Relations between photon scattering, entanglement, and multimode detection are investigated. We first establish a general framework in which one- and two-photon elastic scattering processes can be discussed; then, we focus on the study of the intrinsic entanglement degradation caused by a multimode detection. We show that any multimode scattered state cannot maximally violate the Bell-Clauser-Horne-Shimony-Holt inequality because of the momentum spread. The results presented here have general validity and can be applied to both deterministic and random scattering processes.

We investigate resonance patterns in a stadium-shaped microcavity around $n_{c}kR\simeq 10$, where $n_c$ is the refractive index, $k$ the vacuum wave number, and $R$ the radius of the circular part of the cavity. We find that the patterns of high-$Q$ resonances can be classified, even though the classical dynamics of the stadium system is chaotic. The patterns of the high-$Q$ resonances are consistent with ray dynamical considerations and appear as stationary lasing modes with low pumping rate in a nonlinear dynamical model. All resonance patterns are presented in a finite range of $kR$.

It was shown by Ahn, Wiseman, and Milburn [Phys. Rev. A 67, 052310 (2003)] that feedback control could be used as a quantum error correction process for errors induced by weak continuous measurement, given one perfectly measured error channel per qubit. Here we point out that this method can be easily extended to an arbitrary number of error channels per qubit. We show that the feedback protocols generated by our method encode $n-2$ logical qubits in $n$ physical qubits, thus requiring just one more physical qubit than in the previous case.

We demonstrate how spontaneous emission in a cavity can be controlled by the application of a dc field. The method is especially suitable for Rydberg atoms. We present a simple argument based on Stark shifts for the control of emission.

A single-step operation is proposed to produce multiqubit maximally entangled states in the NMR model. In the scheme, all qubits are initially in the ground state, and one single pulse of a multifrequency coherent magnetic radiation is applied to manipulate simultaneously the “active states” that satisfy the resonant conditions while all other “inactive states” remain unchanged. An effective Hamiltonian is derived in a generalized rotating frame, which allows us to predict the time evolution of “active states” generated by the magnetic pulse. The magnetic pulse parameters, such as frequencies, phases, amplitudes, and duration time, are obtained analytically to implement a Bell state of two qubits and a Greenberger-Horne-Zeilinger state of three qubits. The scheme has been generalized to create an $N$-qubit entangled state. Two rules are found to calculate the magnetic pulse parameters numerically, which are required to realize entangled states for even and odd qubits, respectively. The rules are successfully checked in the cases of $4⩽N⩽10$. The relation of $\ln t_0$ and $\ln N$ is found to be linear, and the duration time $t_0$is approximately proportional to $\sqrt{N}$.

In quantum mechanics, outcomes of measurements on a state have a probabilistic interpretation while the evolution of the state is treated deterministically. Here we show that one can also treat the evolution as being probabilistic in nature and one can measure which unitary acted. In further analogy to states, one can also choose which basis of unitaries to measure. Likewise, one can give an information-theoretic interpretation to evolutions by defining the entropy of a completely positive map. This entropy gives the rate at which the informational content of the evolution can be compressed. One cannot compress this information and still have the evolution act on an unknown state, but we demonstrate a general scheme to do so probabilistically. This allows one to generalize super-dense coding to the sending of quantum information. One can also define the “interaction-entanglement” of a unitary, and concentrate this entanglement.

We analyze the entanglement properties of spins (qubits) attached to the boundary of spin chains near quantum critical points, or to dissipative environments, near a boundary critical point, such as Kondo-like systems or the dissipative two level system. In the first case, we show that the properties of the entanglement are significantly different from those for bulk spins. The influence of the proximity to a transition is less marked at the boundary. In the second case, our results indicate that the entanglement changes abruptly at the point where coherent quantum oscillations cease to exist. The phase transition modifies the entanglement significantly less.

