A scheme is presented to realize transfer of a three-dimensional quantum state between two atoms trapped in distant cavities connected by an optical fiber, whose mode is resonant with the polarization cavity modes. Performing an adiabatic passage along dark states, the fiber mode remains in the vacuum state and the population of the cavities being excited can be negligible under certain conditions. In addition, atomic transitions in our scheme are largely detuned from cavity modes. As a consequence, the atomic spontaneous emission and the fiber decay can be effectively eliminated. Furthermore, we give some discussions about the fidelity of our scheme.

The problem of estimating the thermal corrections to the Casimir and Casimir-Polder interactions in systems involving conducting plates has attracted considerable attention in the recent literature on dispersion forces. Alternative theoretical models, based on distinct low-frequency extrapolations of the plate’s reflection coefficient for transverse electric (TE) modes, provide widely different predictions for the magnitude of this correction. In this paper we examine the most widely used prescriptions for this reflection coefficient from the point of view of their consistency with the Bohr–van Leeuwen theorem of classical statistical physics, stating that at thermal equilibrium transverse electromagnetic fields decouple from matter in the classical limit. We find that the theorem is satisfied if and only if the TE reflection coefficient vanishes at zero frequency in the classical limit. This criterion appears to rule out some of the models that have been considered recently for describing the thermal correction to the Casimir pressure with nonmagnetic metallic plates.

We present an efficient quantum algorithm for preparing a pure state on a quantum computer, where the quantum state corresponds to that of a molecular system with a given number $m$ of electrons occupying a given number $n$ of spin orbitals. Each spin orbital is mapped to a qubit: the states $|1⟩$ and $|0⟩$ of the qubit represent, respectively, whether the spin orbital is occupied by an electron or not. To prepare a general state in the full Hilbert space of $n$ qubits, which is of dimension $2^n$, $O\left(2^n\right)$ controlled-NOT gates are needed, i.e., the number of gates scales exponentially with the number of qubits. We make use of the fact that the state to be prepared lies in a smaller Hilbert space, and we find an algorithm that requires at most $O\left(2^{m+1}{n}^m/m!\right)$ gates, i.e., scales polynomially with the number of qubits $n$, provided $n⪢m$. The algorithm is simulated numerically for the cases of the hydrogen molecule and the water molecule. The numerical simulations show that when additional symmetries of the system are considered, the number of gates to prepare the state can be drastically reduced, in the examples considered in this paper, by several orders of magnitude, from the above estimate.

A simple experimental scheme is proposed to generate a three-photon entangled $W$ state and a genuine four-photon entangled state $|{\chi }^{00}⟩$ that has many interesting entanglement properties and possible applications in quantum information processing. These multipartite entangled states can be generated with a certain success probability. We show that even when the quantum efficiency of the detector is $\eta =0.75$, the success probability can reach $2.8\times {10}^{-3}$ for $W$-state production and $3.0\times {10}^{-9}$ for $|{\chi }^{00}⟩$-state production. The scheme is based only on simple linear optical elements and conventional photon detectors, which greatly decrease the experimental difficulty of implementing linear optical quantum computation.

Any bipartite quantum state has quasiprobability representations in terms of separable states. For entangled states these quasiprobabilities necessarily exhibit negativities. Based on the general structure of composite quantum states, one may reconstruct such quasiprobabilities from experimental data. Because of ambiguity, the quasiprobabilities obtained by the bare reconstruction are insufficient to identify entanglement. An optimization procedure is introduced to derive quasiprobabilities with a minimal amount of negativity. Negativities of optimized quasiprobabilities are necessary and sufficient for entanglement; their positivity proves separability.

The concrete schemes to realize three types of basic quantum logical gates using linear quadripartite cluster states of optical continuous variables are proposed. The influences of noises and finite squeezing on the computation precision are analyzed in terms of the fidelity of propagated quantum information through the continuous variable cluster states. The proposed schemes provide direct references for the design of experimental systems implementing quantum computation with the cluster entanglement of amplitude and phase quadratures of light.

