Most quantum entanglement investigations are focused on two qubits or some finite (small) chain structure, since the infinite chain structure is a considerably cumbersome task. Therefore, the quantum entanglement properties involving an infinite chain structure is quite important, not only because the mathematical calculation is cumbersome but also because real materials are well represented by an infinite chain. Thus, in this paper we consider an entangled diamond chain with Ising and anisotropic Heisenberg (Ising-XXZ) coupling. Two interstitial particles are coupled through Heisenberg coupling or simply two-qubit Heisenberg, which could be responsible for the emergence of entanglement. These two-qubit Heisenberg operators are interacted with two nodal Ising spins. An infinite diamond chain is organized by interstitial-interstitial and nodal-interstitial (dimer-monomer) site couplings. We are able to get the thermal average of the two-qubit operator, called the reduced two-qubit density operator. Since these density operators are spatially separated, we could obtain the concurrence (entanglement) directly in the thermodynamic limit. The thermal entanglement (concurrence) is constructed for different values of the anisotropic Heisenberg parameter, magnetic field, and temperature. We also observed the threshold temperature via the parameter of anisotropy, Heisenberg and Ising interaction, external magnetic field, and temperature.

Recently discovered processes of volume reflection and multiple volume reflection of charged particles in bent crystals are accompanied by specific electromagnetic (e.m.) radiation. The mechanism of radiation emission is quite complex due to the coexistence of different regimes for particle dynamics, resulting in e.m. generation with features characteristic of each regime. We present a simulation taking into consideration both the nondipole nature and arbitrary multiplicity of radiation accompanying volume reflection. This approach is worked out for electrons or positrons of any energy and is based on the local straight-crystal approximation and Baier-Katkov formula for radiation, the integration of which is considerably facilitated by the fast Fourier transform method. The radiation generated by multiple volume reflection with vertical and skew planes has been studied, too. A large axial contribution to the hard part of the radiative energy loss spectrum as well as the strengthening of planar radiation, with respect to the single volume reflection case, in the soft part of the spectrum are demonstrated. Both the wide spectrum and high intensity make this type of radiation quite suitable for the conversion of electron and positron beams to hard $\gamma$ radiation.

We discuss the production of ultracold molecules in their electronic ground state by photoassociation employing electronically excited states with ion-pair character and strong spin-orbit interaction. A short photoassociation laser pulse drives a nonresonant three-photon transition for alkali-metal atoms colliding in their lowest triplet state. The excited-state wave packet is transferred to the ground electronic state by a second laser pulse, driving a resonant two-photon transition. After analyzing the transition matrix elements governing the stabilization step, we discuss the efficiency of population transfer using transform-limited and linearly chirped laser pulses. Finally, we employ optimal control theory to determine the most efficient stabilization pathways. We find that the stabilization efficiency can be increased by one and two orders of magnitude when using linearly chirped and optimally shaped laser pulses, respectively.

The nonadiabatically coupled dynamics of electrons and nuclei is investigated for the ozone molecule on the attosecond time scale. A coherent superposition of nuclear wave packets located on different electronic states in the Chappuis and in the Hartley bands are created by pump pulses. The multiconfiguration time-dependent Hartree method is used to solve the coupled nuclear quantum dynamics in the framework of the adiabatic separation of the time-dependent Schrödinger equation including nonadiabatic couplings. Our nuclear wave-packet calculations demonstrate that the coherence between Hartley state B and one of the Chappuis states (Chappuis 1) is significantly large, while it is almost negligible for the other two cases (between Hartley B and Chappuis 2 or between Chappuis 1 and Chappuis 2). At present we limited our description of the electronic motion to the Franck-Condon region only due to the localization of the nuclear wave packets around this point during the first 5–6 fs.

Using the perturbative approach, it is demonstrated that the widths of the density and second-order correlation function of atoms scattered from a quasicondensate in a Raman process are sensitive to the phase fluctuations induced by the nonzero temperature of the mother cloud. It is also shown how these widths evolve during expansion of the halo of scattered atoms. It is also argued that if the Raman scattering is preceded by expansion of the mother cloud, the density of atoms widens substantially, while the second-order correlation function remains practically unchanged. These results are useful for planning future Raman scattering experiments and indicate the degree of spatial resolution of atom-position measurements necessary to detect the temperature dependence of the quasicondensate. All the calculations are performed using experimental parameters of a metastable ${}^4$He${}^{*}$ quasicondensate of the Palaiseau group.

