The efficiency of converting two-species fermionic atoms into bosonic molecules is investigated in terms of mean-field Lagrangian density. We find that the stimulated Raman adiabatic passage technique aided by Feshbach resonance is more effective than the bare Feshbach resonance for ${}^6\text{L}\text{i}$ atoms rather than ${}^{40}\text{K}$ atoms. We also generally consider the symmetry and its relevant conservation law, which enables us to introduce a natural definition of adiabatic fidelity for a coherent population trapping state. The calculated values of the fidelity are presented and then we provide an explanation of why the conversion efficiencies for ${}^{40}\text{K}$ and ${}^6\text{L}\text{i}$ are distinctly different.

Three-body nonadditive interactions and conical intersections in the lowest ${\mathrm{Li}}_{2}A$ ($A=\mathrm{Na}$, K, Rb, Cs) quartet states are analyzed using high-level ab initio calculations. Near equilibrium geometries large nonadditive effects are stabilizing the systems. Low-lying seams of conical intersection are present at the linear $\mathrm{Li}\text{−}\mathrm{Li}\text{−}A$ arrangement. Qualitatively the ${\mathrm{Li}}_{2}A$ systems are very similar to ${\mathrm{Li}}_3$, although quantitatively the magnitude of these effects is bigger in ${\mathrm{Li}}_3$.

With the present Comment, we would like to clarify several aspects and some claims recently stated by Tumulka, Viale, and Zanghì [Phys. Rev. A 75, 055602 (2007)] in connection with a work published by Sanz, Borondo, and Bastiaans [Phys. Rev. A 71, 042103 (2005)].

We reply to the comment by Sanz and Borondo [Phys. Rev. A 77, 057601 (2008)] on our paper [Tumulka et al., Phys. Rev. A 75, 055602 (2007)] concerning the double-slit diffraction experiment with neutrons published in Zeilinger et al. [Rev. Mod. Phys. 60, 1067 (1988)]. We argue in particular that Sanz and Borondo’s new arguments for the presence of significant decoherence in that experiment are unconvincing.

In this paper, we place bounds on when it is impossible to purify a noisy two-qubit state if all the gates used in the purification protocol are subject to adversarial, local, independent noise. It is found that the gate operations must be subject to less than 5.3% error. An existing proof that purification is equivalent to error correction is used to show that this bound can also be applied to concatenated error correcting codes in the presence of noisy gates, and hence gives a limit to the tolerable error rate for a fault-tolerant quantum computer formed by concatenation. This is shown to apply also to the case where error detection and postselection, as proposed by Knill, is used to enhance the threshold. We demonstrate the trade-off between gate and environmentally induced faulty rotations and qubit loss errors.

We present a quantum algorithm to estimate parameters at the quantum metrology limit using deterministic quantum computation with one bit. When the interactions occurring in a quantum system are described by a Hamiltonian $H=\theta H_0$, we estimate $\theta$ by zooming in on previous estimations and by implementing an adaptive Bayesian procedure. The final result of the algorithm is an updated estimation of $\theta$ whose variance has been decreased in proportion to the time of evolution under $H$. For the problem of estimating several parameters, we implement dynamical-decoupling techniques and use the results of single parameter estimation. The cases of discrete-time evolution and reference frame alignment are also studied within the adaptive approach.

We investigate limitations imposed by detector dead times on the performance of sequential attacks against a differential-phase-shift (DPS) quantum key distribution (QKD) protocol with weak coherent pulses. In particular, we analyze sequential attacks based on unambiguous state discrimination of the signal states emitted by the source and we obtain ultimate upper bounds on the maximal distance achievable by a DPS QKD scheme both for the case of calibrated and uncalibrated devices, respectively.

We examine the entanglement of cyclic spin-1/2 chains with anisotropic $XYZ$ Heisenberg couplings of arbitrary range at transverse factorizing magnetic fields. At these fields the system exhibits a degenerate symmetry-breaking separable ground state (GS). It is shown, however, that the side limits of the GS pairwise entanglement at these fields are actually nonzero in finite chains, corresponding such fields to a GS spin-parity transition. These limits exhibit universal properties like being independent of the pair separation and interaction range, and are directly related to the magnetization jump. Illustrative exact results are shown for chains with (I) full range and (II) nearest-neighbor couplings. Global entanglement properties at such points are also discussed.

