We examine time-domain instabilities in short chains of coupled Kerr-nonlinear resonators. We show that a large parameter region of self-pulsing or chaotic behavior can be realized. We give a detailed description of the possible states in systems with two and three cavities, using phenomenological coupled-mode equations and rigorous simulations. A particular geometry can exhibit a rich range of dynamics, dependent on input conditions. A clear link with the linear transmission properties is shown. This system complements the studies of the nonlinear Bragg reflector and electrons in a nonlinear lattice. Unlike the Bragg grating case, we observe wide detuning ranges that exhibit self-pulsing without first going through a bistable transition.

We estimate the quantum state of a system between two measurements via a maximum likelihood strategy. This incorporates all the information provided by preselection and postselection in a legitimate quantum state. Moreover, it allows to include any other constraint derived from practical considerations.

We propose a multidimensional (MD) pump-probe method to image the coupled rovibrational (RV) nuclear dynamics in small molecules. We apply this imaging method to analyze the laser-induced coherent RV nuclear dynamics in ${\text{D}}_2^{+}$ in terms of MD quantum-beat (QB) spectra, without and including an external laser field, by solving the time-dependent Schrödinger equation in full dimensionality. These spectra resolve the internuclear distance $R$ and molecular orientation and reveal (1) the transition frequencies between stationary vibrational and rotational states, (2) their nodal structures, (3) RV couplings, including (4) their influence on the nuclear dynamics on field-free ground state and laser-dressed potential surfaces, and (5) offer an interpretation for an unexplained structure in recently measured QB spectra.

Memory dephasing and its impact on the rate of entanglement generation in quantum repeaters is addressed. For systems that rely on probabilistic schemes for entanglement distribution and connection, we estimate the maximum achievable rate per employed memory for our optimized partial nesting protocol, when a large number of memories are being used in each node. The above rate scales polynomially with distance, $L$, if quantum memories with infinitely long coherence times are available or if we employ a fully fault-tolerant scheme. For memories with finite coherence times and no fault-tolerant protection, the above rate optimistically degrades exponentially in $\sqrt{L}$, regardless of the employed purification scheme. It decays, at best, exponentially in $L$ if no purification is used.

We prove the unconditional security of the original Bennett 1992 protocol with strong reference pulse. We show that we may place a projection onto suitably defined qubit spaces before the receiver, which makes the analysis as simple as qubit-based protocols. Unlike the single-photon-based qubits, the qubits identified in this scheme are almost surely detected by the receiver even after a lossy channel. This leads to the key generation rate that is proportional to the channel transmission rate for proper choices of experimental parameters.

We address the problem of generating a robust entangling gate between electronic and nuclear spins in a system of a single nitrogen-vacancy center coupled to a nearest ${}^{13}\text{C}$ atom in diamond against certain types of systematic errors such as pulse-length and off-resonance errors. We analyze the robustness of various control schemes: sequential pulses, composite pulses and numerically optimized pulses. We find that numerically optimized pulses, produced by the modified gradient ascent pulse engineering (GRAPE) algorithm, are more robust than the composite pulses and the sequential pulses. The optimized pulses can also be implemented in a faster time than the composite pulses.

The most important processes for the creation of ${\text{S}}^{12+}$ to ${\text{S}}^{14+}$ ions excited states from the ground configurations of ${\text{S}}^{9+}$ to ${\text{S}}^{14+}$ ions in an electron cyclotron resonance ion source, leading to the emission of $K$ x-ray lines, are studied. Theoretical values for inner-shell excitation and ionization cross sections, including double-$KL$ and triple-$KLL$ ionizations, transition probabilities and energies for the de-excitation processes, are calculated in the framework of the multiconfiguration Dirac-Fock method. With reasonable assumptions about the electron energy distribution, a theoretical $K\alpha$ x-ray spectrum is obtained, which is compared to recent experimental data.

