The utilization of a $d$-level partially entangled state, shared by two parties wishing to communicate classical information without errors over a noiseless quantum channel, is discussed. We analytically construct deterministic dense coding schemes for certain classes of nonmaximally entangled states, and numerically obtain schemes in the general case. We study the dependency of the maximal alphabet size of such schemes on the partially entangled state shared by the two parties. Surprisingly, for $d>2$ it is possible to have deterministic dense coding with less than one ebit. In this case the number of alphabet letters that can be communicated by a single particle is between $d$ and $2d$. In general, we numerically find that the maximal alphabet size is any integer in the range $[d,d^2]$ with the possible exception of $d^2-1$. We also find that states with less entanglement can have a greater deterministic communication capacity than other more entangled states.

We present a theory for the estimation of a scalar or a vector magnetic field by its influence on an ensemble of trapped spin-polarized atoms. The atoms interact off resonantly with a continuous laser field, and the measurement of the polarization rotation of the probe light, induced by the dispersive atom-light coupling, leads to spin squeezing of the atomic sample which enables an estimate of the magnetic field that is more precise than that expected from standard counting statistics. For polarized light and polarized atoms, a description of the non classical components of the collective spin angular momentum for the atoms and the collective Stokes vectors of the light field in terms of effective Gaussian position and momentum variables is practically exact. The Gaussian formalism describes the dynamics of the system very effectively and accounts explicitly for the back action on the atoms due to measurement and for the estimate of the magnetic field. Multicomponent magnetic fields are estimated by the measurement of suitably chosen atomic observables and precision and efficiency is gained by dividing the atomic gas in two or more samples which are entangled by the dispersive atom-light interaction.

Quantum-bit-string commitment [A. Kent, Phys. Rev. Lett. 90, 237901 (2003)] (QBSC) is a variant of bit commitment (BC). In this paper, we propose a QBSC protocol that can be implemented using currently available technology and prove its security under the same security criteria as discussed by Kent. QBSC is a generalization of BC, but has slightly weaker requirements, and our proposed protocol is not intended to break the no-go theorem of quantum BC.

Fixed-nuclei $R$-matrix calculations for elastic electron-ozone scattering have been performed in the energy region between 0 and $10\mathrm{eV}$. Electron correlation has been included extensively in the calculations. Applying the Fano-Feshbach projection operator formalism to the $R$-matrix results, we can extract from the calculations the positions and the energy-dependent widths of the resonances. The latter were used to estimate the magnitude of the total ion yield. We can show by comparison with experimental results that the low-lying ${}^{2}A_2$ Feshbach resonance is very likely to be responsible for the observed pronounced maxima in the cross sections for the ${\mathrm{O}}^{-}$ and the $\mathrm{O}_2{}^{-}$ channels at $1.3\mathrm{eV}$.

The process in which a fast passing proton captures an electron from a hydrogen atom and scatters at small angles is considered. In contrast to previous calculations within the peaking approximation the numerical calculations of the second Born as well as second Born-Faddeev diagrams are presented. The results of calculations are compared with the experimental data obtained at 2.8 and $5.0\mathrm{MeV}$ impact energy. It is shown that numerical second Born calculations give quite different results from that in the peaking approximation. In particular, the role of the $pp$ interaction is rather noticeable. On the other hand, the second Born-Faddeev numerical calculations clearly demonstrate the important role of distortion of plane waves.

We study the diffraction of quantum-degenerate fermionic atoms off of quantized light fields in an optical cavity. We compare the case of a linear cavity with standing-wave modes to that of a ring cavity with two counterpropagating traveling wave modes. It is found that the dynamics of the atoms strongly depends on the quantization procedure for the cavity field. For standing waves, no correlations develop between the cavity field and the atoms. Consequently, standing-wave Fock states yield the same results as a classical standing wave field while coherent states give rise to a collapse and revivals in the scattering of the atoms. In contrast, for traveling waves the scattering results in quantum entanglement of the radiation field and the atoms. This leads to a collapse and revival of the scattering probability even for Fock states. The Pauli exclusion principle manifests itself as an additional dephasing of the scattering probability.