We introduce a class of quantum channels with correlations acting on pairs of qubits, where the correlation takes the form of a shift operator onto a maximally entangled state. We optimize the output purity and show that below a certain threshold the optimum is achieved by partially entangled states whose degree of entanglement increases monotonically with the correlation parameter. Above this threshold, the optimum is achieved by the maximally entangled state characterizing the shift. Although, a full analysis can only be done for the 2-norm, both numerical and heuristic arguments indicate that this behavior and the optimal inputs are independent of $p>1$ when the optimal output purity is measured using the $p$-norm.

We provide a simple model to study memory effects in a lossy bosonic quantum channel over arbitrary number of uses. The noise correlation among different uses is introduced by contiguous modes interactions which results in an exponential decay of the correlations over channel uses (modes). This model allows us to characterize the asymptotic behavior of the channel for classical information transmission.

The hyperfine structure of the long-lived $5D_{3/2}$ and $5D_{5/2}$ levels of ${\text{Ba}}^{+}$ ion is analyzed. A procedure for extracting relatively unexplored nuclear magnetic moments $\Omega$ is presented. The relevant electronic matrix elements are computed in the framework of the ab initio relativistic many-body perturbation theory. Both the first- and the second-order (in the hyperfine interaction) corrections to the energy levels are analyzed. It is shown that a simultaneous measurement of the hyperfine structure of the entire $5D_J$ fine-structure manifold allows one to extract $\Omega$ without contamination from the second-order corrections. Measurements to the required accuracy should be possible with a single trapped barium ion using sensitive techniques already demonstrated in ${\text{Ba}}^{+}$ experiments.

Elastic and inelastic properties of weakly bound $s$- and $p$-wave molecules of fermionic atoms that collide with a third atom are investigated. Analysis of calculated collisional properties of $s$-wave dimers of fermions in different spin states permit us to compare and highlight the physical mechanisms that determine the stability of $s$-wave and $p$-wave molecules. In contrast to $s$-wave molecules, the collisional properties of $p$-wave molecules are found to be largely insensitive to variations of the $p$-wave scattering length, and these collisions will usually result in short molecular lifetimes. We also discuss the importance of this result for both theories and experiments involving degenerate Fermi gases.

We report fully differential cross sections for simultaneous double ionization of atomic beryllium by fast electron impact. We analyze separately the two ionization channels that leave the dication in either the $2s^2$ autoionizing excited state or its $1s^2$ bound ground state. For the double continuum, owing to the two slow emitted electrons, we employ the three-pairwise-Coulomb-interaction model (3C) along with the first Born approximation. Aiming to probe the hitherto largely unexplored role of the electron correlation in $\left(e,3e\right)$, we determine and check fully correlated, compact analytical wave functions, satisfying all two-particle Kato cusp conditions for the four-electron Be initial state and the two-electron ${\text{Be}}^{2+}$ final states, as well as other, much or little or noncorrelated, functions, issuing from either analytic global optimization or Hartree-Fock theory. We point out similarities or contrasts with trends observed in the double photoionization, allowing for parallels between $\left(e,3e\right)$ and $\left(\gamma ,2e\right)$. Various numerical examples serve to navigate possible future experiments in the nonrelativistic regime of the ionization problem.

Small gold cluster anions ${{\mathrm{Au}}_n}^{-}$ are known for their unusual two-dimensional (2D) structures, giving rise to properties very different from those of bulk gold. Previous experiments and calculations disagree about the number of gold atoms $n_c$ where the transition to 3D structures occurs. We combine trapped ion electron diffraction and state of the art electronic structure calculations to resolve this puzzle and establish $n_c=12$. It is shown that theoretical studies using traditional generalized gradient functionals are heavily biased towards 2D structures. For a correct prediction of the 2D-3D crossover point it is crucial to use density functionals yielding accurate jellium surface energies, such as the Tao-Perdew-Staroverov-Scuseria (TPSS) functional or the Perdew-Burke-Ernzerhof functional modified for solids (PBEsol). Further, spin-orbit effects have to be included, and large, flexible basis sets employed. This combined theoretical-experimental approach is promising for larger gold and other metal clusters.

