Multiple Compton scattering results in an incorrect evaluation of Compton profile, which usually examines the case when the photon has undergone only one Compton collision in the sample. The probability of a photon being scattered several times may be significant for a target of finite dimensions both in depth and lateral dimensions. The present measurements are carried out to study the energy and intensity distributions of 0.279, 0.662, and $1.12\mathrm{MeV}$ $\gamma$ rays multiply scattered from the copper target of various thicknesses at a scattering angle of 60°. The scattered photons are detected by a NaI(Tl) scintillation detector whose detector response unfolding, converting the pulse-height distribution to a photon energy spectrum, is obtained with the help of an inverse response matrix. We observe that with an increase in target thickness, the number of multiply scattered photons saturates at a particular value of the target thickness (saturation depth). The multiply scattered fraction starts saturating above a particular energy window around the centroid of inelastic scattered peak. The signal-to-noise ratio is found to be decreasing with an increase in target thickness. Monte Carlo calculations based upon the package developed by Bauer and Pattison [Comptom Scattering Experiments at the HMI (1981), HMI-B 364, pp. 1–106] support the present experimental results.

General and analytical expressions for probabilities for center-of-mass scattering into small angles $(0°⩽\vartheta \lesssim 3°)$ in elastic neutral-neutral collisions with low impact energy $(0.05\mathrm{eV}\lesssim E\lesssim 0.5\mathrm{eV})$ are derived. Several theories of the differential scattering into the small angles are analyzed in order to establish the most reliable differential cross section for the collisions. The “best” cross section is then used to derive the corresponding (analytical) scattering probabilities. The approach is used to calculate the absolute values of the small-angle, low-energy differential cross sections and the scattering probabilities for the Li-Hg, Na-Hg, and K-Hg interactions.

Dissociation of the $\mathrm{C}\mathrm{O}_2{}^{+}$ ion has been investigated in an intense ultrafast $\left(55\mathrm{fs}\right)$ laser field by employing an intensity-selective scan technique and comparing the signals from linearly and circularly polarized pulses. Nonsequential contributions have been observed, highlighting the role of rescattering in the dissociative process.

We have investigated the phase-sensitive photoionization process of molecules induced by phase-controlled two-color laser pulses consisting of a fundamental light and a second-harmonic light with a pulse duration of $130\mathrm{fs}$ and an intensity of ${10}^{12}–{10}^{13}\mathrm{W}∕{\mathrm{cm}}^2$. We observed a phase-sensitive forward-backward asymmetry of photofragment emission along the laser polarization direction in the dissociative ionization of four methyl halide molecules ($\mathrm{C}{\mathrm{H}}_{3}X$; $X=\mathrm{F},\mathrm{Cl},\mathrm{Br},\mathrm{I}$). The directional asymmetries observed in the halogen ions and the counter methyl cations were out of phase with each other. This result indicates that detected molecules are oriented with discrimination of their head-and-tail order. We show that under our experimental conditions, selective ionization of oriented molecules based on the molecular Ammosov-Delone-Krainov model is the main process rather than dynamical molecular orientation for the phase-sensitive ionization process in methyl halides.

The dissociation of $\mathrm{H}_2{}^{+}$ in an intense laser field has been experimentally studied using femtosecond laser pulses at $790\mathrm{nm}$ in the intensity range of ${10}^{13}–{10}^{15}\mathrm{W}∕{\mathrm{cm}}^2$. Kinematically complete measurements of both the ionic ${\mathrm{H}}^{+}$ and neutral H fragments dissociated from a vibrationally excited $\mathrm{H}_2{}^{+}$ beam have been achieved by a coincidence three-dimensional momentum imaging system. Angular-resolved kinetic energy release spectra for a series of different intensity ranges have been obtained using the intensity-difference spectrum method, thus disentangling the problem caused by the intensity volume effect. Our results indicate that the dissociation dynamics are drastically different for “long” $\left(135\mathrm{fs}\right)$ and “short” $\left(45\mathrm{fs}\right)$ laser pulses at similar high laser intensities. Specifically, bond softening is found to be the main feature in long pulses, while above threshold dissociation is dominant in short pulses whose durations are comparable with the vibrational period of the molecule. Bond softening in short pulses appears at low kinetic energy release with a narrow angular distribution. The experimental results are well interpreted by solving the time-dependent Schrödinger equation in the Born-Oppenheimer representation without nuclear rotation.

