In tripartite discrete systems, two classes of genuine tripartite entanglement have been discovered, namely, the Greenberger-Horne-Zeilinger (GHZ) class and the $W$ class. To date, much research effort has been concentrated on the polarization entangled three-photon GHZ and $W$ states. Most studies of continuous-variable multiparticle entanglement have been focused on Gaussian states. In this Brief Report, we examine two classes of three-photon entangled states in space and time. One class is a three-mode three-photon entangled state and the other is a two-mode triphoton state. These states show behavior similar to the GHZ and $W$ states when one of the photons is not detected. The three-mode entangled state resembles a $W$ state, while a two-mode three-photon state resembles a GHZ state when one of the photons is traced away. We characterize the distinction between these two states by comparing the second-order correlation functions $G^{\left(2\right)}$ with the third-order correlation function $G^{\left(3\right)}$.

We calculate sensitivity coefficients to $\alpha$ variation for the fine-structure transitions (1,0) and (2,1) within ${{}^{3}P}_J\left[2s^{2}2p^2\right]$ multiplet of the carbonlike ions $\text{C}\text{I}$, $\text{N}\text{II}$, $\text{O}\text{III}$, $\text{Na}\text{VI}$, $\text{Mg}\text{VII}$, and $\text{Si}\text{IX}$. These transitions lie in the far-infrared region and are in principle observable in astrophysics for high redshifts $z\sim 10$. This makes them very promising candidates for the search for possible $\alpha$ variation in a cosmological time scale. In such studies one of the most dangerous sources of systematic errors is associated with isotope shifts. We calculate isotope shifts with the help of relativistic mass shift operator and show that it may be significant for $\text{C}\text{I}$ but rapidly decreases along the isoelectronic sequence and becomes very small for $\text{Mg}\text{VII}$ and $\text{Si}\text{IX}$.

We discuss quantitatively the complementarity of information transmitted by a quantum system prepared in a basis state in one out of several different mutually unbiased bases (MUBs). We obtain upper bounds on the information available to a receiver who has no knowledge of which MUB was chosen by the sender. These upper bounds imply a complementarity of information encoded via different MUBs and ultimately ensure the security in quantum key distribution protocols.

In this paper we report on large-scale multiconfiguration Hartree-Fock and multiconfiguration Dirac-Fock calculations of the fine structures of the ${\text{Cm}}^{4+}$ and ${\text{Am}}^{3+}$ ions, both having nominal $5f^6$ electronic configuration. Correlation effects play an extremely important role for both considered ions, and the single-configuration model is proved to be unfit to account for this complex picture. A comparison of the calculated energy levels with the results of spectroscopic measurements shows that theory and experiment are in quantitatively good agreement; the ground state of both ions is predicted to have nonmagnetic character. The accuracy of the results is estimated and discussed.

The group delay in frustrated total internal reflection (FTIR) is shown to equal the dwell time plus a self-interference delay. The self-interference delay originates from the energy stored in the overlap between incident and reflected waves above the interface. The group delay in FTIR for finite beams is thus shown to be the lifetime of stored energy in the gap between the prisms and in the partial standing wave above the gap. As in other manifestations of tunneling the saturation of group delay with barrier length (Hartman effect) is explained by the saturation of stored energy in the region defined. No superluminal propagation is implied.

An intense and ultrashort laser pulse undergoing filamentation in a molecular gas induces impulsive rotational excitation of the medium. The changes produced in the gas optical properties can dramatically affect a delayed probe pulse propagating in the wake of the pump one. We observe remarkable spatial and spectral reshaping of the probe beam occurring simultaneously at suitable pump-probe delays. These effects are experimentally demonstrated in several molecular gases, with minor changes related to the nature of the medium. A theoretical investigation shows that spatial and temporal domains are strictly related and cannot be disentangled like in experiments performed in a hollow waveguide.