The circular dichroism in the photoelectron angular distribution was investigated for valence photoionization of randomly oriented pure enantiomers of camphor and bromocamphor molecules using circularly polarized light in the vacuum ultraviolet. The forward-backward electron emission spectra were recorded simultaneously with two spectrometers at several opposite angles relative to the propagation direction of the photon beam and compared for each of the two substances. Measurements were also carried out for reversed light helicity and opposite molecular handedness. For the left- and right-handed enantiomers of both molecules we observed asymmetries of comparable magnitude up to several percent. The measured asymmetry parameters vary strongly for different orbital binding energies and also for the selected photon energies in the valence region. The results for both molecules are compared. They suggest a strong influence of the final states on the asymmetry, depending on the chiral geometry of the molecular electronic structure, as well as a significant dependence on the initial states involved. They also confirm theoretical predictions describing the effect in pure electric-dipole approximation.

The intensity evolution of the Cu $K{\beta }_{1,3}$ and $K{\beta }_{2,5}$ emission spectra is measured as a function of excitation energy using resonant inelastic x-ray scattering spectroscopy. The contribution of $\underset{̱}{1s3d}$ shake satellites to the Cu $K{\beta }_{1,3}$ as well as to the Cu $K{\beta }_{2,5}$ emission spectrum is extracted. Both emission spectra indicate an additional contribution of $\underset{̱}{1s3p}$ shake satellites. The intensity evolution of the shake satellites from threshold to saturation is compared to the Thomas model and shows a similar saturation behavior as found for the Cu $K{\alpha }_{1,2}$ $\underset{̱}{1s3d}$ shake satellites, stating that the shake process is independent of the reemission shell. These $\underset{̱}{1s3d}$ shake satellites rapidly reach their saturation limit in contrast to Cu $\underset{̱}{1s2p}$ and $\underset{̱}{1s1s}$ shake satellites. Furthermore, the contribution of the $KMN$ and the $KNN$ radiative Auger satellites to the Cu $K{\beta }_{1,3}$ and Cu $K{\beta }_{2,5}$ emission spectra is obtained, respectively. The $KNN$ radiative Auger satellite is shown to play an important role in the interpretation of valence fluorescence spectra.

A complete nonperturbative close-coupling solution of the helium atom in a box problem is presented by developing two numerical techniques. The first technique is the direct solution by diagonalization of the Hamiltonian, and the second is based on a constrained relaxation of the wave functions. A Feshbach projection operator of the direct solutions to the bound-continuum subspaces allows a comparison of the low-lying autoionization levels obtained in both methods. Time-dependent propagation of these doubly excited wave functions is analyzed, allowing the calculation and the visualization of the autoionization process. In this work, results are presented for the $S$-wave model in which the electrons are restricted to spherical states and all angular correlations have been eliminated.

The optical Bloch equations of a four-level atomic system with three closely spaced upper levels irradiated by a single mode of the electromagnetic radiation field are constructed. Under the rotating wave approximation and by using the slowly varying envelope approximation, we obtain the analytical solution to these optical Bloch equations. These solutions, which are obtained perturbatively, are used to calculate the polarization and hence the absorptive line shape. In particular, we examine the effects of the field-induced coherence arising from the interference between the probability amplitudes of the levels participating in dipole-allowed transitions. We obtain the shifts of the resonance peak positions due to two different reasons. The first is due to the nonlinear effects arising from the closely spaced nature of the peaks. The second is attributed to the field-induced coherence alone. It is found that the shifts of the positions of the resonance peaks due to field-induced coherence may be in the blue or in the red region.

We investigate the effects of a strong laser field on the dynamics of electron capture and emission in ion-atom collisions within a reduced dimensionality model of the scattering system in which the motion of the active electron and the laser electric field vector are confined to the scattering plane. We examine the probabilities for electron capture and ionization as a function of the laser intensity, the projectile impact parameter $b$, and the laser phase $\varphi$ that determines the orientation of the laser electric field with respect to the internuclear axis at the time of closest approach between target and projectile. Our results for the $b$-dependent ionization and capture probabilities show a strong dependence on both $\varphi$ and the helicity of the circularly polarized laser light. For intensities above $5\times {10}^{12}\mathrm{W}∕{\mathrm{cm}}^2$ our model predicts a noticeable circular dichroism in the capture probability for slow proton-hydrogen collisions, which persists after averaging over $\varphi$. Capture and electron emission probabilities defer significantly from results for laser-unassisted collisions. Furthermore, we find evidence for a charge-resonance-enhanced ionization mechanism that may enable the measurement of the absolute laser phase $\varphi$.

