We show that nonlinear optical structures involving a balanced gain-loss profile can act as unidirectional optical valves. This is made possible by exploiting the interplay between the fundamental symmetries of parity ($\mathcal{P}$) and time ($\mathcal{T}$), with optical nonlinear effects. This unidirectional dynamics is specifically demonstrated for the case of an integrable $\mathcal{PT}$-symmetric nonlinear system.

Correlations are important tools in the characterization of quantum fields, as they can be used to describe statistical properties of the fields, such as bunching and antibunching, as well as to perform field state tomography. Here we analyze experiments by Bozyigit et al. [Nat. Phys. (to appear), e-print arXiv:1002.3738] where correlation functions can be observed using the measurement records of linear detectors (i.e., quadrature measurements), instead of relying on intensity or number detectors. We also describe how large amplitude noise introduced by these detectors can be quantified and subtracted from the data. This enables, in particular, the observation of first- and second-order coherence functions of microwave photon fields generated using circuit quantum electrodynamics and propagating in superconducting transmission lines under the condition that noise is sufficiently low.

Through a simple and exact analytical derivation, we show that for a particle on a lattice there is a one-to-one correspondence between the spectrum in the presence of an attractive potential $\mathrm{V̂}$ and its repulsive counterpart $-\mathrm{V̂}$. For a Hermitian potential, this result implies that the number of localized states is the same in both attractive and repulsive cases although these states occur above (below) the band continuum for the repulsive (attractive) case. For a $\mathcal{P}\mathcal{T}$-symmetric potential that is odd under parity, our result implies that, in the $\mathcal{P}\mathcal{T}$-unbroken phase, the energy eigenvalues are symmetric around zero and that the corresponding eigenfunctions are closely related to each other.

Motivated by work [Phys. Rev. Lett. 89, 220404 (2002)] for detecting the vacuum-induced Berry phases with two-mode Jaynes-Cummings models (JCMs), we show here that, for a parameter-dependent single-mode JCM, certain atom-field states also acquired photon-number-dependent Berry phases after the parameter slowly changed and eventually returned to its initial value. This geometric effect related to the field quantization still exists, even if the field is kept in its vacuum state. Specifically, a feasible Ramsey interference experiment with a cavity quantum electrodynamics system is designed to detect the vacuum-induced Berry phase.

The classical-trajectory Monte Carlo method has been used to study the capture of negative kaons by hydrogen and deuterium atoms; subsequently, the elastic scattering, Stark mixing, and Coulomb deexcitation cross sections of $\mathit{Kp}$ and $\mathit{Kd}$ atoms have been determined. The results for kaonic atom formation confirm the initial conditions that have been parametrically applied by most atomic cascade models. Our results show that Coulomb deexcitation in $\mathit{Kp}$ and $\mathit{Kd}$ atoms with $\Delta n>1$ is important in addition to $n=1$. We have shown that the contribution of molecular structure effects to the cross sections of the collisional processes is larger than the isotopic effects of the targets. We have also compared our results with the semiclassical approaches.

We theoretically propose a method of producing single attosecond pulse by high-order harmonic generation with a two-color multicycle driver laser field. It is shown that this scheme can modulate the quantum paths by adjusting the relative phase. In the case of the short quantum path control, a broadband supercontinuum spectrum with a 93 eV spectral width can be produced which directly generates an isolated 46 as pulse without phase compensation. When the long quantum path is picked out, a harmonic continuum spectrum with a bandwidth of 101 eV appears, an intense isolated 39 as pulse with high-signal-to-noise ratio is generated successfully by superposing some properly selected harmonics, the phases of which are locked. In both cases, our simulations show that the value of the time delay between two lasers can be varied between $-0.2$ fs and 0.2 fs to keep the isolated sub-50-as pulse generation. In particular, these results are analyzed by using both classical and three-dimensional quantum-mechanical approaches.

We present a comprehensive analysis of how the properties of two-dimensional lattice (“discrete”) solitons in a Kerr medium are influenced by their peak intensity and width. We are able to quantitatively relate the Townes solution for solitons in a two-dimensional, homogeneous media to two distinct regimes of the lattice solitons: to narrow, high-intensity, highly nonlinear solitons and to broad, low-intensity, weakly nonlinear solitons, which experience the periodic potential as an effective homogeneous medium. Both regimes, although they support a different power flow and are affected by completely different diffraction dynamics, are thus traced back to the same physical phenomenon. They are separated by a range of unstable and stable solutions, directly caused by the periodicity of the lattice.

