We study both dynamical and steady responses of a coherently driven four-level tripod-type atomic system, which may be either open or closed. With all three driving fields being on Raman resonance, this tripod system can quickly evolve into a complicated steady dark state decoupled from both coherent and dissipative interactions. In the open model, this tripod system is sensitive to all parameters in its dynamical evolution while the steady dark state only depends on the initial population distribution and the relative Rabi frequency values. In the closed model, however, relative values of spontaneous decay rates also play an important role in generating the steady dark state. Analytical results based on the quantum jump theory show that the steady dark state is always multiple (from twofold to sixfold) degenerate and usually comes into existence through both coherent and incoherent superpositions.

In this paper we calculate the Casimir-Polder force density (force per unit area acting on the elements of the surface) on a metallic plate placed in front of a neutral atom. To obtain the force density we use the quantum operator associated to the electromagnetic stress tensor. We explicitly show that the integral of this force density over the plate reproduces the total force acting on the plate. This shows that, although the total force is obtained as a sum of surface element-atom contributions, many-body components proportional to the atomic polarizability are included in our result. The local force that we obtain can also be useful in evaluating Casimir torques or stresses on the wall.

We consider the problem of phase-covariant cloning for coherent states. We show that an experimental scheme based on ideal phase measurement and feedforward outperforms the semiclassical procedure of ideal phase measurement and preparation in terms of fidelity. A realistic scheme where the ideal phase measurement is replaced with double-homodyne detection is shown to be unable to overcome the semiclassical cloning strategy. On the other hand, such a realistic scheme is better than semiclassical cloning based on double-homodyne phase measurement and preparation.

We present the first high-resolution spectroscopic study of LiCs. LiCs is formed in a heat-pipe oven and studied via laser-induced fluorescence Fourier-transform spectroscopy. By exciting molecules through the $X{}^1\Sigma ^{+}\text{−}B{}^1\Pi$ and $X{}^1\Sigma ^{+}\text{−}D{}^1\Pi$ transitions vibrational levels of the $X{}^1\Sigma ^{+}$ ground state have been observed up to $3{\mathrm{cm}}^{-1}$ below the dissociation limit enabling an accurate construction of the potential. Furthermore, rovibrational levels in the $a{}^3\Sigma ^{+}$ triplet ground state have been observed via singlet-triplet mixing in the higher excited states. With the help of coupled channels calculations, accurate singlet and triplet ground-state potentials were derived reaching the atomic ground-state asymptote. These potentials yield an accurate determination of the singlet and triplet ground-state dissociation energies and allow first predictions of cold collision properties of $\mathrm{Li}+\mathrm{Cs}$ pairs.

Polarization-entangled photon pairs can be efficiently prepared into pure Bell states with a high fidelity via type-II spontaneous parametric down-conversion (SPDC) of narrow-band pump light. However, the use of femtosecond pump pulses to generate multiphoton states with precise timing often requires spectral filtering to maintain a high quality of polarization entanglement. This typically reduces the efficiency of photon pair collection. We experimentally map the polarization correlations of photon pairs from such a source over a range of down-converted wavelengths with a high spectral resolution and find strong polarization correlations everywhere. A spectrally dependent imbalance between contributions from the two possible decay paths of SPDC is identified as the reason for a reduction in entanglement quality observed with femtosecond pump pulses. Our spectral measurements allow us to predict the polarization correlations for arbitrary filter profiles when the frequency degree of freedom of the photon pairs is ignored.

We obtain an analytical expression for two-photon correlation $G^{\left(2\right)}$ from two atoms driven in a double-Raman (or $\Lambda$) configuration where collective effects such as superradiant, subradiant, and dipole-dipole interaction are included. It is found that the collective effects on the $G^{\left(2\right)}$ can be quenched to some extent by a resonant control laser field. The collective effects provide features via $G^{\left(2\right)}$ that enable the two atoms to be resolved at subwavelength separation. We also identify an effect in the double Raman scheme due to the collective effects and the control field, i.e., the Stokes and anti-Stokes frequencies are increased by fourfold.

We study the effect of spontaneously generated coherence (SGC) on two-photon correlation in a double-cascade scheme. An analytical expression for two-photon correlation is obtained and analyzed. The SGC and asymmetry in the two cascade channels provide a “sinelike” interference term that distorts the profile of the correlation. We show that the SGC term contains nonclassical features that are identical to the spontaneous Raman electromagnetic-induced transparency scheme which shows antibunching. Yet the correlation shows bunching instead of antibunching, and this is explained. We consider all possible transitions and find that photon antibunching is not possible in the double-cascade scheme.

