We study the combined effect of delayed Raman response and bit pattern randomness on pulse propagation in massive multichannel optical fiber communication systems. The propagation is described by a perturbed stochastic nonlinear Schrödinger equation, which takes into account changes in pulse amplitude and frequency as well as emission of continuous radiation. We perform extensive numerical simulations with the model and analyze the dynamics of the frequency moments, the bit-error rate, and the mutual distribution of amplitude and position. The results of our numerical simulations are in good agreement with theoretical predictions based on the adiabatic perturbation approach.

Exact solutions are obtained for all the modes of wave propagation along an anisotropic cylindrical waveguide. Closed-form expressions for the energy flow on the waveguide are also derived. For extremely anisotropic waveguide where the transverse permittivity is negative $\left({\epsilon }_{\perp }<0\right)$ while the longitudinal permittivity is positive $\left({\epsilon }_{\parallel }>0\right)$, only transverse magnetic (TM) and hybrid modes will propagate on the waveguide. At any given frequency the waveguide supports an infinite number of eigenmodes. Among the TM modes, at most only one mode is forward wave. The rest of them are backward waves which can have very large effective index. At a critical radius, the waveguide supports degenerate forward- and backward-wave modes with zero group velocity. These waveguides can be used as phase shifters and filters, and as optical buffers to slow down and trap light.

A simple technique for robust generation of vibrational Fock states in a chain of trapped ions is proposed. The method is fast and easy to implement, since only a single chirped laser pulse, simultaneously addressing all of the ions, is required. Furthermore, because the approach uses collective adiabatic passage, significant fluctuations in the intensity or frequency of the laser pulse can be tolerated, and the technique performs well even on the border of the Lamb-Dicke regime. We also demonstrate how this technique may be extended in order to create nonclassical superposition states of the ions’ collective motion and Greenberger-Horne-Zeilinger states of their internal states. Because only a single laser pulse is required, heating effects arising under realistic experimental conditions are negligibly small.

Recent papers in Physical Review A have considered small quantum circuit decompositions in terms of a minimal number of controlled-NOT (CNOT) gates. Specifically, we point out errors in the papers by Vidal and Dawson [Phys. Rev. A 69, 010301(R) (2004)] and by Shende et al. [Phys. Rev. A 70, 012310 (2004)].

I consider the problem of deterministically distinguishing the state of a multipartite system, from a set of $N\ge d+1$ orthogonal states, where $d$ is the dimension of each party’s subsystem. It is shown that if the set of orthogonal states is chosen at random, then there is a vanishing probability that this set will be perfectly distinguishable under the restriction that the parties use only local operations on their subsystems and classical communication among themselves.

In this paper, we present a $\text{U}\left(1\right)$-invariant expansion theory of the adiabatic process. As its application, we propose and discuss different sufficient adiabatic approximation conditions. In these conditions, we find an interesting invariant quantity referred to as the quantum geometric potential (QGP) contained in all time-dependent processes. Furthermore, we also give detailed discussion and analysis on the properties and effects of QGP.

We present a multipartite entanglement measure for $\mathit{N}$-qubit pure states, using the norm of the correlation tensor which occurs in the Bloch representation of the state. We compute this measure for several important classes of $\mathit{N}$-qubit pure states such as Greenberger-Horne-Zeilinger and $\mathit{W}$ states and their superpositions. We compute this measure for interesting applications like the one-dimensional Heisenberg antiferromagnet. We use this measure to follow the entanglement dynamics of Grover’s algorithm. We prove that this measure possesses almost all the properties expected of a good entanglement measure, including monotonicity. Finally, we extend this measure to $\mathit{N}$-qubit mixed states via convex roof construction and establish its various properties, including its monotonicity. We also introduce a related measure which has all properties of the above measure and is also additive.

We investigate the dependence of the dissociation branching ratio of ${\text{ND}}^{+}$ on intensity in the strong-field regime. ${\text{ND}}^{+}$, unexplored previously using an intense laser, offers an interesting, alternating sequence of dissociation limits to ${\text{N}}^{+}+\text{D}$ and $\text{N}+{\text{D}}^{+}$ separated by less than the photon energy at 795 nm wavelength. By exposing an ${\text{ND}}^{+}$ beam to a 40 fs Ti:sapphire laser pulse and measuring the ion fragments in coincidence with the neutral fragments, we find that the branching ratio of the final dissociation channel depends strongly on intensity in the range $2\times {10}^{13}–3\times {10}^{15}\text{W}/{\text{cm}}^2$. Furthermore, we use measurements of the kinetic energy release and angular distributions for these channels to identify their most probable dissociation pathways.

