 3-level system, generic to ionized diatomic molecules, shows such a strong multiphoton coupling that $\pi$-pulses are possible on at least 12-photon transitions. In this paper, we show that adiabatic transfer of population is also possible on such high-order multiphoton transitions. Full one-dimensional quantum mechanical two-electron calculations confirm this effect even in a system with many excited states and an ionization continuum.

The electron-impact excitation cross sections at low electron energies have been calculated using a fully relativistic $R$-matrix method for transitions between levels of $4p^6,4p^{5}5s$, and $4p^{5}5p$ configurations. To ensure the convergence of results, we have paid special attention to the factors that may affect the convergence of cross sections. For examples, we have included extensive configuration interactions in the wave-function expansion of the target states. A large enough $R$-matrix boundary has been taken to ensure the convergence of atomic wave functions. Contributions to cross sections from a large number of partial waves (up to $J=39.5$) have been explicitly calculated. The final results are in good agreement with recent experimental data by Jung et al. [Phys. Rev. Lett. 94, 163202 (2005)] after shifting the position of electron energy. The relative difference is about 10% for four transitions out of the metastable levels. The results eliminated the significant discrepancies between theory and experimental work on excitation cross sections out of the metastable levels reported in the literature.

A merged beam technique has been used to investigate the fragmentation of the ${\mathrm{Cl}}_2^{-}$ ion in collisions with electrons over an energy range of $0–200\mathrm{eV}$. We have measured absolute cross sections for detachment, detachment plus dissociation and dissociation processes. Over the energy range studied, the dominant breakup mechanism is dissociation. Dissociation is relatively enhanced in the $e^{-}+{\mathrm{Cl}}_2^{-}$ collision system due to the suppression of the normally dominant detachment process, as a result of the large difference between the equilibrium internuclear distances of the ${\mathrm{Cl}}_2$ and ${\mathrm{Cl}}_2^{-}$ ground state potential curves. A prominent structure is observed just above the threshold in the ${\mathrm{Cl}}^{-}+\mathrm{Cl}+e^{-}$ dissociation channel. It is proposed that the structure is a resonance associated with production and rapid decay of an excited state of the doubly charged ${\mathrm{Cl}}_2^{-}$ ion. A plausible mechanism for production of the di-anionic state based on an excitation plus capture process is suggested.

Definitive results for elastic positron-hydrogen collisions are obtained using the Schwinger variational principle in the momentum space at low and intermediate energies corresponding to momenta in the range $k_i=0\text{–}3.5\text{atomic units}$. The discrete basis set used in the calculation makes allowance for long-range dipole polarization, atomic distortion, and infinite-order short-range positron-electron correlation. Results for the scattering parameters, such as the scattering length, phase shifts for all significant partial waves, and total cross section, are in conformity with other accurate variational and nonvariational calculations available in the literature.

A detailed study is made of the elastic differential cross section in positron-hydrogen collisions using the accurate Schwinger variational results for partial waves up to $l=19$ of the preceding paper [Ghoshal and Mandal, Phys. Rev. A, 72, 042709 (2005)]. Dramatic behavior of the differential cross section is evident in the surface plots, suggesting interference of partial-wave contributions at small and large scattering angles for the appearance of rich structure. It is found that the behavior of the critical point basically characterizes the nature of the differential cross section. Further interferences of the scattered waves for different angular momentum states produced by the interaction between the incoming positron and the target hydrogen atom determine such behavior and consequently build up the nature of the differential cross section. These findings reveal the remarkable fact that positron-hydrogen scattering almost does not occur in the direction around 87.3° at an incident energy of about 3.4 eV, corresponding to incident positron momentum of 0.5 atomic units.

Quadrupole radiation of an atom in an arbitrary environment is investigated within classical as well as quantum electrodynamical approaches. Analytical expressions for decay rates are obtained in terms of the Green’s function of Maxwell equations. The equivalence of both approaches is shown. General expressions are applied to analyze the quadrupole decay rate of an atom placed between two half spaces with arbitrary dielectric constant. It is shown that in the case where the atom is close to the surface, the total decay rate is inversely proportional to the fifth power of distance between an atom and a plane interface.

We propose a scheme for the controlled generation of Einstein-Podosky-Rosen entangled photon pairs from an atom coupled to a high $Q$ optical cavity, extending the prototype system as a source for deterministic single photons. A thorough theoretical analysis confirms the promising operating conditions of our scheme as afforded by currently available experimental setups. Our result demonstrates the cavity QED system as an efficient and effective source for entangled photon pairs, and shines new light on its important role in quantum information science.