To advance the QED approach and exploit more details of the high-order harmonic generation (HHG) of atoms and molecules in intense ultrashort laser fields, a QED model for HHG is developed in a time-dependent framework and a multimode-laser setup. By introduction of the classical-field-dressed quantized Volkov states, an analytical expression to calculate HHG for hydrogenlike atoms and ions in an ultrashort intense laser pulse is obtained. This formula provides a simple intuitive interpretation of the mechanism in which the electron is first ionized to the classical-field-dressed quantized Volkov states, and then falls back to the ground state to emit the harmonic photon. Calculations using this formalism demonstrate a good agreement with recent semiclassical computations. The limiting of the existing QED models, which are successful in providing alternative perspectives on the HHG beyond semiclassical treatments, to the time-independent framework and the single-mode laser have been removed to take into account the laser pulse’s length and its shape. Possible extensive applications of this QED approach as well as its potential usefulness for research in various interesting fields are also discussed. The long-wavelength approximation and strong-field approximation are involved in the development of the formalism.

We study single-photon transport in an array of coupled microcavities where two two-level atomic systems are embedded in two separate cavities of the array. We find that a single photon can be totally reflected by a single two-level system. However, two separate two-level systems can also create, between them, single-photon quasibound states. Therefore, a single two-level system in the cavity array can act as a mirror while a different type of cavity can be formed by using two two-level systems, acting as tunable “mirrors,” inside two separate cavities in the array. In analogy with superlattices in solid state physics, we call this “cavity inside a coupled-cavity array” a supercavity. This supercavity is the quantum analog of Fabry-Perot interferometers. Moreover, we show that the physical properties of this quantum supercavity can be adjusted by changing the frequencies of these two-level systems.

With the HOD molecule initially in its vibrational ground state, we theoretically analyze the laser-induced control of the $\mathrm{O}\mathrm{D}∕\mathrm{O}\mathrm{H}$ branching ratio $\mathrm{D}+\mathrm{O}\mathrm{H}\leftarrow \mathrm{H}\mathrm{O}\mathrm{D}\to \mathrm{H}+\mathrm{O}\mathrm{D}$ in the first absorption band. In the weak-field limit, any form of UV-pulse shaping control leads to a branching ratio larger than $\sim 2$. We obtain in the strong-field limit (peak intensities $\sim 10\mathrm{TW}∕{\mathrm{cm}}^2$) a branching ratio significantly less than 2. The optimized pulses operate by a pump-dump-pump mechanism, where the dumping to the electronic ground state creates nonstationary vibrational states in HOD.

We have observed the stationary interference oscillations of a triple-entangled neutron state in an interferometric experiment. Time-dependent interaction with two radio-frequency fields enables coherent manipulation of an energy degree of freedom in a single neutron. The system is characterized by a multiply entangled state governed by a Jaynes-Cummings Hamiltonian. The experimental results confirm coherence of the manipulation as well as the validity of the description.

Quantum systems in Fock states do not have a phase. When two or more Bose-Einstein condensates are sent into interferometers, they nevertheless acquire a relative phase under the effect of quantum measurements. The usual explanation relies on spontaneous symmetry breaking, where phases are ascribed to all condensates and treated as unknown classical quantities. However, this image is not always sufficient: when all particles are measured, quantum mechanics predicts probabilities that are sometimes in contradiction with it, as illustrated by quantum violations of local realism. In this Rapid communication, we show that interferometers can be used to demonstrate a large variety of violations with an arbitrarily large number of particles. With two independent condensates, we find violations of the Bell-Clauser-Horne-Shimony-Holt inequalities, as well as $N$-body Hardy impossibilities. With three condensates, we obtain Greenberger-Horne-Zeilinger-type contradictions.

To date, most efforts to demonstrate quantum nonlocality have concentrated on systems of two (or very few) particles. It is, however, difficult in many experiments to address individual particles, making it hard to highlight the presence of nonlocality. We show how a natural setup with no access to individual particles allows one to violate the Clauser-Horne-Shimony-Holt inequality with many pairs, including in our analysis effects of noise and losses. We discuss the case of distinguishable and indistinguishable particles. Finally, a comparison of these two situations provides insight into the complex relation between entanglement and nonlocality.

