We study a hybrid mode in a composite system consisting of a localized metal nanosphere on a subwavelength fiber. It is found that the hybrid mode resulting from coupling of the fiber propagating mode and the plasmonic resonance is promising for strongly enhanced interaction between light and matter. We then propose a single-nanoparticle detection scheme by monitoring the nanofiber transmission change induced by the plasmonic-enhanced target scattering. The detection limit can be as low as 18 nm in target diameter, showing great potential for sensing single viruses or biomolecules. In the hybrid system, the resonant wavelength is tunable from visible to near-infrared spectral range by employing a metal nanoshell instead of the nanosphere, while the low detection limit of a few tens of nanometers can still remain.

The properties of electron vortex beams are examined when subject to static magnetic fields. The fields are assumed to be applied after the electron vortex beam carrying a well-defined orbital angular momentum has been created as a result of using a holographic mask. The shifts in the electron vortex beam energy momentum as well as its angular momentum due to the presence of an axial uniform magnetic field are evaluated. Order-of-magnitude estimates of the shifts are given with reference to typical electron vortex beams subject to moderate magnetic fields.

We provide analytic insight into the generation of stationary itinerant photon entanglement in a three-mode optomechanical system. We identify the parameter regime of maximal entanglement and show that strong entanglement is possible even for weak many-photon optomechanical couplings. We also show that strong tripartite entanglement is generated between the photonic and phononic output fields; unlike the bipartite photon-photon entanglement, this tripartite entanglement diverges as one approaches the boundary of system stability.

We consider two kinds of solutions of a recently proposed field theory leading to a nonlinear Schrödinger equation exhibiting solitonlike solutions of the power-law form $e_q^{i(kx-wt)}$, involving the $q$ exponential function naturally arising within nonextensive thermostatistics [$e_q^z\equiv {[1+(1-q)z]}^{1/(1-q)}$, with $e_1^z=e^z$]. These fundamental solutions behave like free particles, satisfying $p=\hslash k$, $E=\hslash \omega$, and $E=p^2/2m$ $(1\le q<2)$. Here we introduce two additional types of exact, analytical solutions of the aforementioned field theory. As a first step we extend the theory to situations involving a potential energy term, thus going beyond the previous treatment concerning solely the free-particle dynamics. Then we consider both bound, stationary states associated with a confining potential and also time-evolving states corresponding to a linear potential function. These types of solutions might be relevant for physical applications of the present nonlinear generalized Schrödinger equation. In particular, the stationary solution obtained shows an increase in the probability for finding the particle localized around a certain position of the well as one increases $q$ in the interval $1\le q<2$, which should be appropriate for physical systems where one finds a low-energy particle localized inside a confining potential.

In this paper, we present an extension of the Ghirardi-Rimini-Weber model for the spontaneous collapse of the wave function. Through the inclusion of dissipation, we avoid the divergence of the energy on the long-time scale, which affects the original model. In particular, we define jump operators, which depend on the momentum of the system and lead to an exponential relaxation of the energy to a finite value. The finite asymptotic energy is naturally associated to a collapse noise with a finite temperature, which is a basic realistic feature of our extended model. Remarkably, even in the presence of a low-temperature noise, the collapse model is effective. The action of the jump operators still localizes the wave function and the relevance of the localization increases with the size of the system, according to the so-called amplification mechanism, which guarantees a unified description of the evolution of microscopic and macroscopic systems. We study in detail the features of our model, at the level of both the trajectories in the Hilbert space and the master equation for the average state of the system. In addition, we show that the dissipative Ghirardi-Rimini-Weber model, as well as the original one, can be fully characterized in a compact way by means of a proper stochastic differential equation.

We study angular-momentum couplings in ${}^{87}\mathrm{Rb}{}_2$ Rydberg molecules formed between a Rydberg and $5S_{1/2}$ ground-state atom. We use a Fermi model that includes $S$-wave and $P$-wave singlet and triplet scattering of the Rydberg electron with the $5S_{1/2}$ atom, along with the fine-structure coupling of the Rydberg atom and hyperfine-structure coupling of the $5S_{1/2}$ atom. We discuss the effects of these couplings on the adiabatic molecular potentials. We obtain bound-state energies, lifetimes, and electric and magnetic dipole moments for the vibrational ground states of the ${}^{87}\mathrm{Rb}(nD+5S_{1/2})$ molecules in all adiabatic potentials, with fine and hyperfine structure included. The hyperfine structure gives rise to mixed singlet-triplet potentials. We also study the effect of the hyperfine structure on the deep ${}^{3}S$-wave- and ${}^{3}P$-wave-dominated adiabatic molecular potentials, which support high-$\ell$ ${}^{87}\mathrm{Rb}{}_2$ Rydberg molecules.