We theoretically verify that converting the Hermite-Gaussian modes into the Laguerre-Gaussian can lead to the spatial morphologies of the two-dimensional (2D) coherent states to be transformed from Lissajous figures to trochoidal curves. With this transformational relationship, we experimentally generate various structured light beams by exploiting a cylindrical-lens mode converter to transform the optical Lissajous modes. The present investigation manifests a notable method to generate optical coherent waves with various orbital spatial morphologies.

Single photon Sagnac interferometry as a probe to macroscopic quantum mechanics is considered at the theoretical level. For a freely moving macroscopic quantum mirror susceptible to radiation pressure force inside a Sagnac interferometer, a careful analysis of the input-output relation reveals that the particle spectrum readout at the bright and dark ports encode information concerning the noncommutativity of position and momentum of the macroscopic mirror. A feasible experimental scheme to probe the commutation relation of a macroscopic quantum mirror is outlined to explore the possible frontier between classical and quantum regimes. In the Appendix, the case of Michelson interferometry as a feasible probe is also sketched.

The necessary and sufficient conditions for the equivalence of arbitrary $n$-qubit pure quantum states under local unitary (LU) operations derived in [B. Kraus, Phys. Rev. Lett. 104, 020504 (2010)] are used to determine the different LU-equivalence classes of up to five-qubit states. Due to this classification new parameters characterizing multipartite entanglement are found and their physical interpretation is given. Moreover, the method is used to derive examples of two $n$-qubit states (with $n>2$ arbitrary) which have the properties that all the entropies of any subsystem coincide; however, the states are neither LU equivalent nor can be mapped into each other by general local operations and classical communication.

We consider the security of the Bennett-Brassard 1984 protocol for quantum key distribution, with arbitrary individual imperfections simultaneously in the source and detectors. We provide the secure key generation rate and show that three parameters must be bounded to ensure security; the basis dependence of the source, a detector-blinding parameter, and a detector leakage parameter. The system may otherwise be completely uncharacterized and contain large losses.

Complete solutions are given in a closed analytic form for unambiguous discrimination of three general pure states with real mutual inner products. For this purpose, we first establish some general results on unambiguous discrimination of $n$ linearly independent pure states. The uniqueness of solution is proved. The condition under which the problem is reduced to an ($n-1$)-state problem is clarified. After giving the solution for three pure states with real mutual inner products, we examine some difficulties in extending our method to the case of complex inner products. There is a class of set of three pure states with complex inner products for which we obtain an analytical solution.

A bipartite state, which is secretly chosen from a finite set of known entangled pure states, cannot immediately be useful in standard quantum information processing tasks. To effectively make use of the entanglement contained in this unknown state, we introduce a way to locally manipulate the original quantum system: either identify the state successfully or distill some pure entanglement. Remarkably, if many copies are available, we show that any finite set of entangled pure states, whether orthogonal or not, can be locally distinguished in this way, which further implies that pure entanglement can be deterministically extracted from unknown entanglement. These results make it clear why a large class of entangled bipartite quantum operations including unitary operations and measurements that are globally distinguishable can also be locally distinguishable: They can generate pure entanglement consistently.

We show how, under the influence of non-Markovian environments, two different maximally entangled Bell states give rise to states that have equal classical correlations and the same capacities to violate the Bell-Clauser-Horne-Shimony-Holt inequality, but intriguingly differing usefulness for teleportation and dense coding. We elucidate how different entanglement measures like negativity and concurrence, and two different measures of quantum discord, could account for these behaviors. In particular, we explicitly show how the Ollivier-Zurek measure of discord directly accounts for one state being a better resource for dense coding compared to another. Our study leads to several important issues about these measures of discord.

