In the future, faithful exchange of quantum information can become a key part of many computational algorithms. Some authors suggest using chains of mutually coupled spins as channels for quantum communication. One can divide these proposals into the groups of assisted protocols, which require some additional action from the users, and natural ones, based on the concept of state mirroring. We show that state mirroring is fundamentally not the feature of chains of spins-$\frac{1}{2}$ coupled by the Heisenberg interaction, but without local magnetic fields. This fact has certain consequences in terms of the natural state transfer.

We have measured the ratio $R$ of the vector ac Stark effect (or light shift) in the $6S_{1∕2}$ and $5D_{3∕2}$ states of a single trapped barium ion to 0.2% accuracy at two different off-resonant wavelengths. We earlier found $R={\Delta }_S∕{\Delta }_D=-11.494\left(13\right)$ at $514.531\mathrm{nm}$ where ${\Delta }_{S,D}$ are the vector light shifts of the $6S_{1∕2}$ and $5D_{3∕2}$ $m=\pm 1∕2$ splittings due to circularly polarized light, and now we report the value at $1111.68\mathrm{nm}$, $R=+0.4176\left(8\right)$. These observations together yield a value of the $⟨5D\Vert er\Vert 4F⟩$ matrix element. Also, comparison of our results with an ab initio calculation of dynamic polarizability would yield a test of atomic theory and improve the understanding of atomic structure needed to interpret a proposed atomic parity violation experiment.

The dipole polarizability of the ${\mathrm{Si}}^{2+}$ ground state has been determined by a large-scale configuration-interaction (CI) calculation using the sum-over-states approach. The CI calculation was used to describe the valence electron dynamics with respect to the Hamiltonian which treats core-valence correlations with a semiempirical approach. Various higher-order polarizabilies were computed, and this permitted a more refined analysis of the energy intervals measured by resonant excitation Stark ionization spectroscopy (RESIS). The polarizability derived from the revised analysis of the RESIS data was $11.669\left(9\right)\mathrm{a.u.}$, only 0.03% larger than that reported in Komara et al. [J. Phys. B 38, 87 (2005)]. The CI value of the polarizability, $11.688\mathrm{a.u.}$, was only 0.2% larger than experiment, while the quadrupole polarizability of $35.75\mathrm{a.u.}$ was also consistent with experiment. A resonant oscillator strength of 1.621(3) was derived from the experimental dipole polarizability.

We study multiphoton scattering inside a one-dimensional waveguide caused by a number of resonant two-level atoms located in a region small compared to the photon wavelength. Applying powerful methods of the theory of integrable quantum systems, we analyze photon correlations caused by scattering. In particular, for the important case of two-photon scattering, we describe explicitly the dependence of the scattering parameters on the number of two-level atoms. Also, we outline the method of describing scattering of an arbitrary number of photons and present explicit expressions for the scattering of three photons in the effective chiral one-dimensional model. The photon wave function is represented as a sum of terms of a different degree of “entanglement.” We discuss qualitative distinctions in photon correlations for the single two-level atom case and the case of several atoms.

We show that dark solitons in one-dimensional Bose liquids may be created by absorption of a single quanta of an external ac field, in a close analogy with the Einstein’s photoelectric effect. Similarly to the von Lenard’s experiment with photoexcited electrons, the external field’s photon energy $\hslash \omega$ should exceed a certain threshold. In our case the latter is given by the soliton energy ${\epsilon }_s\left(\hslash q\right)$ with the momentum $\hslash q$, where $q$ is photon’s wave number. We find the probability of soliton creation to have a power-law dependence on the frequency detuning $\omega -{\epsilon }_s∕\hslash$. This dependence is a signature of the quantum nature of the absorption process and the orthogonality catastrophe phenomenon associated with it.