In this article we propose an unexplored mechanism to understand the physics underlying the electron emission and exciton formation occurring during grazing impact of neutral hydrogen atoms on LiF(100) surfaces. Instead of adopting the traditional point of view considering the negative ion formation as the precursor of the processes, we investigate the formation of excited hydrogen $\mathrm{H}(n=2)$ as a catalyzer to produce excitons and electrons. We base our proposal on ab initio calculations that compare quite well with experiments for impact energies larger than 4 keV.

Remote entanglement will inevitably decrease due to the interactions between quantum systems and their environments. Therefore protecting remote entanglement against decoherence is of great importance in realizing quantum communication and quantum comp utation. In this paper, we demonstrate that decoherence caused by weak-measurement-induced damping can be effectively suppressed by adding local unitary operation series on each qubit. The results show that the entanglement of the output state can approach that of the state before amplitude damping. The most distinct advantage of this entanglement protection scheme is that any unitary operation (except the identity operation) has this entanglement reversal effect on the amplitude-damped states. Furthermore, in each local unitary operation series, all the operations can be different from one other, and all the time intervals between any two adjacent operations can be different too. In addition, there is no need for the two remote users to synchronize their operations. Unlike most of the previous schemes, we do not assume the instantaneous local unitary operation, and each operation has a duration. All these advantages suggest that this remote entanglement protection scheme is much simpler and feasible than the previous ones, and we hope it can be implemented in the near future. Recently, Y. S. Kim et al. [Nat. Phys. 8, 117 (2012)] pointed out that “pre-weak measurement $+$ amplitude damping $+$ bit-flipping operation $+$ post-weak measurement” can actively combat specific decoherence. By combing our proposal with Kim et al.'s scheme, the protection performance can be greatly improved by replacing the bit-flipping operation with an arbitrary rotational operation around the $x$ axis, the number of the operations as well as the interval between any two adjacent operations all can be different, and the application range of the scheme can be greatly broadened.

Using a cutoff-free formulation of the coherent transport theory, we show that the interference terms at the origin of localization strongly affect the transport anisotropy. In contrast to the common hypothesis, we then find that the anisotropies of incoherent and coherent diffusion are significantly different, in particular at criticality. There, we show that the coherent transport anisotropy is mainly determined by the properties of the disorder-averaged effective scattering medium while the incoherent transport contributions become irrelevant.

We study the spectrum of a generalized Rabi model in which co- and counter-rotating terms have different coupling strengths. It is also equivalent to the model of a two-dimensional electron gas in a magnetic field with Rashba and Dresselhaus spin-orbit couplings. Like in the case of the Rabi model, the spectrum of our generalized Rabi model consists of the regular and the exceptional parts. The latter is represented by the energy levels which cross at certain parameter values which we determine explicitly. The wave functions of these exceptional states are given by finite order polynomials in the Bargmann representation. The roots of these polynomials satisfy a Bethe ansatz equation of the Gaudin type. At the exceptional points the model is therefore quasiexactly solvable. An analytical approximation is derived for the regular part of the spectrum in the weak- and strong-coupling limits. In particular, in the strong-coupling limit the spectrum consists of two ladders of equidistant levels.

We study time-ordering corrections to the description of spontaneous parametric down-conversion (SPDC), four-wave mixing (SFWM), and frequency conversion using the Magnus expansion. Analytic approximations to the evolution operator that are unitary are obtained. They are Gaussian preserving, and allow us to understand order-by-order the effects of time ordering. We show that the corrections due to time ordering vanish exactly if the phase-matching function is sufficiently broad. The calculation of the effects of time ordering on the joint spectral amplitude of the photons generated in SPDC and SFWM are reduced to quadrature.