We report on the direct observation of spontaneous coherence grating transfer between different pairs of Zeeman sublevels belonging to different cesium hyperfine states. Coherent Bragg diffraction is employed as a signature of the transfer mechanism; this spectrum shows a subnatural linewidth. Theoretically we model the observed effect using a tensorial density matrix formalism to describe the light interaction with a pair of degenerate two-level systems coupled by spontaneous emission. The possibility of transferring a short-lived atomic coherence into a long-lived one might be of considerable importance for the growing field of quantum-information processing.

We report a significant increase of the output of a 41.8-nm ${\mathrm{Xe}}^{8+}$ laser achieved by means of multimode guiding of high-intensity femtosecond laser pulses in a gas-filled dielectric capillary tube. The optimized lasing signal from a 15-mm-long capillary was nearly an order of magnitude higher than that from a gas cell of the same length. Simulations of the propagation of the pump laser pulse in the capillary confirmed that this enhancement is due to reflections from the capillary wall, which increase the length of the ${\mathrm{Xe}}^{8+}$ plasma column generated. The influence of gas pressure and focusing position on the lasing is also presented.

We construct entanglement witnesses using fundamental quantum operators of spin models which contain two-particle interactions and have a certain symmetry. By choosing the Hamiltonian as such an operator, our method can be used for detecting entanglement by energy measurement. We apply this method to the Heisenberg model in a cubic lattice with a magnetic field, the $XY$ model, and other familiar spin systems. Our method provides a temperature bound for separable states for systems in thermal equilibrium. We also study the Bose-Hubbard model and relate its energy minimum for separable states to the minimum obtained from the Gutzwiller ansatz.

Interference patterns due to the coherent electron emission from the atomic centers of a $D_2$ molecule by fast electron impact are experimentally observed in doubly differential cross sections. This behavior is supported by theoretical predictions. Measurements are given for the impact of $2.4\mathrm{keV}$ electrons. The emitted electrons are detected in the energy range from $2\text{to}2000\mathrm{eV}$, at observation angles between 30° and 130°. In addition to the interference phenomenon in electron emission following soft collisions, it is shown that interference effects appear for ejected electron energies corresponding to the binary-encounter mechanism.

The Josephson vortex (JV) between two long parallel coupled atomic Bose-Einstein condensates (BECs) is considered analytically and numerically in the quasi-one-dimensional limit. Its closed analytical solution is presented. It is found that, upon increasing the Josephson coupling, the JV transforms spontaneously into a dark soliton and vice versa. We suggest how the JV and its transformation can be detected. We also propose that the JV can be utilized for controlled and coherent transfer of atomic BECs and as a possible mobile qubit.

We present an experimental and theoretical study of the effect of additive noise on the signal-to-noise ratio (SNR) in a bistable, vertical cavity laser. We show that, the regime of vibrational resonance leads to higher SNR’s than the regime of stochastic resonance. A scaling law for the noise-induced degradation of SNR is obtained analytically and compared with experimental and numerical results, showing a good agreement.

An entanglement purification scheme for arbitrary unknown (mixed and pure nonmaximally) entangled ionic states is proposed by using linear optical elements. The main advantage of the scheme is that not only two-ion maximally entangled pairs but also four-ion maximally entangled pairs can be extracted from the less entangled pairs. The scheme is within current technology.

The multiconfiguration Dirac-Hartree-Fock model has been employed to compute the magnetic dipole hyperfine structure constant and the electric field gradient in the ${}^{3}P_1$ state of neutral mercury. Combined with the experimental electric quadrupole hyperfine interaction constant, the computed electric field gradient yields the nuclear quadrupole moment $Q=387\pm 6\mathrm{mb}$ for ${}^{201}\mathrm{Hg}$. This value is in good agreement with older muonic, atomic, and solid-state values, but differs from the latest muonic result and from the recent $\gamma$ spectroscopy determination.