In this paper we consider an initially excited two-level system coupled to a monomode cavity, and compute exact expressions for the spectra spontaneously emitted by each system in the general case where they have arbitrary linewidths and frequencies. Our method is based on the fact that this problem has an easily solvable classical counterpart, which provides a clear interpretation of the evidenced phenomena. We show that if the cavity linewidth is much lower than the atomic linewidth, photons are emitted at the cavity frequency, even if the atom and the cavity are strongly detuned. We also study the links between the spontaneous emission spectra and the fluorescence spectra emitted when the atom is driven by a classical field of tunable frequency in the low excitation limit.

We address the impact of imprinted optical lattices on the mobility of a strongly nonlocal spatial optical soliton. With the aid of the Ehrenfest theorem in quantum mechanics, we obtain an equation of motion for the soliton center, and find its analytical solution. Comparison with numerical simulations shows that our analytical solution can describe the motion of the soliton center very well when the optical lattice period is much larger than the soliton width. It is found that there exists a critical angle that is a function of the optical lattice parameters. When its incident angle is smaller than the critical angle, the soliton is trapped within the waveguide induced by the optical lattices. Above the critical angle, the soliton leaves the central waveguide and propagates along its original direction traveling across the optical lattices. A way to control the soliton mobility is discussed based on the finding.

We introduce a mathematical framework for the description of measurements of quantum processes. Using this framework, process estimation problems can be treated in a similar way as state estimation problems, only replacing the concept of positive-operator-valued measure (POVM) by the concept of process POVM (PPOVM). In particular, we will show that any measurement of qudit channels can be described by a collection of effects (positive operators) defined on a two-qudit system. However, the effects forming a PPOVM are not normalized in the usual sense. We will demonstrate the use of this formalism in discrimination problems by showing that perfect channel discrimination is equivalent to a specific unambiguous state discrimination.

We develop a theory describing the dynamics of quantum kicked rotators (modeling cold atoms in a pulsed optical field) which are subjected to combined amplitude and timing noise generated by a renewal process (acting as an engineered reservoir). For waiting-time distributions of variable exponent (Lévy noise), we demonstrate the existence of a regime of nonexponential loss of phase coherence. In this regime, the momentum dynamics is subdiffusive, which also manifests itself in a non-Gaussian limiting distribution and a fractional power-law decay of the inverse participation ratio. The purity initially decays with a stretched exponential which is followed by two regimes of power-law decay with different exponents. The averaged logarithm of the fidelity probes the sprinkling distribution of the renewal process. These analytical results are confirmed by numerical computations on quantum kicked rotators subjected to noise events generated by a Yule-Simon distribution.

We investigate the control of a three-level quantum system by laser fields assisted by von Neumann measurements. We consider a system which is not completely controllable by unitary evolution but which becomes controllable if particular measurements are used. The optimal control is defined from a cost functional which takes into account the measurements. The cost corresponds either to the minimization of the duration of the control or to the minimization of the energy of the laser field. Using the Pontryagin maximum principle, we determine the optimal control which steers the system from a given initial state toward a desired target state. This allows one to determine which observable has to be chosen for the measurement and the time at which the measurement has to be performed.

The fully relativistic theory of the Zeeman splitting of the ${\left(1s\right)}^{2}2s$ hyperfine-structure levels in lithiumlike ions with $Z=6–32$ is considered for the magnetic field magnitude in the range from 1 to 10 T. The second-order corrections to the Breit-Rabi formula are calculated and discussed including the one-electron contributions as well as the interelectronic-interaction effects of order $1/Z$. The $1/Z$ corrections are evaluated within a rigorous QED approach. These corrections are combined with other interelectronic-interaction, QED, nuclear recoil, and nuclear size corrections to obtain high-precision theoretical values for the Zeeman splitting in Li-like ions with nonzero nuclear spin. The results can be used for a precise determination of nuclear magnetic moments from $g$-factor experiments.

We experimentally investigate the properties of radio-frequency-dressed potentials for Bose-Einstein condensates on atom chips. The three-dimensional potential forms a connected pair of parallel waveguides. We show that rf-dressed potentials are robust against the effect of small magnetic-field variations on the trap potential. Long-lived dipole oscillations of condensates induced in the rf-dressed potentials can be tuned to a remarkably low damping rate. We study a beam splitter for Bose-Einstein condensates and show that a propagating condensate can be dynamically split in two vertically separated parts and guided along two paths. The effect of gravity on the potential can be tuned and compensated for using a rf-field gradient.

