We investigate the possibility of using a dissipative process to prepare a quantum system in a desired state. We derive for any multipartite pure state a dissipative process for which this state is the unique stationary state and solve the corresponding master equation analytically. For certain states, such as the cluster states, we use this process to show that the jump operators can be chosen quasilocally, i.e. they act nontrivially only on a few, neighboring qubits. Furthermore, the relaxation time of this dissipative process is independent of the number of subsystems. We demonstrate the general formalism by considering arbitrary matrix-product states or projected entangled pair states. In particular, we show that the ground state of the Affleck-Kennedy-Lieb-Tasaki model can be prepared employing a quasi-local dissipative process.

We have completed an extensive study of ${}^{129}\mathrm{Xe}$ longitudinal spin relaxation in the gas phase, involving both intrinsic and extrinsic mechanisms. The dominant intrinsic relaxation is mediated by the formation of persistent ${\mathrm{Xe}}_2$ van der Waals dimers. The dependence of this relaxation on applied magnetic field yields the relative contributions of the spin-rotation and chemical-shift-anisotropy interactions; the former dominates at magnetic fields below a few tesla. This relaxation also shows an inverse quadratic dependence on temperature $T$; the maximum low-field intrinsic relaxation for pure xenon at room temperature (measured here to be $4.6\mathrm{h}$, in agreement with previous work) increases by $\approx 60\%$ for $T=100°\mathrm{C}$. The dominant extrinsic relaxation is mediated by collisions with the walls of the glass container. Wall relaxation was studied in silicone-coated alkali-metal-free cells, which showed long (many hours or more) and robust relaxation times, even at the low magnetic fields typical for spin-exchange optical pumping $(\approx 3\mathrm{mT})$. The further suppression of wall relaxation for magnetic fields above a few tesla is consistent with the interaction of ${}^{129}\mathrm{Xe}$ with paramagnetic spins on or inside the surface coating. At $14.1\mathrm{T}$ and sufficiently low xenon density, we measured a relaxation time $T_1=99\mathrm{h}$, with an inferred wall-relaxation time of $174\mathrm{h}$. A prototype large storage cell ($12\mathrm{cm}$ diameter) was constructed to take advantage of the apparent increase in wall-relaxation time for cells with a smaller surface-to-volume ratio. The measured relaxation time in this cell at $3\mathrm{mT}$ and $100°\mathrm{C}$ was $5.75\mathrm{h}$. Such a cell (or one even larger) could be used to store many liters of hyperpolarized ${}^{129}\mathrm{Xe}$ produced by a flow-through polarizer and accumulator for up to three times longer than currently implemented schemes involving freezing xenon in liquid nitrogen.

For identical bosons with a large scattering length, the dependence of the three-body recombination rate on the collision energy is determined in the zero-range limit by universal functions of a single scaling variable. There are six scaling functions for angular momentum zero and one scaling function for each higher partial wave. We calculate these universal functions by solving the Skorniakov-Ter-Martirosian equation. The results for the three-body recombination as a function of the collision energy are in good agreement with previous results from solving the three-body Schrödinger equation for ${}^4\mathrm{He}$ atoms. The universal scaling functions can be used to calculate the three-body recombination rate at nonzero temperature. We obtain an excellent fit to the data from Grimm and co-workers for ${}^{133}\mathrm{Cs}$ atoms with a large positive scattering length.

We identify an optimized three-dimensional metallic photonic-band-gap filament architecture for electrically pumped, quasithermal, visible light emission. This identification is based on extensive band structure and finite-difference time-domain calculations of metallic photonic crystals. The optimum structure consists of an inverse square-spiral photonic crystal, exhibiting a large bandwidth optical passband below the effective plasma screening frequency of the periodically structured metal. Light emission from the interior surfaces of the filament to the interior air channels occurs exclusively into the passband modes, enabling high-efficiency conversion of electrical energy into visible light.

We study the Fourier relationship between angle and orbital angular momentum of entangled photons. Spatial light modulators allow us to define and control the spatial mode measurement state. We observe strong quantum correlations, establishing that angular position and momentum distributions between the photons are related as conjugate Fourier pairs.