It is shown that the momentum diffusion of free-space laser cooling has a natural correspondence in optical cavities when the internal state of the atom is treated as a harmonic oscillator. We derive a general expression for the momentum diffusion, which is valid for most configurations of interest: The atom or the cavity or both can be probed by lasers, with or without the presence of traps inducing local atomic frequency shifts. It is shown that, albeit the (possibly strong) coupling between atom and cavity, it is sufficient for deriving the momentum diffusion to consider that the atom couples to a mean cavity field, which gives a first contribution, and that the cavity mode couples to a mean atomic dipole, giving a second contribution. Both contributions have an intuitive form and present a clear symmetry. The total diffusion is the sum of these two contributions plus the diffusion originating from the fluctuations of the forces due to the coupling to the vacuum modes other than the cavity mode (the so-called spontaneous emission term). Examples are given that help to evaluate the heating rates induced by an optical cavity for experiments operating at low atomic saturation. We also point out intriguing situations where the atom is heated although it cannot scatter light.

In this paper, we report an experiment, which demonstrates computation of topological charges of two optical vortices via a nondegenerate four-wave-mixing process. We show that the output signal photon carries orbital angular momentum which equals to the subtraction of the orbital angular momenta of the probe light photon and the backward pump light photon. The ${}^{85}\mathrm{Rb}$ atoms are used as the nonlinear medium, which transfers the orbital angular momenta of lights.

We show that under conditions typical of electromagnetically induced transparency (EIT) in an ultracold atomic sample in a magneto-optical trap, a significant portion of the incident probe pulse is transferred into Rayleigh and Raman scattering channels. The light scattered into the Rayleigh channel emerges from the sample with an EIT time delay. We show that a proper description of the probe light propagation in the sample should include, in the diffusion dynamics, a spin polariton generated by the two-photon EIT process. The results have important implications for studies of weak light localization and for manipulation of single and few photon states in ultracold atomic gases.

We present an alternate solution to an eigenvalue problem which arises in the study of the Fokker-Planck equation for generalized Ornstein-Uhlenbeck processes. We obtain the staggered ladder spectra found previously but in addition we obtain the normalized eigenfunctions in terms of associated Laguerre polynomials. The representation of the eigenfunctions in this form greatly simplifies the evaluation of matrix elements required in calculating ensemble averages and correlation coefficients for various observables. The solution to the eigenvalue problem is given in the generic and general cases.

Electron-momentum distributions for above-threshold ionization of argon in a few-cycle, linearly polarized laser pulse are investigated. Spectral features characteristic of multiphoton as well as tunneling ionization coexist over a range of the Keldysh parameter $\gamma$ in the transition regime $\gamma \sim 1$. Surprisingly, the simple strong-field approximation (SFA) is capable of reproducing the key features of the two-dimensional momentum distributions found in the full solution of the time-dependent Schrödinger equation, despite the fact that SFA is known to severely underestimate the total ionization probability.

Using fourth-order perturbation theory, a general formula for the van der Waals potential of two neutral, unpolarized, ground-state atoms in the presence of an arbitrary arrangement of dispersing and absorbing magnetodielectric bodies is derived. The theory is applied to two atoms in bulk material and in front of a planar multilayer system, with special emphasis on the cases of a perfectly reflecting plate and a semi-infinite half space. It is demonstrated that the enhancement and reduction of the two-atom interaction due to the presence of a perfectly reflecting plate can be understood, at least in the nonretarded limit, by using the method of image charges. For the semi-infinite half space, both analytical and numerical results are presented.

We describe a general framework to study covariant symmetric broadcasting maps for mixed qubit states. We explicitly derive the optimal $N\to M$ superbroadcasting maps, achieving optimal purification of the single-site output copy, in both the universal and phase-covariant cases. We also study the bipartite entanglement properties of the superbroadcast states.

Quantum theory can be regarded as a noncommutative generalization of classical probability. From this point of view, one expects quantum dynamics to be analogous to classical conditional probabilities. In this paper, a variant of the well-known isomorphism between completely positive maps and bipartite density operators is derived, which makes this connection much more explicit. This isomorphism is given an operational interpretation in terms of statistical correlations between ensemble preparation procedures and outcomes of measurements. Finally, the isomorphism is applied to elucidate the connection between no-cloning and no-broadcasting theorems and the monogamy of entanglement, and a simplified proof of the no-broadcasting theorem is obtained as a by-product.

