We show how to freeze distributed entanglement that has been created from the natural dynamics of spin chain systems. The technique that we propose simply requires single-qubit operations and isolates the entanglement in specific qubits at the ends of branches. Such frozen entanglement provides a useful resource, for example for teleportation or distributed quantum processing. The scheme can be applied to a wide range of systems—including actual spin systems and alternative qubit embodiments in strings of quantum dots, molecules, or atoms.

A nonadiabatic change of the control field or the low-frequency coherence allows for an almost instantaneous change of the signal field propagating in a thick resonant absorber where electromagnetically induced transparency is realized. This finding is applied for the storage, retrieval, and splitting of the signal into two parts separated by a time interval that is as long as the lifetime of the low-frequency coherence. Pulse shaping is also possible.

We propose an information reconciliation protocol that uses two-way classical communication. The key rates of quantum key distribution (QKD) protocols that use our protocol are higher than those using previously known protocols for a wide range of error rates for the Bennett-Brassard 1984 and six-state protocols. We also clarify the relation between the proposed and known QKD protocols, and the relation between the proposed protocol and entanglement distillation protocols.

We develop a necessary condition of quantum correlation. It is utilized to construct a $d$-level bipartite Bell-type inequality which is strongly resistant to noise and requires only analyses of $O\left(d\right)$ measurement outcomes compared to the previous result $O\left(d^2\right)$. Remarkably, a connection between the arbitrary high-dimensional bipartite Bell-type inequality and entanglement witnesses is found. Through the necessary condition of quantum correlation, we propose that the witness operators to detect truly multipartite entanglement for a generalized Greenberger-Horne-Zeilinger (GHZ) state with two local measurement settings and a four-qubit singlet state with three settings. Moreover, we also propose a robust entanglement witness to detect a four-level tripartite GHZ state with only two local measurement settings.

We characterize entanglement in two-qubit pure states encoded in transverse momenta of twin photons obtained from spontaneous parametric down-conversion. Two alternate methods are employed: (i) measurement of conditional interference patterns and (ii) measurement of the marginal probability that yields the single-photon interference pattern. Both methods are local with classical communication and rely on Schmidt decomposition of the quantum state, which is generated by letting the photons propagate through an appropriate lens system. In both cases we can obtain the concurrence either through the Schmidt coefficients in the first method or directly relating the concurrence to the visibility of the interference pattern in the second one.

The Fisher-Shannon plane which embodies the Fisher information measure in conjunction with the Shannon entropy is tested in its ability to quantify and compare the informational behavior of the process of atomic ionization. We report the variation of such an information measure and its constituents for a comprehensive set of neutral atoms, and their isoelectronic series including the mononegative ions, using the numerical data generated on 320 atomic systems in position, momentum, and product spaces at the Hartree-Fock level. It is found that the Fisher-Shannon plane clearly reveals shell-filling patterns across the periodic table. Compared to position space, a significantly higher resolution is exhibited in momentum space. Characteristic features in the Fisher-Shannon plane accompanying the ionization process are identified, and the physical reasons for the observed patterns are described.

Electron detachment from $\text{PO}_n{}^{-}$ $\left(n=0–3\right)$ by electron impact was studied for incident-electron energies in the range from 0 to 50 eV. In all cases, the cross section for electron detachment exhibits a well-defined onset at a threshold energy that is significantly higher than the electron binding energy. For ${\text{P}}^{-}$, ${\text{PO}}^{-}$, and $\text{PO}_2{}^{-}$, the overall shape of the detachment cross section is represented fairly well by a classical model cross section. For $\text{PO}_3{}^{-}$, however, deviations occur. It is suggested that deviations appear because the detachment process is dependent on the relative orientation of the molecular ion with respect to the incoming electron. For all the molecular ions studied here, the dominant reaction channel is pure detachment without fragmentation of the molecule. At higher energies, electron impact furthermore results in dissociative detachment where a P–O bond is broken. Three- and four-body fragmentation is found to be negligible.