The macroscopic quantum theory of the electromagnetic field in a dielectric medium interacting with a dense collection of embedded two-level atoms fails to reproduce a result that is obtained from an application of the classical Lorentz local-field condition. Specifically, macroscopic quantum electrodynamics predicts that the Lorentz redshift of the resonance frequency of the atoms will be enhanced by a factor of the refractive index $n$ of the host medium. However, an enhancement factor of $(n^2+2)∕3$ is derived using the Bloembergen procedure in which the classical Lorentz local-field condition is applied to the optical Bloch equations. Both derivations are short and uncomplicated and are based on well-established physical theories, yet lead to contradictory results. Microscopic quantum electrodynamics confirms the classical local-field-based results. Then the application of macroscopic quantum electrodynamic theory to embedded atoms is proved false by a specific example in which both the correspondence principle and microscopic theory of quantum electrodynamics are violated.

Within the framework of macroscopic quantum electrodynamics, the resonant van der Waals potential experienced by an excited two-level atom near a planar magnetoelectric two-layer system consisting of a slab of left-handed material and a perfect mirror is studied. It is shown that the disregard of material absorption leads to unphysical results, with divergent values for the potential away from the surface. Under appropriate conditions, the setup is found to feature a barrier near the surface which can be employed to levitate particles or used as a trapping or cooling mechanism. Finally, the problem of spontaneous decay [Kästel and Fleischhauer, Phys. Rev. A 68, 011804(R) (2005)] is reconsidered. Disregard of absorption is shown to drastically falsify the dependence on the atomic position of the decay rate.

We consider Mach-Zehnder and Hong-Ou-Mandel interferometers with nonclassical states of light as input, and study the effect that dispersion inside the interferometer has on the sensitivity of phase measurements. We study in detail a number of different one- and two-photon input states, including Fock, dual Fock, maximally path-entangled $\mid N,0⟩+⟨0_{,}N\mid$ (“N00N”) states, and photon pairs from parametric down-conversion. Assuming there is a phase shift ${\varphi }_0$ in one arm of the interferometer, we compute the probabilities of measurement outcomes as a function of ${\varphi }_0$, and then compute the Shannon mutual information between ${\varphi }_0$ and the measurements. This provides a means of quantitatively comparing the utility of various input states for determining the phase in the presence of dispersion. In addition, we consider a simplified model of parametric down-conversion for which probabilities can be explicitly computed analytically, and which serves as a limiting case of the more realistic down-conversion model.

We present the dynamic evolution of a probe pulse traveling through a cold-atom ensemble composed of a $\Lambda$-type three-level system interacting with counterpropagating control fields. We numerically demonstrate quantum coherent control of the temporal pulse splitting resulting from atom-field interactions for quantum optical information processing. We discuss the underlying physics using four-wave mixing generation and Bragg-grating reflection.

Raman satellites have been observed in the scattering of a Nd:YAG laser $\left(532\mathrm{nm}\right)$ from a laser-ablated Mg plasma plume. We identify them as originating from transitions between the fine-structure components of the metastable $3s3p{}^{3}P_{0,1,2}$ level of Mg. We have calculated the cross sections for Raman and Rayleigh scattering from the metastable state. Comparison of the expected ratio of the satellites to the Rayleigh peak indicates the changing population fraction of the metastable states in the plume.

We study the quantum dynamics of coherent creation of triatomic molecules from a repulsive atomic Bose-Einstein condensate. In a single-mode approach with short pulse duration we consider the squeezing effects for homonuclear and heteronuclear trimers by taking into account an optical photoassociation. The possibility of manipulating the quantum statistical properties of the trimer fields provides an appealing research area for current molecular matter-wave optics.

In this paper, we show how the framework of information geometry, the natural geometry of discrete probability distributions, can be used to derive the quantum formalism. The derivation rests upon a few elementary features of quantum phenomena, such as complementarity and global gauge invariance. When appropriately formulated within an information-geometric framework, and combined with a novel information-theoretic principle, these features lead to the abstract quantum formalism for finite-dimensional quantum systems, and the result of Wigner’s theorem. By means of a correspondence principle, several correspondence rules of quantum theory, such as the canonical commutation relationships, are also systematically derived. The derivation suggests that information geometry is directly or indirectly responsible for many of the central structural features of the quantum formalism, such as the importance of square roots of probability and the occurrence of sinusoidal functions of phases in a pure quantum state. Global gauge invariance is shown to play a crucial role in the emergence of the formalism in its complex form.