We examine an all-optical atomic-polarization-gate scheme using a polarization-selective Kerr phase-shift technique. Using a Kerr $\pi$-phase-shift technique, we selectively write a $\pi$ phase shift to one of the circularly polarized components of a linearly polarized signal field while leaving the other component unchanged. Upon recombination, the signal field acquires a ${90}^{\circ }$ linear-polarization rotation, completing the critical polarization-gate operation. We demonstrate with numerical simulations that a special phase-control light-field detuning can be obtained which results in a complete linear-polarization rotation with a phase-control light.

We present a quantum logic scheme to detect atomic and molecular ions in different states of angular momentum based on their magnetic $g$ factors. The state-dependent magnetic $g$ factors mean that electronic, rotational, or hyperfine states may be distinguished by their Zeeman splittings in a given magnetic field. Driving motional sidebands of a chosen Zeeman splitting enables reading out the corresponding state of angular momentum with an auxiliary logic ion. As a proof-of-principle demonstration, we show that we can detect the ground electronic state of a ${}^{174}\mathrm{Yb}^{+}$ ion using ${}^{171}\mathrm{Yb}^{+}$ as the logic ion. Further, we can distinguish between the ${}^{174}\mathrm{Yb}^{+}$ ion being in its ground electronic state versus the metastable ${}^2{D}_{3/2}$ state. We discuss the suitability of this scheme for the detection of rotational states in molecular ions.

We study the localization of collective pair excitations in weakly interacting Bose superfluids in one-dimensional quasiperiodic lattices. The localization diagram is first determined numerically. For intermediate interaction and quasiperiodic amplitude we find a sharp localization transition, with extended low-energy states and localized high-energy states. We then develop an analytical treatment, which allows us to quantitatively map the localization transition into that of an effective multiharmonic quasiperiodic system.

Although the quantum Zeno effect takes its name from Zeno's arrow paradox, the effect of frequently observing the position of a freely moving particle on its motion has not been analyzed in detail in the frame of standard quantum mechanics. We study the evolution of a moving free particle while monitoring whether it lingers in a given region of space, and explain the dependence of the lingering probability on the frequency of the measurements and the initial momentum of the particle. Stopping the particle entails the emergence of Schrödinger cat states during the observed evolution, closely connected to the high-order diffraction modes in Fabry-Pérot optical resonators.

We show that the two-axis countertwisting interaction squeezes a coherent spin state into three states of interest in quantum information, namely, the equally weighted superposition state, the state close to the twin-Fock state, and the state achieving the Heisenberg limit of optimal sensitivity defined by the Cramér-Rao inequality, in addition to the well-known Heisenberg-limited state of spin fluctuations.

We investigate the geometric phase of a two-level atom (qubit) coupled to a bosonic reservoir with Lorentzian spectral density and find that for the non-Markovian dynamics in which the rotating-wave approximation (RWA) is performed, the geometric phase has a $\pi$-phase jump at the nodal point. However, the exact result without the RWA given by the hierarchical equations of motion method shows that there is no such phase jump or nodal structure in the geometric phase. Thus our results demonstrate that the counterrotating terms significantly contribute to the geometric phase in the multimode Hamiltonian under certain circumstances.

We perform quantum state tomography (QST) in a three-qubit system consisting of strongly coupled nuclear spins, known in the NMR literature as $ABX$ systems. We find that the number of experiments necessary to perform QST in such systems can be reduced with respect to those containing three qubits weakly coupled, which reduces the experimental effort required for the complete density-matrix reconstruction. To test the procedure we implement the full protocol for quantum teleportation. The tomographic results demonstrate that the density matrix can be reconstructed with fewer operations and high fidelity.

Entanglement plays an important role in quantum computation and information. We can only manipulate an entangled quantum state shared among several distant parties by local operations with classical communication (LOCC). Thus, it is of great importance to characterize the conditions by which we can determine whether one pure quantum state can be transformed to another pure state by LOCC. It is well known that separable operations and LOCC are not equivalent. Dozens of results have illustrated this difference. Little do we know about this difference despite these recent results. It is shown in this paper that if the initial state $|\psi \rangle$ and final state $|\varphi \rangle$ are genuine tripartite pure states in the Greenberger-Horne-Zeilinger (GHZ) class then $|\psi \rangle$ can be transformed to $|\varphi \rangle$ by separable operations if and only if $|\psi \rangle$ can be transformed to $|\varphi \rangle$ by deterministic LOCC. That is, SEP equals LOCC on tripartite GHZ states.