We propose an efficient scheme for the implementation of quantum information transfer in a one-dimensional coupled resonator waveguide. We show that, based on the effective long-range dipole-dipole interactions between the atoms mediated by the cavity modes, Raman transitions between the atoms trapped in different nodes can take place. Quantum information could be transferred directly between the opposite ends of the coupled waveguide without involving the intermediate nodes via either Raman transitions or the stimulated Raman adiabatic passages. Since this scheme, in principle, is a one-step protocol, it may provide useful applications in quantum communications.

Results of angle-integrated Stokes-parameter measurements for spin-polarized electron-impact excitation of the $6s6p$ $\left(J=1\right)$ states in mercury, resulting in 185 and 254 nm radiations, are presented. Due to the intermediate-coupling nature of the two states, exchange effects are visible in both excitation processes. However, their influence on the excitation of the 185 nm line $\left({{}^{1}P}_1\to {{}^{1}S}_0\right)$ is significant only for energies very close to the excitation threshold of 6.7 eV, whereas exchange is still relevant for the 254 nm line $\left({{}^{3}P}_1\to {{}^{1}S}_0\right)$ up to at least 50 eV incident energy. Comparison of the experimental data with theoretical predictions reveals a considerable improvement obtained through a fully relativistic 36-state Dirac $R$-matrix (close-coupling) model over a previous 5-state semirelativistic Breit-Pauli $R$-matrix approach. The experimental results at energies above 15 eV for the 254 nm line are likely dominated by cascade effects.

Results from a fully relativistic 36-state Dirac $B$-spline $R$-matrix (close-coupling) calculation for $e$-Hg collisions are reported. Angle-integrated and angle-differential cross sections for elastic scattering and excitation of the $\left(6s6p\right){{}^{3}P}_{0,1,2}^o$ and $\left(6s6p\right){{}^{1}P}_1^o$ states are compared to experimental data and predictions from other theories. We generally obtain a significant improvement in the agreement between experiment and theory compared to previous distorted-wave and close-coupling attempts, in particular, for the near-threshold region that is dominated by various $6s6p^2$ negative-ion resonances.

We report elastic integral, differential, and momentum transfer cross sections for collisions of low-energy electrons with ethylene oxide $\left({\text{C}}_2{\text{H}}_4\text{O}\right)$. We employed the Schwinger multichannel method with pseudopotentials in the static-exchange and static-exchange-polarization approximations, for energies from 0.7 to 20 eV. Experimental studies on vibrational excitation of ${\text{C}}_2{\text{H}}_4\text{O}$ by electron impact reported two shape resonances at around 2.6 and 4.8 eV, while another experimental study on total cross sections reported shape resonances at around 2.7 and 4.6 eV. Our integral cross section shows a peak at around 5 eV which is a shape resonance belonging to the $B_2$ symmetry. We have not found another low-energy shape resonance in our calculations.

Two-dimensional (2D) vector matter waves in the form of soliton-vortex and vortex-vortex pairs are investigated for the case of attractive intracomponent interaction in two-component Bose-Einstein condensates. Both attractive and repulsive intercomponent interactions are considered. By means of a linear stability analysis we show that soliton-vortex pairs can be stable in some regions of parameters while vortex-vortex pairs turn out to be always unstable. The results are confirmed by direct numerical simulations of the 2D coupled Gross-Pitaevskii equations.

We demonstrate two atom interferometric schemes based on Kapitza-Dirac scattering in a magnetic trap. In the first method, two Kapitza-Dirac scattering pulses are applied with a small time delay between them. High contrast interference is observed both using a thermal cloud and a Bose-Einstein condensate (BEC). In the second method, two Kapitza-Dirac scattering pulses are applied to a BEC with a time separation sufficiently large that the interfering orders complete half an oscillation in the magnetic trap; this enables interferometry between spatially separated paths.

Dynamical instabilities in a ${}^{87}\text{R}\text{b}$ $F=1$ spinor Bose-Einstein condensate are used as a parametric amplifier of quantum spin fluctuations. We demonstrate the spectrum of this amplifier to be tunable, in quantitative agreement with theory. We quantify the microscopic spin fluctuations of the initially paramagnetic condensate by applying this amplifier and measuring the resulting macroscopic magnetization. The variance of these fluctuations agrees with predictions of a beyond-mean-field theory. This spin amplifier is thus shown to be nearly quantum limited at a gain as high as 30 dB.

