We argue that long-range photon-photon attraction induced by the dipole interaction of two electron-positron loops can lead to “vacuum self-focusing” of very intense laser beams. The focusing angle ${\theta }_F$ is found to increase with the beam intensity $I$ as ${\theta }_F\sim I^{4∕3}$; for the laser beams available at present or in the near future, ${\theta }_F\simeq {10}^{-10}–{10}^{-7}$.

Collisions, even though they do not limit the lifetime of quantum information stored in ground-state hyperfine coherences, may severely limit the fidelity for quantum memory when they happen during the write and read process. This imposes restrictions on the implementation of Raman-type quantum processes in thermal vapor cells and their performance as a quantum memory. We study the effect of these collisions in our experiment.

Metastable $2S$ muonic-hydrogen atoms undergo collisional $2S$ quenching, with rates which depend strongly on whether the $\mu p$ kinetic energy is above or below the $2S\to 2P$ energy threshold. Above threshold, collisional $2S\to 2P$ excitation followed by fast radiative $2P\to 1S$ deexcitation is allowed. The corresponding short-lived $\mu p\left(2S\right)$ component was measured at $0.6\mathrm{hPa}$ ${\mathrm{H}}_2$ room-temperature gas pressure, with lifetime ${\tau }_{2S}^{\text{short}}={165}_{-29}^{+38}\mathrm{ns}$ (i.e., ${\lambda }_{2S}^{\text{quench}}={7.9}_{-1.6}^{+1.8}\times {10}^{12}{\mathrm{s}}^{-1}$ at liquid-hydrogen density) and population ${\epsilon }_{2S}^{\text{short}}={1.70}_{-0.56}^{+0.80}\%$ (per $\mu p$ atom). In addition, a value of the $\mu p$ cascade time, $T_{\mathrm{cas}}^{\mu p}=(37\pm 5)\mathrm{ns}$, was found.

Single and multiple ionization of ${\mathrm{C}}_{60}$ in collisions with highly charged fast oxygen ions have been studied using the recoil-ion time-of-flight technique. The dependence of multiple-ionization cross sections on projectile charge state $\left(q_p\right)$ was found to be drastically different from those for an atomic target, such as Ne. A model based on the giant dipole plasmon resonance explains quite well the observed $q_p$ dependence for the single- and-double-ionization cross sections. But the same model deviates for triple and quadruple ionizations.

We study families $H_n$ of one-dimensional (1D) quantum spin systems, where $n$ is the number of spins, which have a spectral gap $\Delta E$ between the ground-state and first-excited state energy that scales, asymptotically, as a constant in $n$. We show that if the ground state $\mid {\Omega }_m⟩$ of the Hamiltonian $H_m$ on $m$ spins, where $m$ is an $O\left(1\right)$ constant, is locally the same as the ground state $\mid {\Omega }_n⟩$, for arbitrarily large $n$, then an arbitrarily good approximation to the ground state of $H_n$ can be stored efficiently for all $n$. We formulate a conjecture that, if true, would imply our result applies to all noncritical 1D spin systems. We also include an appendix on quasiadiabatic evolutions.

Atomic collisions are included in an interacting system of optical fields and trapped atoms allowing field amplification. We study the effects of collisions on the system stability. Also a study of the degree of entanglement between atomic and optical fields is made. We found that, for an atomic field initially in a vacuum state and optical field in a coherent state, the degree of entanglement does not depend on the optical field intensity or phase. We show that in conditions of exponential instability the system presents at long times two distinct stationary degree of entanglement with collisions affecting only one of them.

We demonstrate a scheme to spectrally manipulate a collinear, continuous stream of time and energy entangled photons to generate beamlike, bandwidth-limited fluxes of polarization-entangled photons with nearly degenerate wavelengths. Utilizing an ultrashort-pulse shaper to control the spectral phase and polarization of the photon pairs, we tailor the shape of the Hong-Ou-Mandel interference pattern, demonstrating the rules that govern the dependence of this interference pattern on the spectral phases of the photons. We then use the pulse shaper to generate all four polarization Bell states. The singlet state generated by this scheme forms a very robust decoherence-free subspace, extremely suitable for long-distance fiber-optics-based quantum communication.