An analytical description for plane-wave propagation in metamaterials is presented. It follows the usual approach for describing light propagation in homogeneous media on the basis of Maxwell’s equations, although applied to a medium composed of metallic nanostructures. Here, as an example, these nanostructures are double (or cut) wires. In the present approach it is assumed that the carriers perform collective oscillations in a single wire. These oscillations are coupled to those in the adjacent wire; thus, the internal carrier dynamics may be described by a coupled-oscillator model. The multipole expansion technique is used to account for the electric and magnetic dipole as well as the electric quadrupole moments of these carrier oscillations within the nanostructure. It turns out that the symmetric normal mode is related to the electric dipole moment whereas the antisymmetric normal mode evokes simultaneously a magnetic dipole and an electric quadrupole moment. It is shown how effective permittivity and permeability can be derived from analytical expressions for the dispersion relation, the magnetization, and the electric displacement field. The results of the analytical model are compared with rigorous simulations of Maxwell’s equations yielding the limitations and the domain of applicability of the proposed model.

We show that a quantum-beat system with incoherent pump or coherent driving produces an entangled sub-shot-noise laser that operates well above threshold. The numerical results and physical analyses are presented using the combination modes of the lasing fields. The relative mode is decoupled from the active medium and thus remains in its vacuum state, while the sum mode operates well above threshold and has sub-shot-noise. The quantum beat and the sum mode intensity noise reduction combine to yield entanglement between two bright beams and sub-Poissonian photon statistics of the respective beams.

We consider the decoherence of photons suffering in phase-damping channels. By exploring the evolutions of single-photon polarization states and two-photon polarization-entangled states, we find that different frequency spectrum envelopes of photons induce different decoherence processes. A white frequency spectrum can lead the decoherence to an ideal Markovian process. Some color frequency spectrums can induce asymptotical decoherence, while, some other color frequency spectrums can make coherence vanish periodically with variable revival amplitudes. These behaviors result from the non-Markovian effects on the decoherence process, which may give rise to a revival of coherence after complete decoherence.

Graph states are generalized from qubits to collections of $n$ qudits of arbitrary dimension $D$, and simple graphical methods are used to construct both additive and nonadditive, as well as degenerate and nondegenerate, quantum-error-correcting codes. Codes of distance 2 saturating the quantum Singleton bound for arbitrarily large $n$ and $D$ are constructed using simple graphs, except when $n$ is odd and $D$ is even. Computer searches have produced a number of codes with distances 3 and 4, some previously known and some new. The concept of a stabilizer is extended to general $D$, and shown to provide a dual representation of an additive graph code.

We present both differential and total cross sections for the direct ionization of the cytosine molecule by protons in the incident energy range $0.1–100\mathrm{MeV}$. We have used the first Born approximation and included the pairwise Coulomb interactions of the ejected electron with both the scattered proton and the residual ionized target. In this model, the ground state of the cytosine molecule is described by means of an accurate one center molecular wave function.

We have investigated the effects of inhomogeneous Doppler line broadening on the Autler-Townes (AT) splitting in a three-level open molecular cascade system. For moderate Rabi frequencies in the range of $300–600\mathrm{MHz}$, the fluorescence line shape from the uppermost level ∣3⟩ in this system depends strongly on the wave-number ratio of the two laser fields. However, the fluorescence spectrum of the intermediate level ∣2⟩ appears as expected. We provide a description of the conditions for optimally resolved AT splitting in terms of the ratio of the probe and coupling laser wave numbers, and the laser propagation geometry based on our theoretical analysis of the Doppler integral. The low threshold Rabi frequency region for Autler-Townes splitting is important for high-resolution Autler-Townes spectroscopy since the laser E-field amplitudes are modest in continuous-wave laser experiments.

In this paper, the quantum phase transition between the superfluid state and the Mott-insulator state is studied based on an extended Bose-Hubbard model with two- and three-body on-site interactions. By employing the mean-field approximation we find the extension of the insulating “lobes” and the existence of a fixed point in three-dimensional phase space. We investigate the link between experimental parameters and theoretical variables. The possibility to observe our results through some experimental effects in optically trapped Bose-Einstein condensates is also discussed.

