Elastic scattering of low-energy (0–13 eV) electrons from more realistic models of a DNA base-pair decamer is studied using multiple-scattering theory and $T$ matrices obtained from ab initio $R$-matrix calculations. The models include two types of irregularities usually found in cellular DNA: base-pair mismatch and structural water molecules. Furthermore, we include in our calculation inelastic collisions. It is found that the basic interference patterns observed in the ideal and nonideal (i.e., more realistic) decamers are similar but have different amplitudes and are shifted in energy. Substantial inelastic losses, interestingly, cause pronounced local resonances, which could have an important influence in DNA strand breaks.

We study ferromagnetism in a repulsively interacting two-component Fermi gas in a harmonic trap. Within a local density approximation, the two components phase separate beyond a critical interaction strength, with one species having a higher density at the trap center. We discuss several easily observable experimental signatures of this transition. The mean-field release energy, its separate kinetic and interaction contributions, as well as the potential energy all depend on the interaction strength and contain a sharp signature of this transition. In addition, the conversion rate of atoms to molecules, arising from three-body collisions, peaks at an interaction strength just beyond the ferromagnetic transition point. We then go beyond the local density approximation and derive an energy functional that includes a term that depends on the local magnetization gradient and acts as a “surface tension.” Using this energy functional, we numerically study the energetics of some candidate spin textures that may be stabilized in a harmonic trapping potential at zero net magnetization. We find that a hedgehog state has a lower energy than an “in-out” domain-wall state in an isotropic trap. Upon inclusion of trap anisotropy we find that the hedgehog magnetization profile gets distorted due to the surface tension term, this distortion being more apparent for small atom numbers. We estimate that the magnetic dipole interaction does not play a significant role in this system. We consider possible implications for experiments on trapped ${}^6\text{L}\text{i}$ and ${}^{40}\text{K}$ gases.

We consider the creation of superpositions of macroscopically distinct states by a completely positive (CP) operation on a subsystem. We conclude that the subsystem on which the CP operation acts must be macroscopically large if the success probability of the CP operation does not vanish in the thermodynamic limit. In order to obtain this conclusion, we show two inequalities, each of which represents a trade-off relation among the magnitude of an indicator for superpositions of macroscopically distinct states, the success probability of a CP operation, and the volume of the subsystem on which the CP operation acts.

A qubit (a spin-1/2 particle) prepared in the up state is scattered by local spin-flipping potentials produced by the two target qubits (two fixed spins), both prepared in the down state, to generate an entangled state in the latter when the former is found in the down state after scattering. The scattering process is analyzed in three dimensions, both to lowest order and in full order in perturbation, with an appropriate renormalization for the latter. The entanglement is evaluated in terms of the concurrence as a function of the incident and scattering angles, the size of the incident wave packet, and the detector resolution to clarify the key elements for obtaining an entanglement with high quality. The characteristics of the results are also discussed in the context of (in)distinguishability of alternative paths for a quantum particle.

We have studied the $\text{Pb}5d$ photoionization and Auger decay spectrum with high resolution using the new FINEST synchrotron-radiation branchline (on the I3 beamline at MAX–laboratory). We analyzed the experimental spectrum with the help of multiconfigurational Dirac-Fock calculations and analyzed the strong correlation properties of the relativistic $5d_{3/2}$ and $5d_{5/2}$ subshells. We show that the many-electron phenomena in photoionization and Auger decay can be described through mixing with the $np\left(n=7,8,\dots \right)$ orbitals.

We study the elusive transition from exponential to postexponential (algebraic) decay of the probability density of a quantum particle emitted by an exponentially decaying source in one dimension. The main finding is that the probability density at the transition time, and thus its observability, increases with the distance of the detector from the source up to a critical distance beyond which exponential decay is no longer observed. Solvable models provide explicit expressions for the dependence of the transition on resonance and observational parameters, facilitating the choice of optimal conditions.

Collisions between noble-gas atoms and hydrogen molecules are investigated theoretically by solving the time-independent Schrödinger equation. Various initial states of the molecule are considered and the calculations are performed for each system over a large range of collision energies. Cross sections for quenching of rovibrationally excited states of ${\text{H}}_2$ are reported for Ar and Kr colliders and comparisons are made with previous calculations involving He. For both Ar and Kr colliders, the effect of vibrational excitation is found to be more pronounced for ortho-${\text{H}}_2$. The $T\to 0$ limit of the total quenching rate coefficient, which is presented here as the imaginary part of a complex scattering length, is found to increase by about 7 orders of magnitude as the vibrational quantum number of ortho-${\text{H}}_2$ is increased from 1 to 10. Trends in the energy dependence for the heavier systems are very similar, including resonance behavior, which suggest that the dynamics of heavy noble-gas ${\text{H}}_2$ systems are less sensitive to the fine details of the potential.

We investigate the total effect of correlations on photoionization of atomic states with nonzero orbital momentum in the nonrelativistic high-energy asymptotic limit, considering the exclusive case of the dominant final state of an initial neutral atom. We find that the substantial cancellation of the dominant intrashell correlations, which had been reported earlier, can be understood utilizing the closure properties satisfied by the eigenfunctions of the nonrelativistic Hamiltonian. Considering the sum of correlations with all states, occupied or not, we show that complete sum is equal to the contribution of the high-energy part of the continuum. Consequently there is a total cancellation between the contributions of the bound states (occupied and unoccupied) and the low-energy part of the continuum states. This means that the real correlations in the physical atom due to the sum rule over the occupied states can be also obtained as the negative of the sum of contributions of low-energy bound and continuum unoccupied states. We calculate this in the framework of the quantum-defect model. As we would expect, the results are close to those obtained earlier in particular cases by direct summation over the occupied states. However this approach also allows us to see that the sum of correlations over the occupied states is smaller than the dominant intrashell correlation. Hence, there is a partial cancellation between correlations with the occupied states. We can also obtain some limits on the correlation effects by considering calculations with the screened Coulomb functions. The role of correlations in the exclusive photoionization processes, also the modification of correlations in the case of atomic ions, is discussed.