We consider the scheme proposed by Wang [Phys. Rev. A 64, 022302 (2001)] for teleporting bipartite entangled states using beam splitters and phase shifters. It is shown that the proposed scheme can be altered slightly so as to obtain an almost perfect teleportation for an appreciable mean number of photons. For mean number of photons equal to 2, the minimum of average fidelity which is the minimum of sum of the product of probability for occurrence of any case and the corresponding fidelity for an arbitrary bipartite entangled state is less than unity by a quantity of the order of ${10}^{-4}$.

It is found that there exists a hysteretic response between the incident intensity and the lateral shift for a one-dimensional photonic crystal containing a nonlinear defect. The lateral shift can be greatly enhanced when the incident intensity decreases from a value higher than the switch-up threshold of optical bistability and reaches the maximum at the switch-down threshold. It is also found that the hysteresis is strongly dependent on the nonlinear defect, the incident angle, and the incident frequency. The validity of the theoretical analysis is demonstrated by numerical simulations for a Gaussian beam.

We discuss theoretically quantum interface between light and a spin polarized ensemble of atoms with the spin $⩾1$ based on an off-resonant Raman scattering. We present the spectral theory of the light–atom interaction and show how particular spectral modes of quantum light couple to spatial modes of the extended atomic ensemble. We show how this interaction can be used for quantum memory storage and retrieval and for deterministic entanglement protocols. The proposed protocols are attractive due to their simplicity since they involve just a single pass of light through atoms without the need for elaborate pulse shaping or quantum feedback. As a practically relevant example we consider the interaction of a light pulse with hyperfine components of $D_1$ line of ${}^{87}\mathrm{Rb}$. The quality of the proposed protocols is verified via analytical and numerical analysis.

We have used deterministic single-photon two-qubit quantum logic to implement the most powerful individual-photon attack against the Bennett-Brassard 1984 (BB84) quantum key distribution protocol. Our measurement results, including physical source and gate errors, are in good agreement with theoretical predictions for the Rényi information obtained by Eve as a function of the errors she imparts to Alice and Bob’s sifted key bits. The current experiment is a physical simulation of a true attack, because Eve has access to Bob’s physical receiver module. Nevertheless, the physical simulation allows investigation of the fundamental security limit of the BB84 protocol against eavesdropping in the presence of realistic physical errors, and it affords the opportunity to study the effectiveness of error correction and privacy amplification when the BB84 protocol is attacked.

We study elastic scattering of ultracold atoms by a conducting sphere. Loss of flux through inelastic reactions and adsorption is described in an unambiguous and model-independent way by incoming boundary conditions in the semiclassical region near the surface of the sphere. Differential cross sections are presented for Na and metastable $\mathrm{He}\left(2{}^{3}S\right)$ atoms scattered by a nanosphere with radius $200\mathrm{a.u.}$ or $2000\mathrm{a.u.}$ Their near-threshold behavior is determined by a small number of parameters which are properties of the potential tail beyond the semiclassical regime, and it depends sensitively on the characterstic lengths ${\beta }_6$, ${\beta }_7$ which describe the strength of the nonretarded van der Waals part and the highly retarded Casimir part of the atom-sphere interaction, respectively. These lengths can be tuned to probe various regions of the atom-sphere interaction by appropriate choice of the radius of the sphere.

We demonstrate rather interesting manifestations of coexistence of resonance features in characteristics of the photoionization of $3d$ electrons in $\mathrm{Xe}@{\mathrm{C}}_{60}$. It is shown that the reflection of photoelectrons produced by the $3d$ Xe photoionization affects greatly partial photoionization cross sections of $3d_{5∕2}$ and $3d_{3∕2}$ levels and respective angular anisotropy parameters, both dipole and nondipole adding to all of them additional maximums and minimums. The calculations are performed treating the $3∕2$ and $5∕2$ electrons as electrons of different kinds with their spins “up” and “down.” The effect of ${\mathrm{C}}_{60}$ shell is accounted for in the frame of the “orange” skin potential model. It is essential that within the considered photon frequency region the presented resonance features are not affected by the ${\mathrm{C}}_{60}$ polarization.

