A method is proposed to generate entanglement between two distant atoms and to preserve the entanglement for a long time by using the properties of refocusing and phase compensation of left-handed (negative-index) materials and spontaneous emission. A distance of ten wavelengths in our method compared with less than one wavelength in free space is achieved for the generation of concurrence of $1/2$ between two identical atoms from an initially separable state. The concurrence can be kept for a very long time for ideal left-handed materials, and can be kept for tens of (or several) atomic lifetimes for absorptive left-handed materials.

We present a terrestrial measurement of the Landé $g$ factor of the $5D_{5/2}$ state of singly ionized barium. Measurements were performed on single Doppler-cooled ${}^{138}{\mathrm{Ba}}^{+}$ ions in a linear Paul trap. A frequency-stabilized fiber laser with a nominal wavelength of $1.762$ $\mu$m was scanned across the $6S_{1/2}\leftrightarrow 5D_{5/2}$ transition to spectroscopically resolve transitions between Zeeman sublevels of the ground and excited states. From the relative positions of the four narrow transitions observed at several different values for the applied magnetic field, we find a value of $1.2020\pm 0.0005$ for $g_{5D_{5/2}}$.

We establish a theoretical correspondence between spin-one antiferromagnetic spinor condensates in an external magnetic field and quantum rotor models in an external potential. We show that the rotor model provides a conceptually clear picture of the possible phases and dynamical regimes of the antiferromagnetic condensate. We also show that this mapping simplifies calculations of the condensate’s spectrum and wave functions. We use the rotor mapping to describe the different dynamical regimes recently observed in ${}^{23}\mathrm{Na}$ condensates [Phys. Rev. Lett. 102, 125301 (2009); Phys. Rev. Lett. 102, 225301 (2009)]. We also suggest a way to experimentally observe quantum-mechanical effects (collapse and revival) in spinor condensates.

We introduce a random-variable approach to investigate the dynamics of a dissipative two-state system. Based on an exact functional integral description, our method reformulates the problem as that of the time evolution of a quantum state vector subject to a Hamiltonian containing random noise fields. This numerically exact, nonperturbative formalism is particularly well suited in the context of time-dependent Hamiltonians, at both zero and finite temperature. As an important example, we consider the renowned Landau-Zener problem in the presence of an Ohmic environment with a large cutoff frequency at finite temperature. We investigate the “scaling” limit of the problem at intermediate times, where the decay of the upper-spin-state population is universal. Such a dissipative situation may be implemented using a cold-atom bosonic setup.

We show that the quantum states generated by universal optimal quantum cloning of a single photon represent a universal set of quantum superpositions resilient to decoherence. We adopt the Bures distance as a tool to investigate the persistence of quantum coherence of these quantum states. According to this analysis, the process of universal cloning realizes a class of quantum superpositions that exhibits a covariance property in lossy configuration over the complete set of polarization states in the Bloch sphere.

I explore the entanglement dynamics of a three-qubit system in an initial $X$ state undergoing decoherence including the possible exhibition of entanglement sudden death. To quantify entanglement I utilize negativity measures and make use of appropriate entanglement witnesses. The negativity results are then extended to $X$ states with an arbitraty number of qubits. I also demonstrate nonstandard behavior of the tripartite negativity entanglement metric: its sudden appearance after some amount of decoherence, followed quickly by its disappearance. Finally, I solve for a lower bound on the three-qubit $X$-state concurrence, demonstrate when this bound goes to $0$, and outline simplifcations for the calculation of higher-order $X$-state concurrences.

We analyze the possibility of measuring small thermal effects in the Casimir force between metal test bodies in configurations of a sphere above a plate and two parallel plates. For the sphere-plate geometry used in many experiments, we investigate the applicability of the proximity force approximation (PFA) to calculation of thermal effects in the Casimir force and its gradient. It is shown that for real metals the two formulations of the PFA used in the literature lead to relative differences in the results obtained being less than a small parameter equal to the ratio of separation distance to sphere radius. For ideal metals, PFA results for the thermal correction are obtained and compared with available exact results. It is emphasized that in the experimental region in the zeroth order of the small parameter already mentioned, the thermal Casimir force and its gradient calculated using the PFA (and thermal corrections in their own right) coincide with the respective exact results. For real metals, available exact results are outside the application region of the PFA. However, the exact results are shown to converge with the PFA results when the small parameter goes down to experimental values. We arrive at the conclusion that the large thermal effects predicted by the Drude-model approach, if they exist, could be measured in both static and dynamic experiments in sphere-plate and plate-plate configurations. As for the small thermal effects predicted by the plasma-model approach, the static experiment in the configuration of two parallel plates is found to be the best for their observation.