Stimulated by a recent measurement of coherent control in photoionization of atomic barium, we have calculated one- and two-photon ionization cross sections of the aligned $6s6p{{}^{1}P}_1$ state of barium in the energy range between the $5d_{3/2}$ and $5d_{5/2}$ states of ${\text{Ba}}^{+}$. We have also measured these photoionization spectra in the same energy region, driving the one- or two-photon processes with the second or first harmonic of a tunable dye laser, respectively. Our calculations employ the eigenchannel $R$-matrix method and multichannel quantum defect theory to calculate the rich array of autoionizing resonances in this energy range. The nonresonant two-photon process is described using lowest-order perturbation theory for the photon-atom interactions, with a discretized intermediate-state one-electron continuum. The calculations provide an absolute normalization for the experiment, and they accurately reproduce the rich resonance structures in both the one- and two-photon cross sections, and confirm other aspects of the experimental observations. These results demonstrate the ability of these computationally inexpensive methods to reproduce experimental observables in one- and two-photon ionization of heavy alkaline earths, and they lay the groundwork for future studies of the phase-controlled interference between one- and two-photon ionization processes.

We describe analytically and numerically the process of population transfer by stimulated Raman adiabatic passage through a bright state when the pulses propagate in a medium. Limitations of the adiabaticity are analyzed and interpreted in terms of reshaping of the pulses. We find parameters for the pulses for which the population transfer is nearly complete over long distances.

In this paper, we estimate the losses during teleportation processes requiring either two high-$Q$ cavities or a single bimodal cavity. The estimates were carried out using the phenomenological operator approach introduced by de Almeida et al. [Phys. Rev. A 62, 033815 (2000)].

We study the description of the Bloch theorem in the cylindrically periodic structure and we show that it is valid in a good approximation except for the neighborhood of the origin. The cylindrical Bloch theorem is applied to the analysis of a Bragg fiber, a kind of photonic bandgap fibers. It is confirmed that the above exception does not prevent us from using the theorem for a practical use. We show a procedure to obtain the propagation constant and electromagnetic fields of the TE mode in the Bragg fiber. In the present scheme, electromagnetic fields are genuinely treated as cylindrical waves, namely, Bessel functions without the asymptotic expansion. Then we can obtain a clear mathematical description of the Bragg fiber and investigate its physical properties. The present theory gives a criterion to design generalized Bragg fibers.

We performed nuclear dynamics calculations on ${\text{C}}_2{\text{H}}_2$ and ${\text{C}}_2{\text{D}}_2$ to study the isotope effect in dissociative electron attachment (DEA). Our previous calculations at 0 K led to a ratio ${\sigma }_{0\to \text{DEA}}^{\left({\text{C}}_2{\text{H}}^{-}\right)}/{\sigma }_{0\to \text{DEA}}^{\left({\text{C}}_2{\text{D}}^{-}\right)}$ of about 28.9 which is about a factor of 2 higher than recent experimental results. This discrepancy was attributed to the contribution of higher vibrational modes in the yield of the anion fragments. We included the four lowest bending vibrational states presenting nonvanishing populations at the experiment temperature of $T=333\text{K}$. The resulting ratio is found to be 17.9 which is in closer agreement with the measured value.

We demonstrate how to efficiently derive a broad class of inequalities for entanglement detection in multimode continuous variable systems. The separability conditions are established from partial transposition (PT) in combination with several distinct necessary conditions for a quantum physical state, which include previously established inequalities as special cases. Remarkably, our method enables us to support Peres’ conjecture to its full generality within the framework of Cavalcanti-Foster-Reid-Drummond multipartite Bell inequality [Phys. Rev. Lett. 99, 210405 (2007)] that the nonlocality necessarily implies negative PT entangled states.

We report an experiment in which the correlated imaging in an interferometer with chaotic light can be observed in the intensity pattern rather than the intensity correlation measurement. Similar to ghost imaging with thermal light, object and objective lens are not necessarily set in the same path. Moreover, the two imaging schemes satisfy the same imaging equation. The theoretical analysis shows that the correlated imaging of thermal light is ascribed to the first-order field correlation of the two separate paths, and as a result, it can also be performed through one-photon interference in an incoherent interferometer. Our experiment suggests a phase-sensitive imaging technique using a spatially incoherent light source.

We present a framework to treat quantum networks and all possible transformations thereof, including as special cases all possible manipulations of quantum states, measurements, and channels, such as, e.g., cloning, discrimination, estimation, and tomography. Our framework is based on the concepts of quantum comb—which describes all transformations achievable by a given quantum network—and link product—the operation of connecting two quantum networks. Quantum networks are treated both from a constructive point of view—based on connections of elementary circuits—and from an axiomatic one—based on a hierarchy of admissible quantum maps. In the axiomatic context a fundamental property is shown, which we call universality of quantum memory channels: any admissible transformation of quantum networks can be realized by a suitable sequence of memory channels. The open problem whether this property fails for some nonquantum theory, e.g., for no-signaling boxes, is posed.