We followed fast evolution of coherent nuclear wave packets in ${\text{H}}_2$ and ${\text{D}}_2$ molecules after their interaction with 8-fs 800-nm laser pulses. The molecules were probed by another few-cycle pulse time delayed for up to 10 ps with respect to the pump. For neutral molecules we observed coherent rotational dynamics characterized by periodic revivals without noticeable decoherence within the 10 ps time scale. For heavy hydrogen up to four rotational states were involved in the wave packets for each of the two spin isomers. In light hydrogen the resulting dynamics was dominated by beating of just two rotational states. For neutral molecules the experimental results are in excellent agreement with our numerical simulations obtained by solving the time-dependent Schrödinger equation. By measuring time-dependent yields for singly ionized rotating ${\text{D}}_2$ molecules, we conclude that for an 8-fs $3\times {10}^{14}\text{W}/{\text{cm}}^2$ pulse the ionization probability is nearly independent of the angle between the molecular axis and the electric field. For those molecules that were ionized by the pump pulse we observed both vibrational and rotational dynamics. In molecular ions coherent vibrational wave packets evolving on the bound ${\sigma }_g$ potential surface also exhibit revivals. Time-dependent angular distributions for the molecular ions exhibit transient alignment only soon after the pulse (18 fs for $\text{H}_2{}^{+}$ and 35 fs for $\text{D}_2{}^{+}$) with no consequent revivals within the next 10 ps due to broad distribution of active vibrational states with different rotational constants.

We study the conservative dynamics and stationary configurations of a vortex-antivortex pair in a harmonically trapped two-dimensional Bose-Einstein condensate. We establish the conceptual framework for understanding the stationary states and the topological defect trajectories, through considerations of different mechanisms of vortex motion and the bifurcation of solitonlike stationary solutions. Our insights are based on Lagrangian-based variational calculations, numerical solutions of both the time-dependent and time-independent Gross-Pitaevskii equations, and exact solutions for the noninteracting case.

We study photon creation by a deforming mirror immersed in the electromagnetic field. The mirror radiates photons in pairs, and twins may have opposite polarizations. However, the total (angular) spectrum of photons is the sum of the (angular) spectrum of radiation from mirrors in a scalar field subject to the Dirichlet and Neumann boundary conditions. Windows of radiation are independent of the vectorial or scalar nature of the field. For a harmonic traveling wave on the surface with a frequency ${\omega }_m$ and wave vector $k_m$, the spectrum is not symmetric with respect to ${\omega }_m/2$. This asymmetry of the spectrum depends on ${\omega }_m$ and $k_m$, and is specific to the electromagnetic field.

We present a microresonator-based system capable of simultaneously producing time-advanced and time-delayed pulses. The effect is based on the combination of a sharp spectral feature with two orthogonally-polarized propagating waveguide modes. We include an experimental proof-of-concept implementation using a silica microsphere coupled to a tapered optical fiber and use a time-domain picture to interpret the observed delays. We also discuss potential applications for future all-optical networks.

When trapped atoms are illuminated by weak lasers, off-resonant transitions cause shifts in the frequencies of the vibrational-sideband resonances. These frequency shifts may be understood in terms of Stark shifts of the individual levels or, as proposed here, as a vibrational Bloch-Siegert shift, an effect closely related to the usual (radio-frequency or optical) Bloch-Siegert shift and associated with rapidly oscillating terms when the rotating wave approximation is not made. Explicit analytic expressions are derived and compared to numerical results, and the similarities and differences between the usual and the vibrational Bloch-Siegert shifts are also spelled out.

The general transformation of the product of coherent states ${\prod }_{i=1}^N|{\alpha }_i⟩$ to the output state ${\prod }_{i=1}^M|{\beta }_i⟩$ ($N=M$ or $N\ne M$), which is realizable with linear optical circuit, is characterized with a linear map from the vector $\left({\alpha }_1^{\ast },\dots ,{\alpha }_N^{\ast }\right)$ to $\left({\beta }_1^{\ast },\dots ,{\beta }_M^{\ast }\right)$. A correspondence between the transformations of a product of coherent states and those of a single photon state is established with such linear maps. It is convenient to apply this linear transformation method to design any linear optical scheme working with coherent states. The examples include message encoding and quantum database searching. The limitation of manipulating entangled coherent states with linear optics is also discussed.