We study the physics of ultracold dipolar bosons in optical lattices. We show that dipole-dipole interactions lead to the appearance of many insulating metastable states. We study the stability and lifetime of these states using a generalization of the instanton theory. We also investigate possibilities to prepare, control, and manipulate these states using time-dependent superlattice modifications and modulations. We show that the transfer from one metastable configuration to another necessarily occurs via superfluid states, but can be controlled fully at the quantum level. We show how the metastable states can be created in the presence of a trapping potential. Our findings open the way toward applications of the metastable states as quantum memories.

We present an analytic method for predicting the initial self-focusing dynamics of high-power beams and pulses. Using this method we study the filamentation pattern of a variety of input profiles, without solving partial differential equations. In particular, this method shows that in the anomalous regime, high-power pulses with temporal super-Gaussian profiles will split in time into two shorter pulses, and spherically symmetric pulses with super-Gaussian spatiotemporal profiles will evolve into a spatiotemporal shell.

A scheme for backward retrieval in optical quantum memories in which information is stored in collective states of an extended resonant atomic ensemble is developed such that phase conjugation can be implemented by application of an external nonuniform electric (magnetic) field without use of coherent exciting pulses. The possibilities of realizing such a scheme using resonant solid-state materials are discussed.

Spectral linewidth broadening in Rydberg gases, a phenomenon previously attributed to the many-body effect, was observed experimentally almost a decade ago. The observed linewidth was typically 80–100 times larger than the average interaction strength predicted from a binary interaction. The interpretation of such a phenomenon is usually based on the so-called diffusion model, where the linewidth broadening mostly originates from the diffusion of excitations. We present a model calculation to show that diffusion is not the main mechanism of the linewidth broadening. We find that the rare pair fluctuation at small separation is the dominant factor contributing to this broadening. Our results give a width of about 20–30 times larger than the average interaction strength. More importantly, by turning off the diffusion process, we do not observe an order-of-magnitude change in the spectral linewidth.

We present a universal protocol for continuous-variable graph-state teleportation. A mathematical model is established to determine an arbitrary graph state’s properties, including the possibility of teleportation and the relevant analytical criteria. By investigating the properties of several fundamental graph-state types, we identify a general strategy for optimizing excess noise due to finite squeezing. As an application, a general scheme for teleportation between two arbitrary parties on a two-dimensional square lattice cluster state is given, along with a cheaper yet equivalent optimization for fixed source-target teleportation. Finally, a universal way to distill bipartite entanglement from a graph-state entanglement network is presented based on the equivalence between teleportation and entanglement distillation.

Recently, we presented a classical-over-barrier ionization (COBI) model [J. X. Shao et al., Phys. Rev. A 77, 042711 (2008)] to calculate the pure single- and double-ionization cross sections of He. We extend the COBI model to calculate the simultaneous ionization of the projectile and target in the energy range of $20–500\mathrm{keV}∕\mathrm{amu}$. The results of the model are in excellent agreement with the experimental data. The mechanism of the simultaneous ionization is also discussed.

Erbium-doped solids are potential candidates for the realization of a quantum memory for photons at telecommunication wavelengths. The implementation of quantum memory proposals in rare-earth-ion-doped solids require spectral tailoring of the inhomogeneous absorption profile by efficient population transfer between ground-state levels (spin polarization) using optical pumping. In this paper we investigate the limiting factors of efficient optical pumping between ground-state Zeeman levels in an erbium-doped ${\mathrm{Y}}_2\mathrm{Si}{\mathrm{O}}_5$ crystal. We introduce two methods to overcome these limiting factors: stimulated emission using a second laser and spin mixing using radio frequency excitation. Both methods significantly improve the degree of spin polarization. Population transfer between two Zeeman levels with less than 10% of the total population in the initial ground state is achieved, corresponding to a spin polarization greater than 90%. In addition, we demonstrate spectral tailoring by isolating a narrow absorption peak within a large transparency window.