We calculate the geometric phase for an open system (spin-boson model) which interacts with an environment (ohmic or nonohmic) at arbitrary temperature. However there have been many assumptions about the time scale at which the geometric phase can be measured, there has been no reported observation of geometric phases for mixed states under nonunitary evolution yet. We study not only how they are corrected by the presence of the different type of environments but estimate the corresponding times at which decoherence becomes effective as well. These estimations should be taken into account when planning experimental setups to study the geometric phase in the nonunitary regime, particularly important for the application of fault-tolerant quantum computation.

We show that in some cases, catalyst-assisted entanglement transformation cannot be implemented by multiple-copy transformation for pure states. This fact, together with the result we obtained in R. Y. Duan, Y. Feng, X. Li, and M. S. Ying, Phys. Rev. A 71, 042319 (2005), namely that the latter can be completely implemented by the former, indicates that catalyst-assisted transformation is strictly more powerful than multiple-copy transformation. For the purely probabilistic setting we find, however, these two kinds of transformations are geometrically equivalent in the sense that the sets of pure states that can be converted into a given pure state with maximal probabilities not less than a given value have the same closure, regardless of whether catalyst-assisted transformation or multiple-copy transformation is used.

We derive a master equation describing the collective decay of two-level atoms inside a single mode cavity in the dispersive limit. By considering atomic decay in the collective thermostat, we found a decoherence-free subspace of the multiparticle entangled states of the $W$-like class. We present a scheme for writing and storing these states in collective thermostat.

Recent experiments on electronic excitation of molecules by positron impact have shown much larger cross sections than in the electron scattering case. The challenge of understanding the origin of this enhancement, especially just above electronic excitation thresholds, motivates the search of theoretical explanations of the phenomenon as well as new experimental efforts to confirm the data. In an earlier theoretical effort, an application of the Schwinger multichannel method at two-state level of approximation for the $X{}^1\Sigma _g^{+}\to B{}^1\Sigma _u^{+}$ electronic excitation of the ${\mathrm{H}}_2$ molecule, gave cross sections with smaller magnitude but reasonable qualitative agreement with the experimental data. The purpose of this work was to study the numerical stability of the previous calculation and to investigate the influence of open channels (energetically accessible electronic states—multichannel effects) and closed channels (energetically inaccessible electronic states—polarization effects) on this transition. Our results show minor multichannel effects and a significative enhancement of the $X{}^1\Sigma _g^{+}\to B{}^1\Sigma _u^{+}$ cross section at the threshold due to polarization effects if compared to the usual static results.

We have investigated degenerate four-wave mixing in a sample of cold rubidium atoms. A red-blue asymmetry is observed for high intensities of the pumping beams. This asymmetry is explained by the spatial bunching of the atoms in the nodes or antinodes of the strong standing wave of the pump beams. This explanation is confirmed by different experimental configurations and by numerical simulations.

We theoretically investigate the spectral properties of a $V$-type three-level atom driven by two bichromatic fields with a common frequency difference. By decomposing the master equation using harmonic expansions and invoking quantum regression theorem, fluorescence and probe absorption spectra of the strong atomic transition are numerically calculated under the steady state condition. We find that both fluorescence and absorption spectra exhibit two interesting features, which are equidistant comblike structures and phase-dependent line splittings. In the comblike structures, each fluorescence peak can be made subnatural by manipulating the relative intensities of the coupling fields, while for the absorption lines only the central peak can be narrowed. Line splittings are induced by the relative phase delay between the envelopes of the amplitudes of the two bichromatic fields. Interestingly, we find that the manipulation of the relative phase delay results in the emergence of sharp subnatural dips in the absorption spectra. As a natural consequence of the subnatural absorption dips, absorption spectra in atomic vapors exhibit striking subnatural burning holes for the counterpropagating probe beam geometry.

We propose a scheme for observing dark stationary waves in a Tonks-Girardeau (TG) gas. The scheme is based on parity-selective dynamical filtering of the gas via a time-dependent potential, which excites the gas from its ground state towards a desired specially-tailored many-body state. These excitations of the TG gas are analogous to linear partially coherent nondiffracting beams in optics, as evident from the mapping between the quantum dynamics of the TG gas and the propagation of incoherent light in one-dimensional linear photonic structures.