We investigate magnetic properties and effects of pairing fluctuations in the Bardeen-Cooper-Schrieffer–Bose-Einstein condensation (BCS-BEC) crossover regime of an ultracold Fermi gas. Recently, Liu and Hu [Europhys. Lett. 75, 364 (2006)], and Parish et al. [Nat. Phys. 3, 124 (2007)], pointed out that the strong-coupling theory developed by Nozières and Schmitt-Rink (NSR), which has been extensively used to successfully clarify various physical properties of cold Fermi gases, unphysically gives negative spin susceptibility in the BCS-BEC crossover region. The same problem also exists in the ordinary non-self-consistent $T$-matrix approximation. In this paper, we clarify that this serious problem comes from incomplete treatment in term of pseudogap phenomena originating from strong pairing fluctuations, as well as effects of spin fluctuations on the spin susceptibility. Including these two key issues, we construct an extended $T$-matrix theory which can overcome this problem. The resulting positive spin susceptibility agrees well with the recent experiment on a ${}^6$Li Fermi gas done by Sanner et al. [Phys. Rev. Lett. 106, 010402 (2011)]. We also apply our theory to a polarized Fermi gas to examine the superfluid phase transition temperature $T_{\mathrm{c}}$, as a function of the polarization rate. Since the spin susceptibility is an important physical quantity, especially in singlet Fermi superfluids, our results would be useful in considering how singlet pairs appear above and below $T_{\mathrm{c}}$ in the BCS-BEC crossover regime of cold Fermi gases.

We show that off-site processes and multiorbital physics have a crucial impact on the phase diagram of quantum gas mixtures in optical lattices. In particular, we discuss Bose-Fermi mixtures where the intra- and interspecies interactions induce competing density-induced hopping processes, the so-called bond-charge interactions. Furthermore, higher bands strongly influence tunneling and on-site interactions. We apply a multiorbital interaction-induced dressing of the lowest band, which leads to renormalized hopping processes. These corrections give rise to an extended Hubbard model with intrinsically occupation-dependent parameters. The resulting decrease of the tunneling competes with a decrease of the total on-site interaction energy, both affecting the critical lattice depth of the superfluid to Mott-insulator transition. In contrast to the standard Bose-Fermi Hubbard model, we predict a large shift of the transition to shallower lattice depths with increasing Bose-Fermi attraction. The applied theoretical model allows an accurate prediction of the modified tunneling amplitudes and the critical lattice depth, both being recently observed experimentally.

A scheme is proposed to produce (3+1)-dimensional superluminal spatiotemporal optical solitons and vortices in a coherent atomic system working in an active Raman gain regime. It is shown that the evolution of the envelope of a signal field obeys a modified (3+1)-dimensional nonlinear Schrödinger equation, which includes dispersion, diffraction, and Kerr nonlinearity. Various solutions of light bullets, light vortices, light-bullet trains, and light-vortex trains are presented, which display many interesting characters, including superluminal propagating velocity and extremely low generating power. In addition, they can be easily manipulated in a controllable way. Stabilization of such high-dimensional superluminal light bullets and vortices can be realized using the trapping potential formed by an additional far-detuned laser field.

The synchronization of limit-cycle oscillations in anomalously dispersive nonlinear fiber ring resonators via unidirectional and mutual coupling is shown numerically. The limit-cycle oscillations can be synchronized in their phase as well as in their frequency within a respective synchronization interval. The synchronization of limit cycles establishes a further approach to tailor and modify frequency combs by optical means.

We analyze the possibility of creating two-mode spin squeezed states of two separate spin ensembles by inverting the spins in one ensemble and allowing spin exchange between the ensembles via a near resonant cavity field. We investigate the dynamics of the system using a combination of numerical and analytic calculations, and we obtain squeezing for a wide range of parameters. We also investigate the transfer of the squeezing properties to the cavity field and to an output mode from the cavity. Finally, we investigate how the squeezing is affected by effects of inhomogeneities which would be present in solid-state implementations of the spin ensembles.

We give a derivation of Holevo's bound using an important result from nonequilibrium statistical physics, the fluctuation theorem. To do so we develop a general formalism of quantum fluctuation theorems for two-time measurements, which explicitly accounts for the back action of quantum measurements as well as possibly nonunitary time evolution. For a specific choice of observables this fluctuation theorem yields a measurement-dependent correction to the Holevo bound, leading to a tighter inequality. We conclude by analyzing equality conditions for the improved bound.

We propose a quantum theory for the characterization of the momentum of moving media at zero temperature. It is demonstrated that the zero-point quantum fluctuations of the electromagnetic field may cause a net transfer of momentum between polarizable bodies moving at different velocities in Casimir-type geometries. However, the total net momentum induced by the quantum fluctuations, i.e., the total additional momentum imparted to all the matter in the system, vanishes. It is proven that the exchanged momentum can be calculated from the zero-point interaction energy of the system.

The experimental violation of the Bell inequality establishes necessary but not sufficient conditions that any theory must obey. Namely, a theory compatible with the experimental observations can satisfy at most two of the three hypotheses at the basis of Bell's theorem: free will, no signaling, and outcome independence. Quantum mechanics satisfies the first two hypotheses but not the latter. Experiments not only violate the Bell inequality, but show an excellent agreement with quantum mechanics. This fact restricts further the class of admissible theories. In this work, the author determines the form of the hidden-variable models that reproduce the quantum mechanical predictions for a spin singlet while satisfying both the hypotheses of free will and no signaling. Two classes of hidden-variable models are given as an example, and a general recipe to build infinitely many possible models is provided.

