We report measurements of the transition frequencies $4s{}^{2}S_{1∕2}-4p{}^{2}P_{1∕2}$ and $4s{}^{2}S_{1∕2}-4p{}^{2}P_{3∕2}$ of the potassium isotopes 39, 40, and 41 through an atomic beam experiment with a fractional uncertainty of about $2\times {10}^{-10}$. For frequency calibration, a fs-laser comb referenced to a Cs atomic clock was used. Compared to previous results, hyperfine constants for the states $4p{}^{2}P_{1∕2}$ and $4p{}^{2}P_{3∕2}$ and isotope shifts are given with a considerably reduced uncertainty. This paper also resolves the discrepancy of transition frequencies measured by Banerjee et al. [Phys. Rev. A 70, 052505 (2004)] and Scherf et al. [Z. Phys. D 36, 31 (1996)] and the hyperfine constant $A({}^{39}\mathrm{K},{}^{2}P_{1∕2})$ reported by Banerjee et al. [Europhys. Lett. 65, 172 (2004)] and Bendali et al. [J. Phys. B 14, 4231 (1981)].

We construct a time-dependent Ginzburg-Landau (TDGL) theory for the superfluid atomic Fermi-gases near the Feshbach resonance from the fermion-boson model on the basis of the functional integral formalism. We show that the GL coefficients in the TDGL equation are complex numbers except in the BEC limit. The complex TDGL equation describes both damping and propagating dynamics, which leads to very rich superfluid dynamics near the Feshbach resonance. We predict multiple plane-wave modes corresponding to the in-phase and out-of-phase like oscillations between fermionic and bosonic superfluids and a galaxylike spiral pattern for a vortex.

We report on the experimental realization of large fractional pulse delays in a hot, Doppler-broadened rubidium vapor. A pump laser burns a deep spectral hole in the inhomogeneously broadened vapor. The delay is shown to be widely tunable by both power broadening the resonance and frequency modulating the pump laser. The simplicity of the scheme opens up the possibility for practical optical delays and buffers.

We investigate the behavior of $N$ atoms resonantly coupled to a single electromagnetic field mode sustained by a high quality cavity, containing a mesoscopic coherent field. We show with a simple effective Hamiltonian model that the strong coupling between the cavity and the atoms produces an atom-field entangled state, involving $N+1$ nearly coherent components slowly rotating at different paces in the phase plane. The periodic overlap of these components results in a complex collapse and revival pattern for the Rabi oscillation. We study the influence of decoherence due to the finite cavity quality factor. We propose a simple analytical model, based on the Monte Carlo approach to relaxation. We compare its predictions with exact calculations and show that these interesting effects could realistically be observed on a two or three atoms sample in a 15 photon field with circular Rydberg atoms and superconducting cavities.

The reported observation of two-photon ionization of He by $13\mathrm{eV}$ photons of the TESLA Facility in T. Laarmann, A. R. B. de Castro, P. Gürtler, W. Laasch, J. Schulz, H. Wabnitz, and T. Möller, Phys. Rev. A72, 023409 (2005) is ambiguous, as it is not supported by the experimental data presented. We discuss the lack of evidence for the claimed two-photon ionization.

We do not agree with the conclusion of the Comment by Charalambidis et al. questioning our observation of two-photon ionization of helium by intense radiation with $13\mathrm{eV}$ photons from a vuv free-electron laser. Two-photon ionization is clearly established by the detection of low-energy photoelectrons at $\sim 1.7\mathrm{eV}$, which agrees very well with the expected energy for a two-photon ionization process.

We theoretically investigate the production of Rabi-type oscillations in two-photon resonant ionization of ${\mathrm{H}}_2$ by intense ultrashort vuv pulses. We show that, with such pulses, oscillations proceed through a step-ladder mechanism that successively involves higher and higher vibrational levels of the lower and upper electronic states. The consequences of this mechanism are (i) the relatively large proportion of ${\mathrm{H}}_2$ molecules in excited electronic states not accessible by direct photon absorption, (ii) the splitting in the energy distribution of ionized electrons, and (iii) the unusually broad (non-Franck-Condon) vibrational distribution of both ${\mathrm{H}}_2$ and $\mathrm{H}_2{}^{+}$.