It is shown here that the “simple test of quantumness for a single system” of Alicki and Van Ryn has exactly the same relation to the discussion of the problem of describing the quantum system via a classical probabilistic scheme (that is in terms of hidden variables or within a realistic theory) as the von Neumann theorem [Mathematische Grundlagen der Quantenmechanik (Springer, Berlin, 1932); Mathematical Foundations of Quantum Mechanics (Princeton University, Princeton, NJ, 1955)]. The latter one was shown by Bell [Rev. Mod. Phys. 38, 447 (1966)] to stem from an assumption that the hidden variable values for a sum of two noncommuting observables have to be, for each individual system, equal to sums of eigenvalues of the two operators. One cannot find a justification for such an assumption to hold for noncommeasurable variables. On the positive side, the criterion may be useful in rejecting models which are based on stochastic classical fields. Nevertheless, the example used by the authors has a classical optical realization.

Ultraintense lasers are expected to produce, in the near future, relativistic electron-positron plasma droplets. Considering the local photon production rate in complete leading order in quantum electrodynamics, we point out that these droplets are interesting sources of gamma ray flashes.

The notion of geometric phase has been recently introduced to analyze the quantum phase transitions of many-body systems from the geometrical perspective. In this work, we study the geometric phase of the ground state for an inhomogeneous period-two anisotropic $XY$ model in a transverse field. This model encompasses a group of familiar spin models as its special cases and shows a richer critical behavior. The exact solution is obtained by mapping on a fermionic system through the Jordan-Wigner transformation and constructing the relevant canonical transformation to realize the diagonalization of the Hamiltonian coupled in the $k$ space. The results show that there may exist more than one quantum phase transition point at some parameter regions and these transition points correspond to the divergence or extremum properties of the Berry curvature.

This paper presents a quantum key distribution (QKD) protocol named differential-quadrature-phase-shift (DQPS) QKD. A transmitter sends a coherent pulse train in which each pulse is phase modulated by $\left\{0,\pi \right\}$ $\left\{\pi /2,3\pi /2\right\}$, and a receiver measures it with either one of two asymmetric Mach-Zehnder interferometers having phase biases of 0 and $\pi /2$, respectively. This scheme is devised such that the idea of the conventional Bennett-Brassard 1984 protocol (BB84) is introduced to differential-phase-shift (DPS) QKD. Simulations show that the proposed scheme is robust against a sequential attack that is crucial eavesdropping to DPS-QKD.

The gauge problem in the so-called strong-field approximation (SFA) describing atomic or molecular systems exposed to intense laser fields is investigated. By introducing a generalized gauge and partitioning of the Hamiltonian, it is demonstrated that the $S$-matrix expansion obtained in the SFA depends on both gauge and partitioning in such a way that two gauges always yield the same $S$-matrix expansion, if the partitioning is properly chosen.

We demonstrate theoretically that the abstraction reaction $A+B_2\to AB+B$ can be driven coherently and efficiently with quantum-degenerate bosonic or fermionic matter waves. We show that the initial stages of the reaction are dominated by quantum fluctuations, resulting in the appearance of macroscopic nonclassical correlations in the final atomic and molecular fields. The dynamics associated with the creation of bosonic and of fermionic dimer-atom pairs is also compared. This study opens up a promising regime of quantum-degenerate matter-wave chemistry.

We report on the storage of orbital angular momentum of light in a cold ensemble of cesium atoms. We employ Bragg diffraction to retrieve the stored optical information impressed into the atomic coherence by the incident light fields. The stored information can be manipulated by an applied magnetic field and we were able to observe collapses and revivals due to the rotation of the stored atomic Zeeman coherence for times longer than $15\mu \text{s}$.