We construct a nonlocal density functional approximation with full exact exchange, while preserving the constraint-satisfaction approach and justified error cancellations of simpler semilocal functionals. This is achieved by interpolating between different approximations suitable for two extreme regions of the electron density. In a “normal” region, the exact exchange-correlation hole density around an electron is semilocal because its spatial range is reduced by correlation and because it integrates over a narrow range to $-1$. These regions are well described by popular semilocal approximations (many of which have been constructed nonempirically), because of proper accuracy for a slowly varying density or because of error cancellation between exchange and correlation. “Abnormal” regions, where nonlocality is unveiled, include those in which exchange can dominate correlation (one-electron, nonuniform high density, and rapidly varying limits), and those open subsystems of fluctuating electron number over which the exact exchange-correlation hole integrates to a value greater than $-1$. Regions between these extremes are described by a hybrid functional mixing exact and semilocal exchange energy densities locally, i.e., with a mixing fraction that is a function of position $\mathbf{r}$ and a functional of the density. Because our mixing fraction tends to 1 in the high-density limit, we employ full exact exchange according to the rigorous definition of the exchange component of any exchange-correlation energy functional. Use of full exact exchange permits the satisfaction of many exact constraints, but the nonlocality of exchange also requires balanced nonlocality of correlation. We find that this nonlocality can demand at least five empirical parameters, corresponding roughly to the four kinds of abnormal regions. Our local hybrid functional is perhaps the first accurate fourth-rung density functional or hyper-generalized gradient approximation, with full exact exchange, that is size-consistent in the way that simpler functionals are. It satisfies other known exact constraints, including exactness for all one-electron densities, and provides an excellent fit to the 223 molecular enthalpies of formation of the G3/99 set and the 42 reaction barrier heights of the BH42/03 set, improving both (but especially the latter) over most semilocal functionals and global hybrids. Exact constraints, physical insights, and paradigm examples hopefully suppress “overfitting.”

The formation of dark states under an interaction of degenerate atomic states with incoherent broadband radiation (white light) is discussed. A simple coupling scheme in a three level $\Lambda$ system, which allows both qualitative and quantitative analysis, is discussed. We found a stationary solution of the optical Bloch equations in a broad excitation line approximation that describes the dynamics of the atom-white light interaction and demonstrated its identity to a conventional dark state created with coherent laser fields. We than examine the efficiency of the population transfer induced by broadband radiation in a model $\Lambda$ system and revealed that high efficiency (attaining 100%) of stimulated Raman adiabatic passagelike processes can be achieved with certain temporal control of light polarization. The corresponding criterion of adiabaticity was formulated and justified by means of numerical simulations.

A limitation on simultaneous measurement of two arbitrary positive-operator-valued measures is discussed. In general, simultaneous measurement of two noncommutative observables is only approximately possible. Following Werner’s formulation, we introduce a distance between observables to quantify an accuracy of measurement. We derive an inequality that relates the achievable accuracy with noncommutativity between two observables. As a byproduct a necessary condition for two positive-operator-valued measures to be simultaneously measurable is obtained.

Antihydrogen production by charge exchange reaction between positronium (Ps) atoms and antiprotons requires an efficient excitation of Ps atoms up to high-$n$ levels (Rydberg levels). In this study it is assumed that a Ps cloud is produced within a relatively strong uniform magnetic field $\left(1\mathrm{T}\right)$ and with a relatively high temperature $\left(100\mathrm{K}\right)$. Consequently, the structure of energy levels are deeply modified by Zeeman and motional Stark effects. A two-step laser light excitation, the first one from ground to $n=3$ and the second from this level to a Rydberg level, is proposed and the physics of the problem is discussed. We derive a simple formula giving the absorption probability with substantially incoherent laser pulses. A 30% population deposition in high-$n$ states can be reached with feasible lasers suitably tailored in power and spectral bandwidth.