We propose and experimentally demonstrate a simple and robust approach for retrieving arbitrary complex-valued fields from three or more diffraction intensity recordings. We need no a priori knowledge about the object field. The convergence rate is rapid. We obtained good results using experimental data with only 80 iterations (160 fast Fourier transforms). The method does not suffer any stagnation or ambiguity problem, and it also exhibits a high immunity to noise. The technique exhibits great potential in lensless phase-contrast imaging, wave-front sensing, and metrology for a wide spectral range.

In this paper, we study the entanglement properties of a spin-1 model, the exact ground state of which is given by a matrix product (MP) state. The model exhibits a critical point transition at a parameter value $a=0$. The longitudinal and transverse correlation lengths are known to diverge as $a\to 0$. We use three different entanglement measures $S\left(i\right)$ (the one-site von Neumann entropy), $S(i,j)$ (the two-body entanglement), and $G(2,n)$ (the generalized global entanglement) to determine the entanglement content of the MP ground state as the parameter $a$ is varied. The entanglement length, associated with $S(i,j)$, is found to diverge in the vicinity of the quantum critical point $a=0$. The first derivative of the entanglement measure $E$ $[=S\left(i\right),S(i,j)]$ with respect to the parameter $a$ also diverges. The first derivative of $G(2,n)$ with respect to $a$ does not diverge as $a\to 0$ but attains a maximum value at $a=0$. At the quantum critical point itself all three entanglement measures become zero. We further show that multipartite correlations are involved in the quantum phase transitions at $a=0$.

Effective quantum-information processing requires coherent control of large numbers of qubits on a time scale that is short compared to the decoherence time of the system. It is therefore important to extrapolate and measure decoherence times for large quantum registers and to determine the effect of different couplings between system and environment on the decoherence rate. For this purpose, we have experimentally realized a system that allows one to generate model quantum registers with more than 100 qubits and measure the decay of the information in these states while adjusting the strength of the interaction between the quantum register and the environment. Our results indicate a power-law dependence of the decoherence rate on the number of qubits in the system, with an exponent of the order of 0.5. This behavior remains qualitatively unchanged when the coupling strength to the environment is reduced by about an order of magnitude.

We give the derivation of the Fano profile (the resonance energy position, the resonance width $\Gamma$, and $q$ value) from the time-dependent nonrelativistic density-functional theory (DFT) and propose a scheme for calculating the photoabsorption cross section of hot dense plasmas. As a consequence of this derivation, we show the line profile is obtained as a superposition of Fano and Lorentz profiles when the competition of two optically allowed bound-bound and bound-free transitions occurs. We also show the results of the photoabsorption cross section by applying our scheme to an Fe plasma (density is $7.85\mathrm{g}∕{\mathrm{cm}}^3$, temperature is $100\mathrm{eV}$), where the calculation is carried out without numerical divergence for any photon energy. The calculated results are in good agreement with those of Grimaldi.

We study the coherent association of a two-species atomic condensate into a condensate of heteronuclear diatomic molecules, using both a semiclassical treatment and a quantum mechanical approach. The differences and connections between the two approaches are examined. We show that, in this coupled nonlinear atom-molecule system, the population difference between the two atomic species plays a significant role in the ground-state stability properties as well as in coherent population oscillation dynamics.

We derive a master equation for a two-level emitter interacting with a band-gap reservoir at finite temperatures. This equation is able to capture effects of emitter-reservoir entanglement. We show that the entangled field-emitter bound state, which arises in the process of interaction, does not survive indefinitely at finite temperatures. However, such an entangled state may be effectively excited through an intensive incoherent driving.

The vacuum cavity mode induces a potential barrier and a well when an ultraslow excited atom enters the interaction region so that it can be reflected or transmitted with a certain probability. We demonstrate here that a slow-velocity excited particle tunnels freely through a vacuum electromagnetic field mode filled with $N-1$ ground state atoms. The reason for this is the trapping of the moving atom into its upper state due to multiparticle influences and the corresponding decoupling from the interaction with the environment such that the emitter does not “see” the induced potentials.

