Nondegenerate-wavelength (or two-color) ghost imaging using either thermal or quantum light sources is studied theoretically. We demonstrate that a high-quality ghost image can be obtained even when the wavelengths of light used in the object and reference arms are very different. The spatial resolution of the ghost image is found in general to depend on each of these wavelengths, although in many practical situations it depends primarily on the wavelength used to illuminate the object. The resolution of nondegenerate-wavelength thermal ghost imaging can be higher than that of its quantum counterpart despite the fact that the photons have the same degree of spatial correlation in the two cases.

We have studied a rare-earth-doped crystal, ${\text{Pr}}^{3+}:{\text{La}}_2{\left({\text{WO}}_4\right)}_3$, which exhibits a high magnetic-moment density. Although the latter favors rare-earth dephasing, a nuclear-spin coherence lifetime of $250\mu \text{s}$ has been observed, as well as electromagnetically induced transparency (EIT). This suggests that a broad range of materials could be considered for quantum information applications such as quantum memories for light, where solids are especially attractive. Absorption and dispersion curves are independently in very good agreement with EIT theory. Fano-like profiles have also been observed.

Detailed derivation of the master equation and corresponding time evolution of the cavity radiation of coherent beat laser in terms of the $c$-number variables associated with the normal ordering is presented. Although the obtained expressions in general turn out to have the same form as when dephasing is not taken into consideration, there is clear evidence that the quantum features of the cavity radiation can be significantly affected by the rate of dephasing. It is shown based on intuitive argument that the degree of two-mode squeezing and entanglement decrease with the rate of dephasing. Moreover, a thorough study of the effects of the rate at which the atomic coherent superposition is decaying is expected to aid in determining the actual available various quantum properties of the generated radiation.

An off-axis feedback loop originates a two-point nonlocality that changes the instabilities of lasers of classes A and B: after crossing a first threshold these devices amplify while they lase above a second one. We obtain the instability diagram in presence of off-axis feedback and show that transverse phase and group velocities can be tuned to have the same or opposite sign, depending on control parameters. Counterpropagating noise sustained patterns and localized perturbations are found for an out-of-phase feedback when the laser operates as a signal splitter. We predict these effects in a broad class of devices, with either positive or negative refractive index, diffusion, and in presence of noise.

Sufficiently strong scattering can cause light propagation to stop, not due to absorption, but due to interference effects, thus localizing light. Efforts to observe localization in three dimensions with visible light have focused on high-scattering-disordered nanometer-sized powders with the unwanted side effect of strong absorption. The strong absorption mimics the signature of localization—exponential decay of transmitted intensity—complicating the interpretation of results. Nematic liquid crystals are more effective at scattering light and can have low absorption. We report experimental observation of strong photon localization in a nematic liquid crystal in which the effects of absorption are minimal.

We discuss the complete teleportation of the unknown quantum state of a paraxial single-photon field. The term complete signifies that the teleportation procedure transmits the entire quantum state, which is codified in all the degrees of freedom of a paraxial single-photon field: the polarization, the transverse wave vector, and the frequency. We analyze several illustrative examples and propose a possible experimental scheme.

The dynamics of the quantum entanglement is a fundamental characteristic for various quantum systems. Since the computable entanglement measure for higher dimensional quantum states itself is absent, the dynamics of the entanglement expressed in an operational method will be of interest. We study the dynamics of $\tau$, an analytical lower bound of squared concurrence, of a bipartite $d\otimes d$ quantum state when one party goes through an arbitrary noisy channel. For a pure input state, the range of $\tau$ is obtained explicitly. For a mixed input state, an upper bound of $\tau$ is found. Interestingly, the tangle ${\tau }^{\prime }$, as an upper bound of squared concurrence, also has a similar dynamical property. Our results are similar to that of Konrad et al. and can help the estimation of high-dimension bipartite entanglement in experiments.

We consider dense coding with partially entangled states on bipartite systems of dimension $d\times d$, studying the conditions under which a given number of messages, $N$, can be deterministically transmitted. It is known that the largest Schmidt coefficient, ${\lambda }_0$, must obey the bound ${\lambda }_0\le d/N$, and considerable empirical evidence points to the conclusion that there exist states satisfying ${\lambda }_0=d/N$ for every $d$ and $N$ except the special cases $N=d+1$ and $N=d^2-1$. We provide additional conditions under which this bound cannot be reached—that is, when it must be that ${\lambda }_0<d/N$—yielding insight into the shapes of boundaries separating entangled states that allow $N$ messages from those that allow only $N-1$. We also show that these conclusions hold no matter what operations are used for the encoding, and in so doing, identify circumstances under which unitary encoding is strictly better than nonunitary.

The Rydberg and autoionization Rydberg spectrum of scandium via intermediate state [Ar] $3d4s4p{{}^{2}F}_{5/2}$ have been measured in the energy region $52625–54046{\text{cm}}^{-1}$ using two-laser resonance excitation. Four autoionization Rydberg series are observed. Eigenquantum defects ${\mu }_{\alpha }$ and transformation matrices $U_{i\alpha }$ are calculated from first principles by relativistic multichannel theory within the framework of multichannel quantum defect theory, while the dipole matrix elements $D_{\alpha }$ are obtained by comparing with experimental data. With these parameters ${\mu }_{\alpha }$, $U_{i\alpha }$ and $D_{\alpha }$, Rydberg and autoionization Rydberg spectrum of scandium are calculated and the experimental peaks are then clearly assigned. More specifically, one autoionization Rydberg series is converged to ${\text{Sc}}^{+}$ $3d4s{{}^{3}D}_3$, the other three series are converged to ${\text{Sc}}^{+}$ $3d4s{{}^{1}D}_2$, and one autoionization state converging to ${\text{Sc}}^{+}$ $3d^2\left({{}^{3}F}_2\right)$ and/or ${\text{Sc}}^{+}$ $3d^2\left({{}^{1}D}_2\right)$ are assigned. In addition, the Fano parameters $E_r$, $q$, and $\Gamma$ for some of these states are fitted and analyzed.

