We examine the creation and annihilation dynamics for electron-positron pairs in a time-dependent but subcritical electric force using a simplified model system. Numerical and semianalytical solutions to computational quantum field theory show that despite the continuity of the quantum field operator in time, the actual number of created particles can change in a discontinuous way if the field changes abruptly. The number of permanently created particles after the pulse, however, increases continuously with the duration of the electric field pulse, suggesting a transition from an exclusive annihilation to a creation regime.

We show how radiation emitted from a collection of noninteracting two-level particles, which can be treated as qubits, can be used to detect a wide class of entangled states of this system. This result is an example of a macroscopical manifestation of entanglement that can be inferred from direct measurements. Some examples of physical effects directly related to the collective emission of radiation, such as photon echoes and superradiance, are discussed, suggesting some ways of how to experimentally witness entanglement with simple and standard techniques.

We theoretically study measurement-induced dephasing of a superconducting qubit in the circuit QED architecture and compare the results to those obtained experimentally by Schuster et al. [Phys. Rev. Lett. 94, 123602 (2005)]. Strong coupling of the qubit to the resonator leads to a significant ac Stark shift of the qubit transition frequency. As a result, quantum fluctuations in the photon number populating the resonator cause dephasing of the qubit. We find good agreement between the predicted line shape of the qubit spectrum and the experimental results. Furthermore, in the strongly dispersive limit, where the Stark shift per photon is large compared to the cavity decay rate and the qubit linewidth, we predict that the qubit spectrum will be split into multiple peaks, with each peak corresponding to a different number of photons in the cavity.

The electron affinity of cerium is determined from relativistic configuration interaction photodetachment calculations through a reinterpretation of available experimental data [V. T. Davis and J. S. Thompson, Phys. Rev. Lett. 88, 073003 (2002)]. This analysis yields a $4f5d^{2}6s^2{}^{4}H_{7∕2}$ ${\mathrm{Ce}}^{-}$ ground state (a $5d$ attachment relative to the $4f5d6s^2{}^{1}G_4$ neutral Ce ground state) with binding energy of $0.660\mathrm{eV}$ as well as 18 other bound states of the same configuration. The lowest-lying state of seven bound states of the opposite parity ($6p$ attachments) is $4f5d6s^{2}6p{}^{2}H_{9∕2}$, which is bound by $0.300\mathrm{eV}$.

We report the results of fixed-nuclei complex Kohn variational calculations of elastic electron scattering by formic acid, HCOOH. Momentum transfer and angular differential cross sections for incident electron energies ranging from $0.1\text{to}15\mathrm{eV}$ are presented and compared to available experimental data. The low-energy behavior of the cross section is analyzed and found to be consistent with the existence of a virtual state.

We perform a detailed study of the collective mode across the whole crossover from the Bose-Einstein condensate (BEC)-to the BCS regime in fermionic gases at zero temperature, covering the whole range of energy beyond the linear regime. This is done on the basis of the dynamical BCS model. We recover first the results of the linear regime in a simple form. Then specific attention is paid to the nonlinear part of the dispersion relation and its interplay with the continuum of single-fermionic excitations. In particular we consider in detail the merging of the collective mode into the continuum of single-fermionic excitations. This occurs not only on the BCS side of the crossover, but also slightly beyond unitarity on the BEC side. Another remarkable feature is the very linear behavior of the dispersion relation in the vicinity of unitarity almost up to merging with the continuum. Finally, while on the BEC side the mode is quite analogous to the Bogoliubov mode, a difference appears at high wave vectors. On the basis of our results we determine the Landau critical velocity in the BEC-BCS crossover which is found to be largest close to unitarity. Our investigation has revealed interesting qualitative features which deserve experimental exploration as well as further theoretical studies by more sophisticated means.

Stark maps and the distribution of oscillator strengths within a Stark manifold in the ${\mathrm{H}}_3$ molecule have been calculated by diagonalization of the Hamiltonian matrix using a molecular quantum defect orbital (MQDO) basis set. Our perturbative treatment, in which only experimental quantum defects have been used and no fitting parameters are required, explicitly accounts for the coupling between the rotation of the molecular core and the Rydberg-electron motion. A pure Hund’s case (d) has been adopted for all the Rydberg states except for the $np$ series, for which Hund’s case (b) is considered to be more realistic. A mixing of some of the $np$ Stark states with those belonging to other Rydberg series appears to occur. The presently achieved results conform very well with other reliable theoretical values, in the absence of experimental data. The adequacy of the MQDO procedure for accurately dealing with properties related to the Stark effect in molecules is suggested.

Using the Green’s function approach we investigate separability of the vacuum state of a massless scalar field with a single Dirichlet boundary. Separability is demonstrated using the positive partial transpose criterion for effective two-mode Gaussian states of collective operators. In contrast to the vacuum energy, entanglement of the vacuum is not modified by the presence of the boundary.