Angular distributions, fragmentation, and charge fractions are studied for grazing scattering of ${\text{C}}_{60}^{+}$ fullerenes with keV energies from a clean and flat Al(001) surface. At low energies for the motion along the surface normal, ${\text{C}}_{60}^{+}$ ions are scattered nearly elastically, whereas for larger normal energies energy loss is substantial. We compare our experimental results with classical trajectory simulations exploiting the Tersoff potential between atoms in the cluster and different types of interaction potentials for the cluster with the surface. The internal energy of scattered clusters is deduced from the analysis of fragments. We observe that the loss of kinetic energy for the motion along the surface normal is transferred to internal excitations of the cluster, whereas the energy transfer to the metal surface is negligible. The charge state distributions for scattered projectiles can be understood by a full neutralization of incident ions at the surface and subsequent delayed electron emission.

We investigate theoretically the tunneling of a dilute Bose-Einstein condensate through a potential barrier. This scenario is closely related to recent experimental studies of condensates trapped in one-dimensional optical lattices. We derive analytical results for the tunneling rate of the condensate with emphasis on the effects of atom-atom interactions. Furthermore, we consider the effect of fluctuating barrier height to the tunneling rate. We have computed the tunneling rate as a function of the characteristic frequency of the noise. The result is seen to be closely related to the excitation spectrum of the condensate. These observations should be experimentally verifiable.

Repulsively interacting particles in a periodic potential can form bound composite objects, whose dissociation is suppressed by a band gap. Nearly pure samples of such repulsively bound pairs of cold atoms—“dimers”—have recently been prepared by Winkler et al. [Nature (London) 441, 853 (2006)]. We here derive an effective Hamiltonian for a lattice loaded with dimers only and discuss its implications for the many-body dynamics of the system. We find that the dimer-dimer interaction includes strong on-site repulsion and nearest-neighbor attraction which always dominates over the dimer kinetic energy at low temperatures. The dimers then form incompressible, minimal-surface “droplets” of a quantum lattice liquid. For low lattice filling, the effective Hamiltonian can be mapped onto the spin-$1∕2$ $XXZ$ model with fixed total magnetization which exhibits a first-order phase transition from the droplet to a gas phase. This opens the door to studying first-order phase transitions using highly controllable ultracold atoms.

Propagation of intense, ultrashort laser pulses through condensed media, like crystals of $\mathrm{Ba}{\mathrm{F}}_2$ and sapphire, results in the formation of filaments. We demonstrate that the onset of filamentation may be controlled by rotating the plane of polarization of incident light. We directly visualize filamentation in $\mathrm{Ba}{\mathrm{F}}_2$ via six-photon-absorption-induced fluorescence and, concomitantly, by probing the spectral and spatial properties of white light that is generated.

We address the interactions between optical solitons in the system with longitudinally varying nonlocality degree and nonlinearity strength. We consider a physical model describing light propagation in nematic liquid crystals featuring a strongly nonlocal nonlinear response. We reveal that the variation of the nonlocality and nonlinearity along the propagation direction can substantially enhance or weaken the interaction between out-of-phase solitons. This phenomenon manifests itself as a slowdown or acceleration of the soliton collision dynamics in one-dimensional geometries or of the soliton spiraling rate in bulk media. Therefore, one finds that by engineering the nonlocality and nonlinearity variation rate one can control the output soliton location.

The theories defined by Lagrangians containing a second time derivative are considered. It is shown that if the second derivatives enter only the terms multiplied by coupling constant one can consistently define the perturbative sector via Dirac procedure. The possibility of introducing standard canonical variables is analyzed in detail. The ambiguities in quantization procedure are pointed out.

We propose and experimentally demonstrate an optimal nonunity gain Gaussian scheme for partial measurement of an unknown coherent state that causes minimal disturbance of the state. The information gain and the state disturbance are quantified by the noise added to the measurement outcomes and to the output state, respectively. We derive the optimal trade-off relation between the two noises and we show that the tradeoff is saturated by nonunity gain teleportation. Optimal partial measurement is demonstrated experimentally using a linear optics scheme with feedforward.