A transient interference during the collision of a broad quantum particle wave packet with a properly chosen potential barrier suppresses, for energies well above the barrier top, the main incident momentum. This collision enhances the momenta below and above the central one, violating the classical conservation of energy. For the double square barrier the effect is due to the interference between three components (incident, transmitted, and interbarriers terms) and survives much longer than for the single barrier, facilitating its observation. A quantum optical realization with two-level atoms and effective laser barriers is discussed.

An electron scattering apparatus, combining a laser photoelectron source, a triply differentially pumped supersonic beam target, and several electron multipliers for simultaneous detection of elastically scattered electrons and of metastable atoms due to inelastic scattering, has been used for an improved study of electron-neon scattering over the energy range $16–19\mathrm{eV}$ at experimental energy widths of $4–6\mathrm{meV}$. Accurate values for the energies and widths of the low-lying ${\mathrm{Ne}}^{-}\left(2p^{5}3s^2{}^{2}P_{3∕2,1∕2}\right)$ Feshbach resonances have been determined from detailed analyses of the resonance profiles (measured at the five scattering angles 22.5°, 45°, 90°, 112.5°, 135°). In addition, the excitation function for the production of metastable ${\mathrm{Ne}}^{*}\left(2p^{5}3s{}^{3}P_{2,0}\right)$ atoms has been measured from threshold to $19\mathrm{eV}$; from a fit to its onset, the absolute electron energy scale is established to better than $0.5\mathrm{meV}$. While our resonance widths agree with the recommended values, the resonance energies differ by an amount larger than the quoted respective uncertainties. The experimental data are also compared with theoretical results, calculated with an improved $R$-matrix approach. Very good overall agreement between the experimental and the theoretical results is observed. A very sharp Feshbach resonance, associated with the ${\mathrm{Ne}}^{*}\left(2p^{5}3p{[5∕2]}_2\right)$ level, has been theoretically predicted and experimentally confirmed at $18.527\mathrm{eV}$.

In a previous slow photoionization experiment, modulations of ionization rings were manifested for Xe in a constant electric field. The present quantum calculation reveals that the modulation is an effect of the multichannel core scattering and of tunneling waves through the Coulomb-Stark potential barrier: the barrier reduces the number of oscillations that is observed relatively to the number of oscillations of the short range wave functions, and the nonhydrogenic core phase shifts modify the position of the ionization rings. We find a hidden difference, in the ionization process, for two close values of the energy depending on the resonance with the barrier. The ionization intensity is interpreted as a Runge-Lenz wave packet; thus, we can relate the quantum modulation to the classical Coulomb-Stark trajectories. The Runge-Lenz wave packet differs from a usual temporal wave packet because its components are eigenstates of the Runge-Lenz vector $z$ projection and its evolution is not temporal but spatial.

Using an NMR quantum computer, we experimentally simulate the quantum phase transition of a Heisenberg spin chain. The Hamiltonian is generated by a multiple-pulse sequence, the nuclear-spin system is prepared in its (pseudopure) ground state, and the effective Hamiltonian varied in such a way that the Heisenberg chain is taken from a product state to an entangled state, and finally to a different product state.

We present a general analysis for the criteria to stop and store light coherently. We show that a light pulse can be stopped in any physical system, provided that (i) the system bandwidth can be compressed to zero; (ii) the system has sufficient degrees of freedom to accommodate the pulse, and the bandwidth compression occurs while the pulse is in the system; and (iii) the bandwidth compression is done reversibly in an adiabatic fashion that preserves the phase space and the information in the original photon pulse during the entire duration of the stopping process. Based upon this general criterion, we present a brief discussion of stopping-light schemes using atomic resonances, and a detailed analysis of the all-optical scheme that we recently proposed. We show that the all-optical scheme can achieve arbitrarily small group velocities for large bandwidth pulses, and opens up new opportunities in both fundamental sciences and technological applications.