We give a theoretical analysis of the experimental scheme for creating macroscopic superpositions of different superfluid flow states with Bose-Einstein condensates trapped in an optical lattice [J. Dunningham et al., Phys. Rev. A 74, 023601 (2006)]. It is shown that a class of macroscopic superposition states can be created for an arbitrary number of particles by dynamical evolution with a properly prepared initial state. When the number of particles is a multiple of 3, such states are exactly reduced to macroscopic superpositions of different superfluid flow states.

Quantum optical coherence tomography (Q-OCT) offers a factor-of-2 improvement in axial resolution and the advantage of even-order dispersion cancellation when it is compared to conventional OCT (C-OCT). These features have been ascribed to the nonclassical nature of the biphoton state employed in the former, as opposed to the classical state used in the latter. Phase-conjugate OCT (PC-OCT) shows that nonclassical light is not necessary to reap Q-OCT’s advantages. PC-OCT uses classical-state signal and reference beams, which have a phase-sensitive cross correlation, together with phase conjugation to achieve the axial resolution and even-order dispersion cancellation of Q-OCT with a signal-to-noise ratio that can be comparable to that of C-OCT.

We consider interacting Fermi systems close to the unitary regime and compute the corrections to the energy density that are due to a large scattering length and a small effective range. Our approach exploits the universality of the density functional and determines the corrections from the analyical results for the harmonically trapped two-body system. The corrections due to the finite scattering length compare well with the result of Monte Carlo simulations. We also apply our results to symmetric neutron matter.

We observed continuous-variable entanglement between the bright beams emitted above threshold by an ultrastable optical parametric oscillator (OPO), classically phase locked at a frequency difference of $161.8273240\left(5\right)\mathrm{MHz}$. The amplitude-difference squeezing is $-3\mathrm{dB}$ and the phase-sum one is $-1.35\mathrm{dB}$. Besides proving entanglement in a phase-locked OPO, such outstanding frequency-difference stability paves the way for transferring entanglement between different optical frequencies and densely implementing continuous-variable quantum information in the frequency domain.

By modeling a linear polarizable and magnetizable medium (magnetodielectric) with two quantum fields, namely $E$ and $M$, the electromagnetic field is quantized in such a medium consistently and systematically. A Hamiltonian is proposed from which, using the Heisenberg equations, Maxwell and constitutive equations of the medium are obtained. For a homogeneous medium, the equation of motion of the quantum vector potential, $\vec{A}$, is derived and solved analytically. Two coupling functions which describe the electromagnetic properties of the medium are introduced. Four examples are considered showing the features and the applicability of the model to both absorptive and nonabsorptive magnetodielectrics.

We propose a Procrustean entanglement concentration scheme for continuous-variable states inspired by the scheme proposed in J. Fiuràšek et al. [Phys. Rev. A 67, 022304 (2003)]. We show that the eight-port homodyne measurement of J. Fiuràšek et al. [Phys. Rev. A 67, 022304 (2003)] can be replaced by a balanced homodyne measurement with the advantage of providing a success criterion that allows Alice and Bob to determine if entanglement concentration was achieved. In addition, it facilitates a straightforward experimental implementation.

We analyze a scheme to manipulate quantum states of neutral atoms at individual sites of optical lattices using focused laser beams. Spatial distributions of focused laser intensities induce position-dependent energy shifts of hyperfine states, which, combined with microwave radiation, allow selective manipulation of quantum states of individual target atoms. We show that various errors in the manipulation process are suppressed below ${10}^{-4}$ with properly chosen microwave pulse sequences and laser parameters. A similar idea is also applied to measure quantum states of single atoms in optical lattices.

Two nonidentical atoms may be entangled by inducing simultaneous cooperative absorption of a photon pair from a light field that is nonresonant to either atom in isolation. The atoms may be of two different elements, of different isotopes of the same element, or of a single isotope undergoing different transitions. Utilizing this cooperative process, we propose a protocol for a conditional operation with neutral atoms in an optical lattice. Current practical capabilities permit execution of the scheme.

A dimensional-scaling method for the calculation of excited states of multielectron atoms is introduced. By including the principle and orbital quantum numbers in the dimension parameter, we obtain an energy expression for excited states including high angular momentum states. The method is tested on He, Li, and Be. We obtain good agreement with more orthodox quantum mechanical treatments even in the zeroth order.

A vibrational adiabatic approach to Franck-Condon analysis is presented for systems with a few highly displaced oscillators coupled to a bath of harmonic oscillators. The model reduces the many-coupled-oscillator problem to few-body problems, albeit with corrections due to coupling to harmonic modes. The theory is applied with very good results to the carbon $1s$ photoelectron spectrum of ethanol, which is strongly influenced by change in conformation from gauche to anti when ethanol is ionized at the methyl site.