Absolutely normalized ($e,2e$) measurements for ${\mathrm{H}}_2$ and He covering the full solid angle of one ejected electron are presented for 16 eV sum energy of both final state continuum electrons. For both targets rich cross-section structures in addition to the binary and recoil lobes are identified and studied as a function of the fixed electron’s emission angle and the energy sharing among both electrons. For ${\mathrm{H}}_2$ their behavior is consistent with multiple scattering of the projectile as discussed before [Al-Hagan et al., Nature Phys. 5, 59 (2009)]. For He the binary and recoil lobes are significantly larger than for ${\mathrm{H}}_2$ and partly cover the multiple scattering structures. To highlight these patterns we propose a alternative representation of the triply differential cross section. Nonperturbative calculations are in good agreement with the He results and show discrepancies for ${\mathrm{H}}_2$ in the recoil peak region. For ${\mathrm{H}}_2$ a perturbative approach reasonably reproduces the cross-section shape but deviates in absolute magnitude.

We investigated by electron spectroscopy the strong-field multiphoton ionization of ${\text{O}}_2$ molecules with ultrashort laser pulses in the intensity range between the multiphoton and tunneling regimes. The ionization proceeds by at least three different mechanisms, in addition to the eight- and nine-photon nonresonant pathways. Transient multiphoton resonances with vibrational Rydberg levels give rise to direct Freeman-type peaks with sublaser linewidth and spin-orbit splitting. Some resonance levels actually become populated and yield extremely narrow lines because of postpulse vibrational autoionization. When the lowest photon order resonance channel for the Rydberg states is closed, a third contribution becomes dominant with a main peak at 0.4 eV that shares its main properties with the recently discovered universal low-energy structure in the electron spectra of atoms and molecules [C. I. Blaga et al., Nat. Phys. 5, 335 (2009); W. Quan et al., Phys. Rev. Lett. 103, 093001 (2009)]. The variation of the Freeman resonance spectrum with the laser peak intensity is well correlated with the vibronic Franck-Condon factors for the overlap of the intermediate Rydberg state with the ${\text{O}}_2$ ground state. Accordingly, the Freeman peaks could be unambiguously assigned to individual vibronic multiphoton resonances, and the disappearance of the Freeman resonances at a certain laser intensity could be explained. The population of the autoionizing Rydberg states could be assigned similarly to such vibronic resonances.

We present an efficient approach for computing high-order harmonic propagation based on the discrete dipole approximation. In contrast with other approaches, our strategy is based on computing the total field as the superposition of the driving field with the field radiated by the elemental emitters of the sample. In this way we avoid the numerical integration of the wave equation, as Maxwell’s equations have an analytical solution for an elementary (pointlike) emitter. The present strategy is valid for low-pressure gases interacting with strong fields near the saturation threshold (i.e., partially ionized), which is a common situation in the experiments of high-order harmonic generation. We use this tool to study the dependence of phase matching of high-order harmonics with the relative position between the beam focus and the gas jet.

Dark solitons of ultracold bosons in the vicinity of the Mott-insulator–superfluid phase transition are studied. Making use of the Gutzwiller ansatz we have found antisymmetric eigenstates corresponding to standing solitons, as well as propagating solitons created by phase imprinting. Near the phase boundary, superfluidity has either a particle or a hole character depending on the system parameters, which greatly affects the characteristics of both types of solitons. Within the insulating Mott regions, soliton solutions are prohibited by lack of phase coherence between the lattice sites. Linear and modulational stability show that the soliton solutions are sensitive to small perturbations and, therefore, unstable. In general, their lifetimes differ for on-site and off-site modes. For the on-site modes, there are small areas between the Mott-insulator regions where the lifetime is very large, and in particular much larger than that for the off-site modes.

We compute the thermal conductivity and sound attenuation length of a dilute atomic Fermi gas in the framework of kinetic theory. Above the critical temperature for superfluidity, $T_c$, the quasiparticles are fermions, whereas below $T_c$, the dominant excitations are phonons. We calculate the thermal conductivity in both cases. We find that at unitarity the thermal conductivity $\kappa$ in the normal phase scales as $\kappa \propto T^{3/2}$. In the superfluid phase we find $\kappa \propto T^2$. At high temperature the Prandtl number, the ratio of the momentum and thermal diffusion constants, is 2/3. The ratio increases as the temperature is lowered. As a consequence we expect sound attenuation in the normal phase just above $T_c$ to be dominated by shear viscosity. We comment on the possibility of extracting the shear viscosity of the dilute Fermi gas at unitarity using measurements of the sound absorption length.