We consider another version of the twin paradox. The twins move along the same circular free photon path around the Schwarzschild center. In this case, despite their different velocities, all twins have the same nonzero acceleration. On the circular photon path, the symmetry between the twins situations is broken not by acceleration (as it is in the case of the classic twin paradox), but by the existence of an absolute standard of rest (timelike Killing vector). The twin with the higher velocity with respect to the standard of rest is younger on reunion. This closely resembles the case of periodic motions in compact (nontrivial topology) three-dimensional (3D) space recently considered in the context of the twin paradox by Barrow and Levin [Phys. Rev. A 63, 044104 (2001)], except that in that paper accelerations of all twins were equal to zero and that, in the case considered here, the 3D space has trivial topology.

Total charge-transfer cross sections for ${\mathrm{H}}^{+}$ ions in collisions with Ne atoms have been measured in the energy range of $0.60\text{to}3.6\mathrm{keV}$ with the initial growth rate method and found to agree with the present rigorous calculation based on a molecular-orbital expansion method with the electronic states obtained by the multireference single- and double-excitation configuration interaction method. These observed cross sections are also compared with previously published data and theoretical prediction. It has been found that previous experimental data deviate significantly from the present observed and calculated cross sections as the collision energy decreases.

When molecules are driven by intense laser fields linearly polarized along the internuclear axis, their ionization rate vs the internuclear separation $R$ is known to have a maximum at some critical intermediate separation $R_c$. We show that such a maximum should occur only in molecules whose highest occupied molecular orbitals (HOMOs) are such that the electron cloud is concentrated along the internuclear axis (i.e., $\sigma$-type HOMOs). For molecular systems whose HOMO symmetry prevents electron probability distribution along this axis, we find that the ionization rate increases monotonically with increasing $R$ until saturation occurs. In other words, for such molecular systems, there is no critical $R_c$ at which molecular ionization is maximum.

Entanglement and entanglement assisted are useful resources to enhance the mutual information of the Pauli channels, when the noise on consecutive uses of the channel has some partial correlations. In this paper, we study quantum-communication channels in $d$-dimensional systems and derive the mutual information of the quantum channels for maximally entangled states and product states coding with correlated noise. Then, we compare fidelity between these states. Our results show that there exists a certain fidelity memory threshold, which depends on the dimension of the Hilbert space $\left(d\right)$ and the properties of noisy channels. We calculate the classical capacity of a particular correlated noisy channel and show that in order to achieve Holevo limit, we must use $d$ particles with $d$ degrees of freedom. Our results show that entanglement is a useful means to enhance the mutual information. We choose a special nonmaximally entangled state and show that in the quasiclassical depolarizing and quantum depolarizing channels, maximum classical capacity in the higher memory channels is given by the maximally entangled state. Hence, our results show that for high error channels in every degree of memory, maximally entangled states have better mutual information.

We demonstrate theoretically how photon-number statistics of a driven damped oscillator at finite temperature can be extracted by measuring the dephasing spectrum of a two-level system dispersively coupled to the oscillator; we thus extend the work of M. I. Dykman and M. A. Krivoglaz [Sov. Phys. Solid State 29, 210 (1987)] and J. Gambetta et al. [Phys. Rev. A 74, 042318 (2006)]. We carefully consider the fidelity of this scheme—to what extent does the measurement reflect the initial number statistics of the mode? We also derive analytic results for the dephasing of a qubit by a driven thermal mode, and compare results obtained at different levels of approximation. Our results have relevance both to experiments in circuit cavity QED, as well as to nanoelectromechanical systems.

Graph states (or cluster states) are the entanglement resource that enables one-way quantum computing. They can be grown by projective measurements on the component qubits. Such measurements typically carry a significant failure probability. Moreover, they may generate imperfect entanglement. Here we describe strategies to adapt growth operations in order to cancel incurred errors. Nascent states that initially deviate from the ideal graph states evolve toward the desired high fidelity resource without impractical overheads. Our analysis extends the diagrammatic language of graph states to include characteristics such as tilted vertices, weighted edges, and partial fusion, which arise from experimental imperfections. The strategies we present are relevant to parity projection schemes such as optical path erasure with distributed matter qubits.