We present the theory for a technique, based on multistate variants of the stimulated Raman adiabatic passage (STIRAP) process, that allows efficient and robust preparation of a preselected superposition of two or three degenerate states (magnetic sublevels of an atom) and the measurement of their relative amplitudes and phases. Because the preparation utilizes adiabatic passage it is robust against small fluctuations of the Rabi frequencies and temporal shapes of the coupling fields. We here describe and, in the following companion paper we demonstrate, an approach to the experimental characterization of the superposition state, i.e., the measurement of the relative phases and the ratios of amplitudes of the components. That technique, termed phase-to-population mapping, is applicable to the characterization of a stream of identically prepared atoms and is based on laser-induced fluorescence after the atoms have undergone optical pumping cycles induced by an additional laser. The optical pumping process maps the phase into populations of a subset of levels by means of a filtering laser field, and is robust against variations in the intensity and detuning of that field. We describe four linkage patterns appropriate to the creation of superpositions of two or three degenerate states of angular momentum $J=2$, starting from $J=0$. We offer an interpretation, from three different perspectives, of the subsequent characterization procedure.

We present experimental demonstrations of a technique, based on multistate variants of the stimulated Raman adiabatic passage (STIRAP) process, that allows efficient and robust preparation of a preselected superposition of two or three degenerate states (magnetic sublevels of an atom) and the measurement of their relative amplitudes and phases. The experiments verify theoretical results described in the preceding paper. We perform our experiments with neon atoms in a supersonic beam: atoms in the metastable ${}^3{P}_0$ state are transferred to a superposition of the metastable ${}^3{P}_2$ magnetic sublevels, using four different linkage patterns. Because the preparation utilizes adiabatic passage, it is robust against small fluctuations of the Rabi frequencies and temporal shape of the coupling fields. The relative phases and the amplitude ratios of the components of the prepared superpositions are experimentally analyzed using a technique termed phase-to-population mapping. Phase-to-population mapping is based on laser-induced fluorescence after the atoms have undergone optical pumping cycles induced by an additional laser. The optical pumping process maps the superposition phases into populations of a subset of levels by means of a filtering laser field, and is robust against variations in the intensity and detuning of that field.

We investigate the use of the Stark shift in atomlike systems in order to control the interaction with a high-$Q∕V$ microcavity. By applying a Stark shift pulse to a single atomlike system, in order to affect and control its detuning from a cavity resonance, the cavity QED interaction can be carefully controlled so as to allow stochastic pumping of the emitting state without causing random timing jitter in the output photon. Using a quantum trajectory approach, we conduct simulations that show this technique is capable of producing indistinguishable single photons that exhibit complete Hong-Ou-Mandel interference. Furthermore, Stark tuning control allows for the generation of arbitrary pulse envelopes. We demonstrate this by showing that a simple asymmetric Stark shifting pulse can lead to the emission of symmetric Gaussian single-photon pulse envelopes, rather than the usual exponential decay. These Gaussian pulses also exhibit complete Hong-Ou-Mandel interference. The use of Stark shifting in solid-state systems could ultimately provide the cheap miniature high quality single-photon sources that are currently required for applications such as all-optical quantum computing.

The nonlocal theory of accelerated systems is extended to the propagation of Dirac particles. The implications of nonlocality for the phenomenon of spin-rotation coupling are discussed. The Lorentz-invariant nonlocal Dirac equation is presented for certain special classes of accelerated observers.

The time-resolved photoelectron spectra of the $\mathrm{NaI}$ molecule, as well as the populations of the ${\mathrm{Na}}^{+}$ ion and the ${\mathrm{NaI}}^{+}$ ion have been calculated in this paper by using the time-dependent wave packet method. The results agree well with the experimental ones [J. Phys. Chem. A 101, 2555 (1997)]. It is found that exchange of the wave packets in the crossing area plays a prominent role in the process of photoionization. Two conclusions can be derived from the calculation: First, the population of the ${\mathrm{Na}}^{+}$ ion is approximately seven times that of the ${\mathrm{NaI}}^{+}$ ion when the pump and probe delay is $150\mathrm{fs}$ and, second, the vibrational period is about $700\mathrm{fs}$ and has an increasing trend with the increase of time.

The hydride anion ${\mathrm{H}}^{-}$ would not be bound in the absence of electron correlation. Since the double-photoionization process is exquisitely sensitive to electron correlation, one expects electron correlation effects to leave a strong signature on the double-ionization cross sections for ${\mathrm{H}}^{-}$. We present fully differential cross sections for the three-body breakup of ${\mathrm{H}}^{-}$ by single-photon absorption. The absolute triply and singly differential cross sections were obtained from ab initio calculations making use of exterior complex scaling within a discrete-variable-representation partial-wave basis. Results calculated at photon energies of 18 and $30\mathrm{eV}$ are compared with reported cross sections for helium calculated at $20\mathrm{eV}$ above the double-ionization threshold. These comparisons show a clear signature of initial-state correlations that differentiate the He and ${\mathrm{H}}^{-}$ cases.