Based on quantum complementary relations (QCRs) and a purification scenario, we analyze a class of $N$-qubit mixed states that are entangled but do not have two-, and genuine three-, four-,…,$N$-qubit entanglements. It is shown that entanglement (one-tangle or negativity) in these mixed states is closely related to the QCR entanglement of their purified states. In particular, it is elaborated that when the mixed state does not have multipartite tangles (two- and higher tangles), its entanglement is actually a kind of genuine multipartite QCR entanglement between the system and its environment.

Theoretical studies on hyperfine-interaction-dependent lifetimes of the $4s4p{}^{3}P_2$ states of Zn-like ions are reported in this paper. A systematic investigation along the isoelectronic sequence shows that the lifetime of the $4s4p{}^{3}P_2$ level is very sensitive to hyperfine interaction at the beginning of the sequence, but gets less sensitive going to the higher end. The intensity ratio between the $4s^2{}^{1}S_0\text{−}4s4p{}^{3}P_2$ and the $4s4p{}^{3}P_1\text{−}4s4p{}^{3}P_2$ transitions was also investigated and it is shown that this ratio is very sensitive to hyperfine interaction. In some isotopes the two branches are dominated by transitions from different upper hyperfine levels. Our research also reveals that the hyperfine-induced electric dipole transitions contribute to the transitions of $4s^2{}^{1}S_0\text{−}4s4p{}^{3}P_2$ at about the same level as, or even higher than, $M2$ transitions whenever the nucleus has a spin. This is true all along the isoelectronic sequence.

We present lifetime measurements of the $5D_{3∕2}$ and $5D_{5∕2}$ states of rubidium using the time-correlated single-photon-counting method. We perform the experiment in a magneto-optical trap of ${}^{87}\mathrm{Rb}$ atoms using a two-step excitation with the trap laser at $780\mathrm{nm}$ as the first step. We record the $761.9\mathrm{nm}$ fluorescence from the decay of the $5D_{3∕2}$ state to the $5P_{1∕2}$ state, and measure the lifetime of the $5D_{3∕2}$ state $\tau =246.3\left(1.6\right)\mathrm{ns}$. We record the $420.2\mathrm{nm}$ fluorescence from the cascade decay of the $5D_{5∕2}$ state to the $5S_{1∕2}$ state through the $6P_{3∕2}$ state, and extract the lifetime of the $5D_{5∕2}$ state $\tau =238.5\left(2.3\right)\mathrm{ns}$.

The time-dependent Dirac equation was solved for zero-impact-parameter bare U-U collisions in the monopole approximation using a mapped Fourier grid matrix representation. A total of 2048 states including bound, as well as positive- and negative-energy states for an $N=1024$ spatial grid were propagated to generate occupation amplitudes as a function of internuclear separation. From these amplitudes spectra were calculated for total inclusive positron and electron production, and also the correlated spectra for $(e^{+},e^{-})$ pair production. These were analyzed as a function of nuclear sticking time in order to establish signatures of spontaneous pair creation, i.e., QED vacuum decay. Subcritical Fr-Fr and highly supercritical Db-Db collisions both at the Coulomb barrier were also studied and contrasted with the U-U results.

We present a geometrical representation of the process of sum frequency generation in the undepleted pump approximation, in analogy with the known optical Bloch equations. We use this analogy to propose a technique for achieving both high efficiency and large bandwidth in sum frequency conversion using the adiabatic inversion scheme. The process is analogous with rapid adiabatic passage in NMR, and adiabatic constraints are derived in this context. This adiabatic frequency conversion scheme is realized experimentally using an aperiodically poled potassium titanyl phosphate (KTP) device, where we achieved high efficiency signal-to-idler conversion over a bandwidth of $140\mathrm{nm}$.

We use numerically exact computer simulations of multiple scattering in physically realistic models of sparse discrete random media to quantify the errors of the scalar approximation (SA) in computations of coherent backscattering (CB) assuming that the incident light is unpolarized. We show that while the SA errors in the diffuse backscattered intensity are often small, those in the CB enhancement factor can reach 25% and often exceed 20%. We attribute this to the fact that the computation of the enhancement factor involves all diagonal elements of the diffuse backscattering Stokes matrix rather than only its (1, 1) element. Therefore, the coherent enhancement of backscattered intensity appears to be the result of a complex interplay of various polarization effects involved in the process of multiple scattering. Thus our numerical data make a strong case against the use of the SA in theoretical computations of CB in the case of unpolarized incident light.