We present a quantum algorithm based on classical fully polynomial randomized approximation schemes (FPRASs) for estimating partition functions that combine simulated annealing with the Monte Carlo Markov chain method and use nonadaptive cooling schedules. We achieve a twofold polynomial improvement in time complexity: a quadratic reduction with respect to the spectral gap of the underlying Markov chains and a quadratic reduction with respect to the parameter characterizing the desired accuracy of the estimate output by the FPRAS. Both reductions are intimately related and cannot be achieved separately. First, we use Grover’s fixed-point search, quantum walks, and phase estimation to efficiently prepare approximate coherent encodings of stationary distributions of the Markov chains. The speed up we obtain in this way is due to the quadratic relation between the spectral and phase gaps of classical and quantum walks. The second speed up with respect to accuracy comes from generalized quantum counting used instead of classical sampling to estimate expected values of quantum observables.

We study the one-dimensional $S=\frac{1}{2}$ $XXZ$ model on a finite lattice at zero temperature, varying the exchange anisotropy $\Delta$ and the number of sites $N$ of the lattice. Special emphasis is given to the model with $\Delta =\frac{1}{2}$ and $N$ odd, whose ground state, the so-called Razumov-Stroganov state, has a peculiar structure and no finite-size corrections to the energy per site. We find that such model corresponds to a special point on the $\Delta$ axis which separates the region where adding spin-pairs increases the energy per site from that where the longer the chain the lower the energy. Entanglement properties do not hold surprises for $\Delta =\frac{1}{2}$ and $N$ odd. Finite-size corrections to the energy per site non trivially vanish also in the ferromagnetic $\Delta \to -1^{+}$ isotropic limit, which is consequently addressed; in this case, peculiar features of some entanglement properties, due to the finite length of the chain and related with the change in the symmetry of the Hamiltonian, are evidenced and discussed. In both the above models the absence of finite-size corrections to the energy per site is related to a peculiar structure of the ground state, which has permitted us to provide new exact analytic expressions for some correlation functions.

We apply our technique of using a Rb-stabilized ring-cavity resonator to measure the frequencies of various spectral components in the 555.8-nm ${{}^{1}S}_0\to {{}^{3}P}_1$ line of Yb. We determine the isotope shifts with 60 kHz precision, which is an order-of-magnitude improvement over the best previous measurement on this line. There are two overlapping transitions, ${}^{171}\text{Y}\text{b}\left(1/2\to 3/2\right)$ and ${}^{173}\text{Y}\text{b}\left(5/2\to 3/2\right)$, which we resolve by applying a magnetic field. We thus obtain the hyperfine constants in the ${{}^{3}P}_1$ state of the odd isotopes with a significantly improved precision. Knowledge of isotope shifts and hyperfine structure should prove useful for high-precision calculations in Yb necessary to interpret ongoing experiments testing parity and time-reversal symmetry violation in the laws of physics.

Casimir interactions are not pairwise additive. This property leads to collective effects that we study for a pair of objects near a conducting wall. We employ a scattering approach to compute the interaction in terms of fluctuating multipoles. The wall can lead to a nonmonotonic force between the objects. For two atoms with anisotropic electric and magnetic dipole polarizabilities, we demonstrate that this nonmonotonic effect results from a competition between two- and three-body interactions. By including higher-order multipoles we obtain the force between two macroscopic metallic spheres for a wide range of sphere separations and distances to the wall.

Recent measurements have been made of the angle-integrated Stokes parameters for the spin-polarized electron impact excitation of the $6s6p$ $\left(J=1\right)$ states in mercury, resulting in the emission of 185 and 254 nm radiation to the ground state through the ${{}^{1}P}_1\text{−}{{}^{1}S}_0$ and ${{}^{3}P}_1\text{−}{{}^{1}S}_0$ transitions, respectively. These measurements give information on the electron exchange and spin-orbit coupling effects of the target and projectile electrons. We have evaluated the integrated Stokes parameters using our relativistic distorted-wave method which takes direct account of the fine structure of the excited atomic states as well as the spin of the incident electrons. We compare these results with the recent experimental and theoretical data. In general the agreement between our theoretical results and the experimental measurements is very good.