Thermal escape out of a metastable well is considered in the weak friction regime, where the bottleneck for decay is energy diffusion, and at lower temperatures, where quantum tunneling becomes relevant. Within a systematic semiclassical formalism an extension of the classical diffusion equation is derived starting from a quantum mechanical master equation. In contrast to previous approaches finite barrier transmission also affects transition probabilities. The decay rate is obtained from the stationary nonequilibrium solution and captures the intimate interplay between thermal and quantum fluctuations above the crossover to the deep quantum regime.

Necessary and sufficient conditions for bipartite entanglement are derived, which apply to arbitrary Hilbert spaces. Motivated by the concept of witnesses, optimized entanglement inequalities are formulated solely in terms of arbitrary Hermitian operators, which makes them useful for applications in experiments. The needed optimization procedure is based on a separability eigenvalue problem, whose analytical solutions are derived for a special class of projection operators. For general Hermitian operators, a numerical implementation of entanglement tests is proposed. It is also shown how to identify bound entangled states with positive partial transposition.

We investigate the two-photon decay rate of a highly excited atomic state which can decay to bound states of lower energy via cascade processes. We show that a naïve treatment of the process, based on the introduction of phenomenological decay rates for the intermediate, resonant states, leads to lower-order terms which need to be subtracted in order to obtain the coherent two-photon correction to the decay rate. The sum of the lower-order terms is exactly equal to the one-photon decay rate of the initial state, provided the naïve two-photon decay rates are summed over all available two-photon channels. A quantum electrodynamics treatment of the problem leads to an “automatic” subtraction of the lower-order terms.

We study the energy redistribution of interacting bosons in a ring-shaped quantum trimer as the coupling strength between neighboring sites of the corresponding Bose-Hubbard Hamiltonian undergoes a sudden change $\delta k$. Our analysis is based on a threefold approach combining linear response theory calculations as well as semiclassical and random matrix theory considerations. The $\delta k$ borders of applicability of each of these methods are identified by direct comparison with the exact quantum-mechanical results. We find that while the variance of the evolving quantum distribution shows a remarkable quantum-classical correspondence (QCC) for all $\delta k$ values, other moments exhibit this QCC only in the nonperturbative $\delta k$ regime.

We investigate theoretically the electron dynamics in a single-channel mesoscopic ring with a localized impurity subjected to picosecond linearly polarized asymmetric electromagnetic pulses. A nonequilibrium coherent population of electronic states that possesses a time-dependent polarization is induced. The associated radiation emission decays on a time scale determined by the system relaxation and hence lasts long after the pulses have perished. We derive analytically and confirm numerically that the presence of an impurity influences strongly the time-dependent charge polarization and allows the generation of higher harmonics in the terahertz range.

Inertia of optical nonlinearity is shown to distort the temporal profile of the intensity-dependent phase shift of an ultrashort laser pulse relative to the phase induced by the instantaneous optical nonlinearity. We demonstrate that a weakly inertial optical nonlinearity, whose response time is smaller than the pulse width, tends to redshift the light field and generates a cubic-phase correction to the quadratic phase, thus complicating the chirp of the spectrally broadened field. For very short light pulses with pulse widths shorter than the response times of optical nonlinearity, the redshift and cubic-phase terms often dominate the nonlinear phase shift, giving rise to phase profiles significantly different from those induced by instantaneous optical nonlinearity.

Solitons forming inside optical fibers are perturbed by several higher-order dispersive and nonlinear effects, especially when ultrashort optical pulses are used to excite them. We study, both analytically and numerically, how the radiation emitted by solitons in the form of dispersive waves (sometimes called Cherenkov radiation) is affected by these higher-order effects. Our results show that a certain minimum amount of third-order dispersion is needed before the amplitude of the dispersive wave becomes large enough for a spectral peak to appear in the output spectrum. This minimum value depends on the soliton order $N$ and decreases with increasing $N$. The amplitude of the radiation peak increases initially with both the magnitude of the third-order dispersion and the soliton order, but then saturates to a relative power level that is typically below 10% of the launched power. Our results reveal several interesting features that should be of relevance for applications requiring an ultrabroadband optical source.