We investigate intensity of the parametric down conversion of ultrashort, ultraviolet pulse both in a low- and high-conversion regime. In the first regime, we develop a simple analytical expressions for the photon flux of the fluorescence. Numerical simulations using three-dimensional stochastic Wigner method provide results in the latter regime. We find that the perturbative approximation for the photon flux as a function of the wavelength of the signal and the observation angle, quantitatively describes even the nonperturbative regime. For short pump pulses the intensity of the fluorescence is highest at frequencies where the group velocities of the pump and the down-conversion are equal. For longer pump pulses this requirement is gradually relaxed. Additionally, for small pump beams, the intensity strongly depends on the spatial divergence between interacting fields.

The concept of reciprocity symmetry for matter-wave propagation is established for nonrelativistic quantum mechanics with previous results in the literature extended to include nonlocal interactions. Examples are given for cases with both local and nonlocal potentials, where we show in particular that reciprocity can be violated for the motion of a charged particle in an external electromagnetic field. In addition, this symmetry is applied to interpret a recent analysis [Phys. Rev. A 64, 042716 (2001)] on the symmetry of transmission through one-dimensional complex potentials, with the emphasis that the validity of reciprocity can go beyond that of time-reversal symmetry, such as in the presence of absorption in which the latter symmetry breaks down.

We highlight the direct link between the time-dependent entanglement and single-qubit excited-state population for two independent qubits, each coupled to a zero-temperature bosonic environment. We show that, in environments structured so as to inhibit spontaneous emission, entanglement trapping and thus prevention of entanglement sudden death occur. We explicitly show that, for photonic band-gap materials as environment, high values of entanglement trapping can be achieved. We finally discuss how, under these conditions, coherent quantum operations can be implemented.

We formulate and numerically simulate the single control qubit Shor algorithm for the case of static imperfections induced by residual couplings between qubits. This allows us to study the accuracy of Shor’s algorithm with respect to these imperfections using numerical simulations of realistic quantum computations with up to $n_q=18$ computational qubits allowing to factor numbers up to $N=205193$. We confirm that the algorithm remains operational up to a critical coupling strength ${ϵ}_c$ which drops only polynomially with ${\log }_{2}N$. The obtained numerical dependence of ${ϵ}_c$ on ${\log }_{2}N$ is in a good agreement with the analytical estimates that allows to obtain the scaling for functionality of Shor’s algorithm on realistic quantum computers with a large number of qubits.

We consider the problem of creating a long-distance entangled state between two stations of a network, where neighboring nodes are connected by noisy quantum channels. We show that any two stations can share an entangled pair if the effective probability for the quantum errors is below a certain threshold, which is achieved by a local encoding of the qubits and a global bit-flip correction. In contrast to the conventional quantum-repeater schemes, we do not need to store the qubits in quantum memory for a long time: our protocol is a one-shot process (i.e., the elementary entangled pairs are used only once) involving one-way classical communication. Furthermore, the overhead of local resources increases only logarithmically with the size of the network, making our proposal favorable to long-distance quantum communication.

Dispersion interactions of the ground and resonantly excited states of Cu and Ag with a number of buffer gases are determined. The valence excitation spectrum was calculated by diagonalizing a semiempirical Hamiltonian in a large-dimension single-electron basis. The core made a significant contribution to the $C_6$ and $C_8$ dispersion coefficients for both copper and silver. Oscillator strengths and static scalar and tensor polarizabilities are given for some of the low-lying states.

We describe a protocol to generate transiently localized circular wave packets in very-high-$n$ Rydberg states. These are created from strongly polarized quasi-one-dimensional Rydberg states by applying a transverse pulsed electric field. The resulting wave packet becomes transiently localized as the result of focusing and travels in a nearly circular Bohr-like orbit around the nucleus for several orbital periods. The localization properties can be controlled by carefully choosing the shape of the field pulse, in particular, its rise and fall times. Remarkably, the wave packets exhibit classical revivals after the initial dephasing on time scales shorter than those expected for quantum revivals.

We introduce the concept of noncoherent optical pulse discrimination from a coherent (or partially coherent) signal of the same energy using the phenomenon of soliton generation. The impact of randomization of the optical signal content on the observable characteristics of soliton generation is examined and quantified for the particular example of a rectangular pulse.