We study the near-threshold behavior of scattering phase shifts, quantum reflection amplitudes, and quantization functions for systems described by a circularly symmetric potential in two spatial dimensions. In contrast to the three-dimensional case, the centrifugal potential $(m^2-\frac{1}{4}){\hslash }^2∕\left(2M{r}^2\right)$ is nonvanishing and even attractive for $s$ waves, $m=0$, and the leading near-threshold energy dependence of phase shifts and amplitudes for scattering and quantum reflection is logarithmic in this case. The near-threshold behavior of the $s$-wave phase shifts and amplitudes can nevertheless be characterized by a well-defined scattering length (for potentials falling off faster than $1∕r^2$) and an effective range (for potentials falling off faster than $1∕r^4$). For a potential with a bound state at energy $E_n=-{\hslash }^2{\kappa }_n^2∕\left(2M\right)$ very near threshold, the scattering length obeys $a\stackrel{{\kappa }_n\to 0}{\sim }2e^{-{\gamma }_{\mathrm{E}}}∕{\kappa }_n+O\left({\kappa }_n\right)$, with no term of order ${\kappa }_n^0$—in contrast to the three-dimensional case. Analytical results are derived for homogeneous potentials, and the necessary modification of the effective-range expansion is given for potentials proportional to $1∕r^4$. For $m\ne 0$ we give analytical expressions for the near-threshold behavior of phase shifts as well as scattering and quantum reflection amplitudes, which are generally valid, even when $m$ is neither integer nor half-integer.

A combined experimental and theoretical study of the collision-induced dissociation of $\mathrm{C}\mathrm{O}_2{}^{+}$ ions colliding with He in the energy range of $1\text{to}9\mathrm{keV}$ has been carried out in this work. Total cross sections for the dissociation products ${\mathrm{O}}^{+}$, ${\mathrm{C}}^{+}$, and $\mathrm{C}{\mathrm{O}}^{+}$ are presented as a mean to investigate the collision dynamics of this molecular ion. A remarkable change on the dissociation dynamics is resolved at about $\sim 4\mathrm{keV}$ collision energy. From the comparison of the measured cross sections to a theoretical treatment of the collision process using electron-nuclear dynamics, we infer that electronic multireference configurations are required in the dynamics at low collision energies. For the ${\mathrm{C}}^{+}$ dissociation product, the collision dynamics is fully described by theory with the possible isomerization of the $\mathrm{C}\mathrm{O}_2{}^{+}$ being discarded at these collision energies. As a consequence of the calibration procedure in the experimental method, double electron capture cross section for protons on argon are also reported in good agreement with other available experiments.

We study the properties of a dilute low-energy two-component Fermi gas. To this end, we approximate the Fermi gas by the Hubbard lattice Hamiltonian and solve it using the dynamical mean-field approximation. At unitarity, this approximation yields the value $\xi \approx 0.44$ for the ratio between the ground-state energy per particle of the interacting and free systems $(E∕N=\xi E_{FG})$, in remarkable agreement with full quantum Monte Carlo simulations. Investigating the evolution of the system with temperature, we have found that below the critical temperature $T_c$, the system is a superfluid and the energy gap is decreasing monotonously. For temperatures above $T_c$, the system is an insulator and the corresponding energy gap is monotonously increasing.

Second-order intensity correlation measurements were made using a two-element superconducting nanowire single photon detector (SNSPD) without the need for an optical beam splitter. This approach can be used to obtain a $50\text{‐}\mathrm{ps}$ full width at half maximum timing resolution over a wide range of visible and near-infrared wavelengths and can be extended to measure higher-order intensity correlations. Measurements of the second-order intensity correlation of a pulsed laser and an InGaAs quantum dot were made using both a two-element SNSPD and a conventional Hanbury Brown–Twiss interferometer to demonstrate the accuracy and advantages of the multielement SNSPD.