We propose a way to significantly enhance selectivity and efficiency of coherent population transfer in a $\Lambda$-type four-level system with a closely spaced doublet in the final state by using a train of counterintuitively ordered pump-Stokes pulse pairs. Due to temporal constructive and destructive quantum interference between the sequential transitions and subsequent coherent accumulation and annihilation in the time domain, the spectral resolution of selective and perfect population transfer to either of the final states can be improved by more than one order of magnitude as compared to the conventionally widely used stimulated Raman adiabatic passage and chirped adiabatic passage techniques, which can also be well understood as a result of the formation of a comblike structure of the pulse train spectrum in the frequency domain. Moreover, an arbitrary coherent superposition between the closely separated doublet can be achieved by suitably tuning the repetition period and detuning of the laser fields. This method has potential applications in control of chemical reactions and quantum information processing.

By using spontaneous Raman processes in the high-gain regime, we produce two independent Raman Stokes fields from an atomic ensemble. Temporal beating is observed between the two directly generated Stokes fields in a single realization. The beat frequency is found to be a result of an ac Stark frequency shift effect. However, due to the spontaneous nature of the process, the phases of the two Stokes fields change from one realization to another so that the beat signal disappears after averaging over many realizations. On the other hand, the beat signal is recovered in an intensity correlation measurement, showing a two-photon interference effect. The beat signal in intensity correlation enables us to obtain dephasing information in the Raman process. The dephasing effect is found to depend on the temperature of the atomic medium.

We investigate the ionization of ${\mathrm{N}}_2$ and ${\mathrm{CO}}_2$ using the strong-field approximation (SFA) in both the length gauge (LG) and the velocity gauge (VG). Comparisons have been made among the different versions of SFA, the molecular Ammosov-Delone-Krainov (ADK), and recent experiments. The study also shows that the VG SFA displays a larger laser parameter (intensity and wavelength) dependence than the LG SFA, which is not observed in experiments and first principal calculations. The LG SFA is preferable for description of the strong-field ionization of ${\mathrm{N}}_2$ and ${\mathrm{CO}}_2$.

Inconsistencies are pointed out in a recent proposal [L. Diósi, Phys. Rev. A 80, 064104 (2009)] for a quantum version of the classical linear Boltzmann equation.

Hornberger and Vacchini [Phys. Rev. A 82, 036101 (2010)] claim that the specific collisional momentum decoherence, pointed out in my recent work [Phys. Rev. A 80, 064104 (2009)], is already described by their theory. However, I have performed a calculation whereby I disprove the authors’ claim and refute their conclusion that my recent work had no advantage over theirs.

The quantum fluctuation effects in the time-of-flight (TOF) experiment for a condensate released from an optical-lattice potential is studied within the truncated Wigner approximation. By investigating both the spatial and momentum density distributions, we find that the condensate fraction decreases monotonically in time and hence cannot be measured in the standard TOF image. We then propose a semiquantitative analysis for such dynamical quantum-depletion process. Our study shows a universal algebraic decay of the true condensate fraction, and has a very good agreement with numerical results. We also discuss possible methods to determine the condensate fraction inside the optical lattice, and its implication to the TOF experiments in higher dimensional systems.

Quantum decoherence, which appears when a system interacts with its environment in an irreversible way, plays a fundamental role in the description of quantum-to-classical transitions and has been successfully applied in some important experiments. Here, we study the decoherence in noninertial frames. It is shown that the decoherence and loss of the entanglement generated by the Unruh effect will influence each other remarkably. It is interesting to note that, in the case of the total system under decoherence, the sudden death of entanglement may appear for any acceleration. However, in the case of only Rob’s qubit undergoing decoherence, sudden death may only occur when the acceleration parameter is greater than a “critical point.”

We discuss the limitations for the applicability of the Lifshitz theory to describe the temperature dependence of the Casimir force between bulk lossy metal slabs of finite sizes. We pay attention to the important fact that Lifshitz’s theory is not applicable when the characteristic wavelength of the fluctuating field, responsible for the temperature-dependent terms in the Casimir force, are longer than the size of the sample. As a result, the widely discussed linearly decreasing temperature dependence of the Casimir force can be observed only for dirty and large metal samples at high enough temperatures. Moreover, for the correct description of the Casimir effect at low enough temperatures, a careful consideration of the concrete geometry of the interacting samples is essential.