Pair operators for boson and fermion atoms generate $\text{SU}\left(1,1\right)$ and $\text{SU}\left(2\right)$ Lie algebras, respectively. Consequently, the pairing of boson and fermion atoms into diatomic molecules via Feshbach resonances, produces $\text{SU}\left(1,1\right)$ and $\text{SU}\left(2\right)$ coherent states, making bosonic pairing the matter-wave equivalent of parametric coupling and fermion pairing equivalent to the Dicke model of quantum optics. We discuss the properties of atomic states generated in the dissociation of molecular Bose-Einstein condensates into boson or fermion constituent atoms. The $\text{SU}\left(2\right)$ coherent states produced in dissociation into fermions give Poissonian atom-number distributions, whereas the $\text{SU}\left(1,1\right)$ states generated in dissociation into bosons result in super-Poissonian distributions, in analogy to two-photon squeezed states. In contrast, starting from an atomic gas produces coherent number distributions for bosons and super-Poissonian distributions for fermions.

We propose and demonstrate a linear optical device which deterministically performs optimal quantum measurement or minimum disturbance measurement on a single-photon polarization qubit with the help of an ancillary path qubit introduced to the same photon. We show theoretically and experimentally that this device satisfies the minimum disturbance measurement condition by investigating the relation between the information gain (estimation fidelity) and the state disturbance due to measurement (operation fidelity). Our implementation of minimum disturbance measurement is postselection-free in the sense that all detection events are counted toward evaluation of the estimation fidelity and the operation fidelity, i.e., there is no need for coincidence postselection of the detection events.

Spontaneous emission of an excited two-level nonrelativistic atom is examined in the Schrödinger picture by treating the atom’s external and internal degrees of freedom on the same quantum footing. After all atomic transitions that lead to the evolution of this atom-vacuum system are analyzed, it is found that the atom’s spontaneous emission rate is reduced by its center-of-mass motion. It is also found that, as a result of the entanglement between atomic and photonic states, the spontaneous emission process cannot be explained as a process that takes place in one coordinate system moving with the atom but is observed in another stationary system.

We have studied by mass spectrometry the fragmentation pathways of a molecule of biological interest: adenine ${\mathrm{H}}_5{\mathrm{C}}_5{\mathrm{N}}_5\left(\mathrm{Ad}\right)$. The adenine molecules are ionized and excited in a single collision event mode with slow highly charged ${\mathrm{Ar}}^{8+}$ ions. A method based on the detection in coincidence of neutral and charged fragments has been developed for studying delayed fragmentation channels. The measured spectra show that the well-stated process, i.e., the successive emission of $\mathrm{HCN}$ fragments, is not the only fragmentation pathway. Although in the first fragmentation step the dominant channel is the emission of a neutral ${\mathrm{HCN}}^0(88\%)$, in the second fragmentation step the branching ratios for the delayed emission of an ${\mathrm{HCN}}^0$ and an ${\mathrm{H}}_2{\mathrm{CNCN}}^0$ from the intermediate parent ion ${\mathrm{H}}_4{\mathrm{C}}_4{\mathrm{N}}_4^{+}\left[\right(\mathrm{Ad}\text{−}\mathrm{HCN}{)}^{+}]$ are surprisingly similar, 54% and 46%, respectively. As a consequence, the role of these two latter channels in the reversal process, i.e., the formation of adenine in gas phase, should be considered. Ab initio calculations of molecular energies show that the energy levels of these two fragmentation channels are very close. This is in qualitative agreement with the measured comparable branching ratios.

Interaction-induced properties, including the interaction energy itself, are difficult to compute using explicitly correlated basis functions since bases corresponding to noninteracting atoms are not defined in this case. We propose a direct method of calculation in which no subtraction of exact or approximate atomic properties is required. To test the method we computed the one-electron Darwin and mass-velocity contributions to the interaction energy of two ground-state hydrogen atoms at separations close to the van der Waals minimum in the triplet state. To verify the accuracy at larger distances, we also computed the constants determining the asymptotic long-range behavior of the investigated contributions. Our results, obtained with a Gaussian geminal basis, appear to be more accurate than the results of earlier variational calculations employing explicitly correlated exponential functions. When the same basis sets are employed, our method gives results very close to the so-called “monomer-contraction method” proposed recently by Cencek et al. [W. Cencek et al., Phys. Rev. Lett. 95, 23304 (2005)], which confirms the soundness of the latter approach. To extend the validity of our calculations to very large interatomic separations $R$, we provide damping functions accounting for the retardation effects and producing the correct $1/R^7$ long-range decrease of the computed relativistic corrections.