The dynamic Casimir effect, which concerns two photon radiation processes due to time dependent frequency modulations, is computed in the one photon loop approximation. An instability is signaled by the production of an unphysically large number of photons. We show how it is tamed and a saturation in the number of photons reached through higher order processes. Explicit results are obtained for a recently proposed experiment.

We present a theoretical study of the spectra produced by optical–radio-frequency double resonance devices, in which resonant linearly polarized light is used in the optical pumping and detection processes. We extend previous work by presenting algebraic results which are valid for atomic states with arbitrary angular momenta, arbitrary rf intensities, and arbitrary geometries. The only restriction made is the assumption of low light intensity. The results are discussed in view of their use in optical magnetometers.

We consider electron-positron pair creation by the impact of very powerful laser pulses with highly charged ions. In contrast to our foregoing work with rather limited angular configurations of pair creation, we extend these calculations to even higher laser intensities, and we use the Monte Carlo method to numerically analyze the rates of pair creation for arbitrary angular distributions. We also evaluate the intensity dependence of the total rates of pair creation. Thus we demonstrate that our laser-induced process shows stabilization, because beyond a specific laser power the total rates of pair creation decreases. Our analysis of the angular distributions of the created electron-positron pairs leads to the conclusion that pairs are predominantly emitted in the direction of laser pulse propagation.

For two-component Fermi gases at zero temperature, through a general derivation based on the BCS wave function, our research shows that the nonuniform molecular Bose-Einstein condensate has off-diagonal long-range order and can be described well by an order parameter when the size of molecules is much smaller than the size of the whole system. We also give the equation of state and nonlinear evolution equation for the order parameter of nonuniform molecular condensates, which is similar to the Gross-Pitaevskii equation of atomic Bose condensate. The nonlinear evolution equation is applied to consider the Josephson effect for two weakly linked molecular condensates in the presence of nonuniform magnetic field. We find clear particle-number oscillation where spatially dependent binding energy of molecules plays an important role.

We present a solution to the multichannel quantum control problem, where selective and complete population transfer from an initial bound state to $M$ energetically degenerate continuum channels is achieved under loss-free conditions. The control is affected by adiabatic passage proceeding via $N$ bound intermediate states, where even in the presence of real loss from these states, the control efficiency remains significant, about $40–50\%$. We illustrate the viability of the method by computationally controlling the ${\mathrm{CH}}_3\left(v\right)+{\mathrm{I}}^{*}\left({}^{2}P_{1∕2}\right)\leftarrow {\mathrm{CH}}_3\mathrm{I}\to {\mathrm{CH}}_3\left(v\right)+\mathrm{I}\left({}^{2}P_{3∕2}\right)$ multichannel photodissociation process.

We analyze in detail the proposal for a two-qubit gate for travelling single-photon qubits recently presented by Ottaviani et al. [Phys. Rev. A 73, 010301(R) (2006)]. The scheme is based on an ensemble of five-level atoms coupled to two quantum and two classical light fields. The two quantum fields undergo cross-phase modulation induced by electromagnetically induced transparency. The performance of this two-qubit quantum phase gate for travelling single-photon qubits is thoroughly examined in the steady-state and transient regimes, by means of a full quantum treatment of the system dynamics. In the steady-state regime, we find a general trade-off between the size of the conditional phase shift and the fidelity of the gate operation. However, this trade-off can be bypassed in the transient regime, where a satisfactory gate operation is found to be possible, significantly reducing the gate operation time.

We propose efficient-phase-encoding protocols for continuous-variable quantum key distribution using coherent states and postselection. By these phase encodings, the probability of basis mismatch is reduced and total efficiency is increased. We also propose mixed-state protocols by omitting a part of classical communication steps in the efficient-phase-encoding protocols. The omission implies a reduction of information to an eavesdropper and possibly enhances the security of the protocols. We investigate the security of the protocols against individual beam splitting attack.