We have performed a relativistic many-body perturbation theory calculation up to second order to study the transition amplitudes, Einstein $A$ coefficients, and oscillator strengths for the spin-allowed electric-dipole transitions in the Zn-like ions Zn, Ge${}^{2+}$, As${}^{3+}$, Kr${}^{6+}$, Nb${}^{11+}$, and Mo${}^{12+}$. All possible transitions among the first 13 levels of these ions have been studied. The total transition amplitudes obtained in different gauges are in excellent agreement. The gauge dependences of the first and second order, and the total transition amplitudes are derived. A theoretical justification for the small gauge dependence of the total transition amplitude is given. The present calculations agree well with experiment for all ions except for the neutral Zn atom.

In the article density-orbital embedding (DOE) theory is proposed. DOE is based on the concept of density orbital (DO), which is a generalization of the square root of the density for real functions and fractional electron numbers. The basic feature of DOE is the representation of the total supermolecular density ${\rho }_s$ as the square of the sum of the DO ${\phi }_a$, which represents the active subsystem $A$ and the square root of the frozen density ${\rho }_f$ of the environment $F$. The correct ${\rho }_s$ is obtained with ${\phi }_a$ being negative in the regions in which ${\rho }_f$ might exceed ${\rho }_s$. This makes it possible to obtain the correct ${\rho }_s$ with a broad range of the input frozen densities ${\rho }_f$ so that DOE resolves the problem of the frozen-density admissibility of the current frozen-density embedding theory. The DOE Euler equation for the DO ${\phi }_a$ is derived with the characteristic embedding potential representing the effect of the environment. The DO square ${\phi }_a^2$ is determined from the orbitals of the effective Kohn-Sham (KS) system. Self-consistent solution of the corresponding one-electron KS equations yields not only ${\phi }_a^2$, but also the DO ${\phi }_a$ itself.

We present an analytic description of the effects of dephasing processes on stimulated Raman adiabatic passage in a tripod quantum system. To this end, we develop an effective two-level model. Our analysis makes use of the adiabatic approximation in the weak dephasing regime. An effective master equation for a two-level system formed by two dark states is derived, where analytic solutions are obtained by utilizing the Demkov-Kunike model. From these, it is found that the fidelity for the final coherent superposition state decreases exponentially for increasing dephasing rates. Depending on the pulse ordering and for adiabatic evolution, the pulse delay can have an inverse effect.

The emission of above-ionization-threshold harmonics results from the recombination of two-electron wave packets moving along a “short” and a “long” trajectory in the atomic continuum. Attosecond pulse train generation has, so far, been attributed to the short trajectory, attempted to be isolated through targeted trajectory-selective phase-matching conditions. Here, we provide experimental evidence for the contribution of both trajectories to the harmonic emission, even under phase-matching conditions unfavorable for the long trajectory. This is finger printed in the interference modulation of the harmonic yield as a function of the driving laser intensity. The effect is also observable in the sidebands, which arise from the frequency mixing of the harmonics and the driving laser field, an effect with consequences in cross-correlation pulse metrology approaches.

Taking into account the deformation of the Fermi surface, we investigate the spin-singlet and -triplet BCS pairings in a mixture of fermionic polar molecules with two different hyperfine states. In particular, we explore the relation between the critical temperatures and the Fock-exchange interaction. We also show that, by tuning short-range interaction between interspecies molecules, the singlet- and triplet-paired superfluids may coexist.

We investigate the effects of dipole-dipole (DD) interactions on immiscibility-miscibility transitions (IMT’s) in two-component Bose-Einstein condensates (BEC’s) trapped in the harmonic-oscillator (HO) potential, with the components linearly coupled by a resonant electromagnetic field (accordingly, the components represent two different spin states of the same atom). The problem is studied by means of direct numerical simulations. Different mutual orientations of the dipolar moments in the two components are considered. It is shown that, in the binary BEC formed by dipoles with the same orientation and equal magnitudes, the IMT cannot be induced by the DD interaction alone, being possible only in the presence of the linear coupling between the components, while the miscibility threshold is affected by the DD interactions. However, in the binary condensate with the two dipolar components polarized in opposite directions, the IMT can be induced without any linear coupling. Further, we demonstrate that those miscible and immiscible localized states, formed in the presence of the DD interactions, which are unstable evolve into robust breathers, which tend to keep the original miscibility or immiscibility, respectively. An exception is the case of a very strong DD attraction, when narrow stationary modes are destroyed by the instability. The binary BEC composed of copolarized dipoles with different magnitudes are briefly considered as well.