We obtain a variety of predictions for the properties of population-imbalanced (or polarized) fermionic superfluids near their tricritical point. In the vicinity of the high-symmetry tricritical point, observable quantities such as the cloud shape, heat capacity, local polarization, and correlation length should exhibit distinct behavior arising from the tricritical scaling laws, as well as logarithmic corrections to scaling reflecting the marginal nature of interactions.

We theoretically and experimentally investigate the light dynamics in plane-mirror Fabry-Pérot resonators filled by two-dimensional (2D) and three-dimensional (3D) photonic crystals. It has been predicted that diffraction of such resonators can be manipulated [K. Staliunas, M. Peckus, V. Sirutkaitis, Phys. Rev. A 76, 051803(R) (2007)]; we study the phenomenon here in detail. In particular, we show the hyperbolic shape angular transmission profiles in case of 2D photonic structure (obtained by one-dimensional modulation of the surface of the mirrors), and quadratic shape transmission profiles in case of 3D photonic structure (obtained by 2D modulation of the mirrors). We develop the theoretical-numerical description of the system using the scattering matrix method and compare numerical results following from the model with the experimental ones.

On basis of numerical simulation of fiber laser passive mode locking, we have determined the quantum binding-energy levels for a pair of interacting structural solitons. These solitons have powerful wings and correspondingly large binding energies. It has been found that the field amplitude functions for steady states corresponding to neighboring energy levels have opposite parity. We have pointed out the analogy between the energy quantization for laser bound solitons and for a particle moving in potential well. The possibility of a coexistence of in-, opposite-, and $\pi /2$-phase soliton pairs has been found. In the case of multiple soliton trains, we have demonstrated the realization of highly stable soliton sequences with any required distribution along the soliton train of various types of bonds between neighboring pulses.

High-peak-power ultrafast lasers are widely used in nonlinear spectroscopy, but often limit its spectral resolution because of the broad frequency bandwidth of ultrashort laser pulses. Improving the resolution by achieving spectrally narrow excitation of, or emission from, the resonant medium by means of multiphoton interferences has been the focus of many recent developments in ultrafast spectroscopy. We demonstrate an alternative approach in which high resolution is exercised by detecting narrow spectral correlations between broadband excitation and emission optical fields. All-optical correlation analysis, easily incorporated into a traditional spectroscopic setup, enables direct, robust, and simultaneous detection of multiple narrow resonances in a single measurement.

The van der Waals–Casimir-Polder potential is calculated for two atoms out of equilibrium with external thermal electromagnetic field. The effects of absorption and spontaneous or stimulated emission are shown to strongly modify the potential. We demonstrate that the potential decays like $R^{-2}$ at large distances (much slower than at the equilibrium) can be attractive, repulsive, and resonant depending on the initial states of the atoms and electromagnetic field.

We study the dynamics of the entanglement between two oscillators that are initially prepared in a general two-mode Gaussian state and evolve while coupled to the same environment. In a previous paper, we showed that there are three qualitatively different dynamical phases for the entanglement in the long-time limit: sudden death, sudden death and revival, and no sudden death [Paz and Roncaglia, Phys. Rev. Lett. 100, 220401 (2008)]. Here we generalize and extend those results along several directions: We analyze the fate of entanglement for an environment with a general spectral density providing a complete characterization of the evolution for Ohmic, sub-Ohmic, and super-Ohmic environments. We consider two different models for the interaction between the system and the environment (one where the coupling is through position and another where the coupling is symmetric in position and momentum). Finally, we show that for nonresonant oscillators, the final entanglement is independent of the initial state and may be nonzero at very low temperatures.

Weak measurements offer insights into the behavior of quantum systems. Combined with postselection, quantum mechanics predicts a range of experimentally testable phenomena. In this paper I consider weak measurements performed on time-dependent preselected and postselected ensembles, with emphasis on the decay of excited states. The results show that the standard exponential decay law is a limiting case of a more general law that depends on both the time of postselection and the choice of final state. The generalized law is illustrated for two interesting choices of postselection.

We present a polymer quantization of the $-\lambda ∕r^2$ potential on the positive real line and compute numerically the bound state eigenenergies in terms of the dimensionless coupling constant $\lambda$. The singularity at the origin is handled in two ways: first, by regularizing the potential and adopting either symmetric or antisymmetric boundary conditions; second, by keeping the potential unregularized but allowing the singularity to be balanced by an antisymmetric boundary condition. The results are compared to the semiclassical limit of the polymer theory and to the conventional Schrödinger quantization on $L_2\left({\mathbb{R}}_{+}\right)$. The various quantization schemes are in excellent agreement for the highly excited states but differ for the low-lying states, and the polymer spectrum is bounded below even when the Schrödinger spectrum is not. We find, as expected, that for the antisymmetric boundary condition the regularization of the potential is redundant: the polymer quantum theory is well defined even with the unregularized potential and the regularization of the potential does not significantly affect the spectrum.