Molecular two-electron reduced density matrices (2-RDMs) are computed variationally without the many-electron wave function by constraining the 2-RDM with a set of three-particle $N$-representability conditions known as three-positivity conditions. These constraints restrict four distinct three-particle probability distributions, which can be defined for any $N$-particle system, to be nonnegative. The variational calculation of the 2-RDM with full three-positivity conditions is implemented with the first-order semidefinite programming algorithm [D.A. Mazziotti, Phys. Rev. Lett. 93, 213001 (2004)]. We also derive and implement a generalization of the $T_2$ representability condition, which is a subset of the three-positivity conditions. Energies and 2-RDMs are computed for several molecules as well as the nitrogen molecule at both equilibrium and nonequilibrium geometries. The ground-state energies for nitrogen are within $0.0015\mathrm{a.u.}$ of the values from full configuration interaction at all internuclear distances.

We search for the existence of the weakly bound He-He-K molecules where the binding energies are expected to be of the order of degrees Kelvin. By employing the best empirical interaction between each pair of particles, the Schrödinger equation for the triatomic systems is solved using hyperspherical coordinates in the adiabatic approximation. A bound state is found for each of the ${}^4\mathrm{He}\text{−}{}^4\mathrm{He}\text{−}{}^{39}\mathrm{K}$, ${}^4\mathrm{He}\text{−}{}^3\mathrm{He}\text{−}{}^{39}\mathrm{K}$, and ${}^3\mathrm{He}\text{−}{}^3\mathrm{He}\text{−}{}^{39}\mathrm{K}$ trimers, respectively. The bound state for the ${}^3\mathrm{He}\text{−}{}^3\mathrm{He}\text{−}{}^{39}\mathrm{K}$ molecule is the halo state found in realistic molecular systems for there are no bound states in both the ${}^3\mathrm{He}\text{−}{}^3\mathrm{He}$ and ${}^3\mathrm{He}\text{−}{}^{39}\mathrm{K}$ dimers.

The multiple ionization and fragmentation of ${\mathrm{N}}_2$ induced by ion impact is studied using time- and position-sensitive detectors in multihit coincidence mode. With ${\mathrm{Ar}}^{9+}$ at a projectile velocity of $1\mathrm{a.u.}$, we observe a total of seven fragmentation channels originating from multiply charged transient molecular ions. The preference of symmetric charge breakup channels over the asymmetric ones is clearly observed. A signature of core excitation of the target followed by Auger emission is observed in the ${\mathrm{N}}^{3+}\text{−}{\mathrm{N}}^{3+}$ fragmentation channel. The kinetic energy release spectra of all the observed fragmentation pathways is explained on the basis of calculated ab initio (using the multiconfiguration self-consistent-field method) potential-energy curves.

We present an approach for the analysis of Bose-Einstein condensates in a few mode approximation. This method has already been used to successfully analyze the vibrational modes in various molecular systems and offers a perspective on the dynamics in many particle bosonic systems. We discuss a system consisting of a Bose-Einstein condensate in a triple well potential. Such systems correspond to classical Hamiltonian systems with three degrees of freedom. The semiclassical approach allows a simple visualization of the eigenstates of the quantum system referring to the underlying classical dynamics. From this classification we can read off the dynamical properties of the eigenstates such as particle exchange between the wells and entanglement without further calculations. In addition, this approach offers insights into the validity of the mean-field description of the many particle system by the Gross-Pitaevskii equation, since we make use of exactly this correspondence in our semiclassical analysis. We choose a three mode system in order to visualize it easily and, moreover, to have a sufficiently interesting structure, although the method can also be extended to higher dimensional systems.

The existence of unconditionally secure quantum bit commitment (QBC) is excluded by the Mayers-Lo-Chau no-go theorem. Here we look for the second-best: a QBC protocol that can defeat certain quantum attacks. By breaking the knowledge symmetry between the participants with quantum algorithm, a QBC protocol is proposed and is proven to be secure against a major kind of coherent attacks—the dummy attack, in which the participant makes an empty promise instead of committing to a specific bit. Therefore it surpasses previous QBC protocols which are secure against individual attacks only.