Measurement-based quantum computation is an efficient model to perform universal computation. Nevertheless, theoretical questions have been raised, mainly with respect to realistic noise conditions. In order to shed some light on this issue, we evaluate the exact dynamics of some single-qubit-gate fidelities using the measurement-based quantum computation scheme when the qubits which are used as a resource interact with a common dephasing environment. We report a necessary condition for the fidelity dynamics of a general pure $N$-qubit state, interacting with this type of error channel, to present an oscillatory behavior, and we show that for the initial canonical cluster state, the fidelity oscillates as a function of time. This state fidelity oscillatory behavior brings significant variations to the values of the computational results of a generic gate acting on that state depending on the instants we choose to apply our set of projective measurements. As we shall see, considering some specific gates that are frequently found in the literature, the fast application of the set of projective measurements does not necessarily imply high gate fidelity, and likewise the slow application thereof does not necessarily imply low gate fidelity. Our condition for the occurrence of the fidelity oscillatory behavior shows that the oscillation presented by the cluster state is due exclusively to its initial geometry. Other states that can be used as resources for measurement-based quantum computation can present the same initial geometrical condition. Therefore, it is very important for the present scheme to know when the fidelity of a particular resource state will oscillate in time and, if this is the case, what are the best times to perform the measurements.

In optical implementations of the phase-encoded BB84 protocol, the bit information is usually encoded in the phase of two consecutive photon pulses generated in a Mach-Zehnder interferometer. In the actual experimental realization, the loss in the arms of the Mach-Zehnder interferometer is not balanced, for example because only one arm contains a lossy phase modulator. Therefore, the amplitudes of the pulses are not balanced, and the structure of the signals and measurements no longer corresponds to the (balanced) ideal BB84 protocol. Hence, the BB84 security analysis no longer applies in this scenario. We provide a security proof of the unbalanced phase-encoded BB84. The resulting key rate turns out to be lower than the key rate of the ideal BB84 protocol. Therefore, in order to guarantee security, the loss due to the phase modulator cannot be ignored.

We report hyperfine-resolved scalar polarizabilities for cesium's $8s{}^2{S}_{1/2}$ and $9s{}^2{S}_{1/2}$ states using resonant two-photon spectroscopy in both an effusive beam and a vapor cell. Electric-field strengths are measured in situ, and the frequency scale is directly referenced to the ground-state hyperfine splitting of atomic rubidium. The measured $8s{}^2{S}_{1/2}$ scalar polarizability is $38370\pm 380a_0^3$, which agrees well with previously reported theoretical and experimental values. The measured $9s{}^2{S}_{1/2}$ scalar polarizability is $150700\pm 1500a_0^3$, which agrees within $2\sigma$ with the theory, but we are unaware of previous measurements. We verify that the $8s{}^2{S}_{1/2}$ state polarizability is independent of the hyperfine level, placing an upper limit of $200\pm 260a_0^3$ on the differential polarizability—this agrees with previous observations. We also measure a null differential polarizability between the hyperfine levels for the $9s{}^2{S}_{1/2}$ of $490\pm 450a_0^3$.

Photon-in/photon-out experiments at thin specimens have been carried out to determine $L$-subshell fluorescence yields as well as Coster-Kronig transition probabilities of Au, Pb, Mo, and Pd using radiometrically calibrated instrumentation in the Physikalisch-Technische Bundesanstalt (PTB) laboratory at the electron storage ring BESSY II in Berlin. An advanced approach was developed in order to derive the fluorescence line intensities by means of line sets of each subshell that were corrected for self-absorption and broadened with experimentally determined detector response functions. The respective photoelectric cross sections for each subshell were determined by means of transmission measurements of the same samples without any change in the experimental operating condition. All values derived were compared to those of earlier works. A completely traceable uncertainty budget is provided for the determined values.

We calculate, based on first-order perturbation theory, the total and differential ionization probabilities from a dynamic periodic Rydberg wave packet of a given $n$-shell exposed to a train of femtosecond laser pulses. The total probability is shown to depend crucially on the laser repetition rate: For certain frequencies the ionization probability vanishes, while for others it becomes very large. The origin of this effect is the strong dependence of the ionization probability on the Stark quantum number. Correspondingly, the angular electronic distribution also changes significantly with the increasing number of pulses for certain repetition rates.

In this work, combining the Bethe ansatz approach with the variational principle, we calculate the ground-state energy of the relative motion of a system of two fermions with spin up and spin down interacting via a delta-function potential in a one-dimensional (1D) harmonic trap. Our results show good agreement with the analytical solution of the problem, and provide a starting point for the investigation of more complex few-body systems where no exact theoretical solution is available.

We investigate the zero-temperature quantum phases of a Bose-Bose mixture on a triangular lattice using the bosonic dynamical mean-field theory (BDMFT). We consider the case of total filling where geometric frustration arises for asymmetric hopping. We map out a rich ground-state phase diagram including $xy$-ferromagnetic, spin-density wave, superfluid, and supersolid phases. In particular, we identify a stripe spin-density wave phase for highly asymmetric hopping. On top of the spin-density wave, we find that the system generically shows weak charge (particle) density wave order.