Traditional optical phase imprinting of matter waves is of a dynamical nature. Here we show that both Abelian and non-Abelian geometric phases can be optically imprinted onto matter waves, yielding a number of interesting phenomena such as wave-packet redirecting and wave-packet splitting. In addition to their fundamental interest, our results open up opportunities for robust optical control of matter waves.

A completely analytical and unified approach to the theory of chirped-pulse oscillators is presented. The approach developed is based on the approximate integration of the generalized nonlinear complex Ginzburg-Landau equation and demonstrates that a chirped-pulse oscillator is controlled by only two parameters. It makes it easy to trace spread of the real-world characteristics of both solid-state and fiber oscillators operating in the positive-dispersion regime.

The characterization and conditional preparation of multiphoton quantum states require the use of photon-number resolving detectors. We study the use of detectors based on multiple avalanche photodiode pixels in this context. We develop a general model that provides the positive operator value measures for these detectors. The model incorporates the effect of cross talk between pixels which is unique to these devices. We validate the model by measuring coherent-state photon-number distributions and reconstructing them with high precision. Finally, we evaluate the suitability of such detectors for quantum state tomography and entanglement-based quantum state preparation, highlighting the effects of dark counts and cross talk between pixels.

A general scenario for electronic charge migration in finite samples illuminated by an intense laser pulse is given. Microscopic calculations for neon clusters under strong short pulses as produced by x-ray free-electron laser sources confirm this scenario and point to the prominent role of field ionization by strong internal fields. The latter leads to the fast formation of a core-shell system with an almost static core of screened ions while the outer shell explodes. Substituting the shell ions with a different material such as helium as a sacrificial layer leads to a substantial improvement of the diffraction image for the embedded cluster thus reducing the consequences of radiation damage for coherent diffractive imaging.

Let us consider the set of joint quantum correlations arising from two-outcome local measurements on a bipartite quantum system. We prove that no finite dimension is sufficient to generate all these sets. We approach the problem in two different ways by constructing explicit examples for every dimension $d$, which demonstrates that there exist bipartite correlations that necessitate $d$-dimensional local quantum systems in order to generate them. We also show that at least ten two-outcome measurements must be carried out by the two parties altogether so as to generate bipartite joint correlations not achievable by two-dimensional local systems. The smallest explicit example we found involves 11 settings.

What is the relation between spin squeezing and entanglement? To clarify this, we derive the full set of generalized-spin-squeezing inequalities for the detection of entanglement. These are inequalities for the mean values and variances of the collective angular-momentum components $J_k$. They can be used for the experimental detection of entanglement in a system of spin-$\frac{1}{2}$ particles in which the spins cannot be individually addressed. We present various sets of inequalities that can detect all entangled states that can be detected based on the knowledge of (i) the mean values and variances of $J_k$ in three orthogonal directions, or (ii) the variances of $J_k$ in three orthogonal directions, or (iii) the mean values of $J_k^2$ in three orthogonal directions, or (iv) the mean values and variances of $J_k$ in arbitrary directions. We compare our inequalities to known spin-squeezing entanglement criteria and discuss to which extent spin squeezing is related to entanglement in the reduced two-qubit states. Finally, we apply our criteria for the detection of entanglement in spin models, showing that they can be used to detect bound entanglement in these systems.

We examine state mixing in a dense, cold Rydberg gas. Cold Rb atom clouds in an optical dipole trap are excited into $n{D}_{5/2}$ Rydberg levels using narrow-bandwidth laser pulses. For $n=43$, two atoms excited to $43D_{5/2}$ states can be converted to $41F_{7/2}$ and $45P_{3/2}$ product states via a Förster resonance. We find, unexpectedly, that up to 50% of the Rydberg atoms are detected in such product states after only 100 ns of interaction time. The experiment is modeled using many-body quantum simulations. To reproduce the mixing fractions measured near Förster resonances, we use an exact, nonperturbative many-atom Hamiltonian in which product states are treated on an equal basis with the laser-excited $n{D}_{5/2}$ Rydberg level. Simplified many-body interaction models based on sums over pairwise atomic potentials cannot reproduce the measured mixing fractions.