We study conditions for the deterministic transformation $\mid \psi ⟩\to \mid \varphi ⟩$ of a bipartite entangled state by a separable operation. If the separable operation is a local operation with classical communication (LOCC), Nielsen’s majorization theorem provides necessary and sufficient conditions. For the general case, we derive a necessary condition in terms of products of Schmidt coefficients, which is equivalent to the Nielsen condition when either of the two factor spaces is of dimension 2, but is otherwise weaker. One implication is that no separable operation can reverse a deterministic map produced by another separable operation, if one excludes the case where the Schmidt coefficients of $\mid \psi ⟩$ are the same as those of $\mid \varphi ⟩$. The question of sufficient conditions in the general separable case remains open. When the Schmidt coefficients of $\mid \psi ⟩$ are the same as those of $\mid \varphi ⟩$, we show that the Kraus operators of the separable transformation restricted to the supports of $\mid \psi ⟩$ on the factor spaces are proportional to unitaries. When that proportionality holds and the factor spaces have equal dimension, we find conditions for the deterministic transformation of a collection of several full Schmidt rank pure states $\mid {\psi }_j⟩$ to pure states $\mid {\varphi }_j⟩$.

We propose the general scheme of remote one-qubit information concentration. To achieve the task, the Bell-correlated mixed states are exploited. In addition, the nonremote one-qubit information concentration is equivalent to the decoding of the quantum error-correction code. Here we propose how to decode the stabilizer codes. In particular, the proposed scheme can be used for the operator quantum error-correction codes. The encoded state can be recreated on the errorless qubit, regardless how many bit-flip errors and phase-flip errors have occurred.

We were previously successful in representing a series of low-lying $\mathrm{Ps}+\mathrm{H}$ scattering resonances as due to quasibound Coulomb states in the closed-channel $e^{+}+{\mathrm{H}}^{-}$ system. Here we investigate the analogous Coulomb states of ${\mathrm{Ps}}^{-}+{\mathrm{H}}^{+}$, which are also accessible from the $\mathrm{Ps}+\mathrm{H}$ scattering system, but at considerably higher collision energies. We locate several such states and suggest that they may be scattering resonances, but because there are many open-channel thresholds lying below they may have large widths and shifts.

The dispersion coefficients for the van der Waals interactions between the rare gases Ne, Ar, Kr, and Xe and the low-lying states of Li, Na, K, and Rb are estimated using a combination of ab initio and semiempirical methods. The rare-gas oscillator strength distributions for the quadrupole and octupole transitions were derived by using high-quality calculations of rare-gas polarizabilities and dispersion coefficients to tune Hartree-Fock single-particle energies and expectation values.

We investigate the conditions under which quantum fluctuations are relevant for the quantitative interpretation of experiments with ultracold Bose gases. This requires to go beyond the description in terms of the Gross-Pitaevskii and Hartree-Fock-Bogoliubov mean-field theories, which can be obtained as classical (statistical) field-theory approximations of the quantum many-body problem. We employ functional-integral techniques based on the two-particle irreducible (2PI) effective action. The role of quantum fluctuations is studied within the nonperturbative 2PI $1∕\mathcal{N}$ expansion to next-to-leading order. At this accuracy level memory integrals enter the dynamic equations, which differ for quantum and classical statistical descriptions. This can be used to obtain a classicality condition for the many-body dynamics. We exemplify this condition by studying the nonequilibrium evolution of a one-dimensional Bose gas of sodium atoms, and discuss some distinctive properties of quantum versus classical statistical dynamics.

We propose a method for measuring the spectral phase of attosecond pulses by means of angle-resolved photoelectron spectroscopy of alkali-metal atoms in the ${}^{2}P$ state. A coherent superposition of the ${}^{2}P_{1∕2}$ and ${}^{2}P_{3∕2}$ states allows us to observe the quantum beat, whose phase offset is obtained as a function of photoelectron energy. The formulation derived in this study elucidates that the phase offset is straightforwardly utilized for determining the spectral phase of attosecond pulses. This method is applicable not only to near transform-limited attosecond pulses, but also to significantly chirped pulses with the same spectrum. Numerical simulation for Cs atoms demonstrates the feasibility in the experiment and the simplicity in the analysis. From a practical point of view, there is no need for the most advanced technologies like nonlinear optics by high harmonics, phase stabilization of fundamental pulses, and sub-optical-cycle resolution of a delay line, some of which have been required in the other methods.