We reconsider the idea of a pulsed optically pumped frequency standard conceived in the early 1960s to eliminate the light-shift effect. The development of semiconductor lasers and of pulsed electronic techniques for atomic fountains and new theoretical findings allow an implementation of this idea which may lead to a frequency standard whose frequency stability is limited only by the thermal noise in the short term and by the temperature drift in the long term. We shall also show both theoretically and experimentally the possibility of doubling the atomic quality factor with respect to the classical Ramsey technique approach.

The superfluid–Mott-insulator transition of spin-2 boson atoms with repulsive interaction in an optical lattice in a magnetic field is presented. By using the mean-field theory, the Mott ground states and phase diagrams of the superfluid–Mott-insulator transition at zero temperature are revealed. An applied magnetic field leads to splitting of some phase boundaries. For all the initial Mott ground states containing multiple spin components, different spin components take on different phase boundaries. It is found that in this system phase boundaries with different magnetization can be moved in different ways by changing only the intensity of the applied magnetic field.

Three-level atom optics is introduced as a simple, efficient, and robust method to coherently manipulate and transport neutral atoms. The tunneling interaction among three trapped states allows us to realize the spatial analog of the stimulated Raman adiabatic passage, coherent population trapping, and electromagnetically induced transparency techniques and offers a wide range of possible applications. We investigate an implementation in optical microtrap arrays and show that under realistic parameters the coherent manipulation and transfer of neutral atoms among dipole traps could be realized in the millisecond range.

We consider a model of a Fermi-Bose mixture with strong hard-core repulsion between particles of the same sort and attraction between particles of different sorts. In this case, besides the standard anomalous averages of the type $⟨b⟩$, $⟨bb⟩$, and $⟨cc⟩$, a pairing between fermions and bosons of the type $bc$ is possible. This pairing corresponds to the creation of composite fermions in the system. At low temperatures and equal densities of fermions and bosons composite fermions are further paired in quartets. At higher temperatures trios, which consist of composite fermions and elementary bosons, are also present in the system. Our investigations are important in connection with the recent observation of weakly bound dimers in magnetic and optical dipole traps at ultralow temperatures and with the observation of the collapse of a Fermi gas in an attractive Fermi-Bose mixture of neutral particles.

We demonstrate full selective control over the constructive or destructive character of fourth-order recurrent interferences in a modified version of a Hong-Ou-Mandel interferometer using comblike two-photon states. The comb spectral/temporal structure is obtained by inserting an etalon cavity in the signal path of an entangled photon pair obtained by pulsed spontaneous parametric down-conversion. Both a simple qualitative discussion and a complete theoretical derivation are used to explain and analyze the experimental data.

It is shown that a linear superposition of two macroscopically distinguishable optical coherent states can be generated using a single photon source and simple all-optical operations. Weak squeezing on a single photon, beam mixing with an auxiliary coherent state, and photon detecting with imperfect threshold detectors are enough to generate a coherent state superposition in a free propagating optical field with a large coherent amplitude $(\alpha >2)$ and high fidelity $(F>0.99)$. In contrast to all previous schemes to generate such a state, our scheme does not need photon number resolving measurements nor Kerr-type nonlinear interactions. Furthermore, it is robust to detection inefficiency and exhibits some resilience to photon production inefficiency.

The angular distribution of photoelectron emission is explored from a theoretical perspective using time-dependent density-functional theory as a tool. We discuss the principal features of one- and two-photon processes for $\mathrm{Na}_3{}^{+}$ with a simple jellium background. As more realistic test cases, we consider $\mathrm{Na}_9{}^{+}$ and $\mathrm{Na}_{41}{}^{+}$ with detailed ionic structure and compare the results with a jellium description. We find that high-frequency photons are required to resolve ionic details. Moreover, it is desirable to have simultaneously energy-resolved angular distributions to disentangle the details of the electronic structure.