We demonstrate a technique for performing stochastic simulations of conditional master equations. The method is scalable for many quantum-field problems and therefore allows first-principles simulations of multimode bosonic fields undergoing continuous measurement, such as those controlled by measurement-based feedback. As examples, we demonstrate a 53-fold speed increase for the simulation of the feedback cooling of a single trapped particle, and the feedback cooling of a quantum field with 32 modes, which would be impractical using previous brute force methods.

The output fields of a pair of coherently coupled nonlinear optical cavities are found to exhibit strong optical entanglement. For sufficiently strong coupling time-delayed quantum correlations are observed providing a resource for quantum information protocols such as all-optical quantum memories. A straightforward experimental implementation applicable to whispering gallery mode resonators such as microtoroids is proposed.

We determine the superfluid transition temperature $T_c$ and related finite temperature phase diagrams for the entire BCS–Bose-Einstein-condensation crossover in a three-dimensional homogeneous mixture of ${}^6\text{L}\text{i}$ and ${}^{40}\text{K}$ atoms with population imbalance. Our work is motivated by the recent observation of an interspecies Feshbach resonance. Pairing fluctuation effects, which significantly reduce $T_c$ from the onset temperature for pairing $\left(T^{\ast }\right)$, provide reasonable estimates of $T_c$ and indicate that the interspecies superfluid phase should be accessible in future experiments. Although a homogeneous polarized superfluid is not stable in the ground state near unitarity, our phase diagrams show that it stabilizes at finite temperature.

We show that quantum subdynamics of an open quantum system can always be described by a linear, Hermitian map irrespective of the form of the initial total system state. Since the theory of quantum error correction was developed based on the assumption of completely positive (CP) maps, we present a generalized theory of linear quantum error correction, which applies to any linear map describing the open system evolution. In the physically relevant setting of Hermitian maps, we show that the CP-map-based version of quantum error correction theory applies without modifications. However, we show that a more general scenario is also possible, where the recovery map is Hermitian but not CP. Since non-CP maps have nonpositive matrices in their range, we provide a geometric characterization of the positivity domain of general linear maps. In particular, we show that this domain is convex and that this implies a simple algorithm for finding its boundary.

We analyze the orientation dependence of harmonic amplitudes and phases from laser driven ${\text{H}}_2^{+}$. We use the Lewenstein model, with and without employing the saddle-point approximation for the summation over electron momenta. This means that the direction of the electron motion is not necessarily restricted to the laser polarization axis in contrast to previous implementations. The model predicts smooth phase jumps by almost $\pi$ in the orientation dependence. This demonstrates that the smoothness can be explained without Coulomb effects, but these may be relevant for the size of the phase jumps observed in recent experiments.

We show that atom-molecule coupling with large detuning can cause effective hopping of pairs of bosonic atoms in a state-dependent optical lattice. Taking advantage of the high controllability of all relevant parameters in such systems, we discuss the pair-superfluid (PSF) to Mott insulator (MI) transition using the effective model within mean-field theory. In the presence of on-site disorder, simultaneous tunneling of bosonic atoms can result in a compressible weak Mott insulating phase. We have also investigated the coexistence of superfluid (SF) and PSF in the lattice, and found that the competition between the two hopping mechanisms can cause a first-order PSF(SF)-MI transition.

We report on the experimental observation of high peak power stretched-pulse emission in a dispersion-managed (DM) fiber laser with large positive net cavity dispersion. As stretched pulses are normally observed in DM fiber lasers with around zero cavity dispersion, it suggests that the effective cavity dispersion of a DM fiber laser is not a fixed value but varies with the laser operation conditions. Based on numerical simulations we further show that a stretched pulse could indeed be obtained in our laser under the laser operation condition when a high peak power pulse is formed, suggesting that the effective cavity dispersion shifts with the cavity nonlinearity.

We propose a quantum feedback scheme for the preparation and protection of photon-number states of light trapped in a high-$Q$ microwave cavity. A quantum nondemolition measurement of the cavity field provides information on the photon-number distribution. The feedback loop is closed by injecting into the cavity a coherent pulse adjusted to increase the probability of the target photon number. The efficiency and reliability of the closed-loop state stabilization is assessed by quantum Monte Carlo simulations. We show that, in realistic experimental conditions, the Fock states are efficiently produced and protected against decoherence.

We present a general model to account for the multimode nature of the quantum electromagnetic field in projective photon-counting measurements. We focus on photon-subtraction experiments, where non-Gaussian states are produced conditionally. These are useful states for continuous-variable quantum-information processing. We present a general method called mode reduction that reduces the multimode model to an effective two-mode problem. We apply this method to a multimode model describing broadband parametric down-conversion, thereby improving the analysis of existing experimental results. The main improvement is that spatial and frequency filters before the photon detector are taken into account explicitly. We find excellent agreement with previously published experimental results, using fewer free parameters than before, and discuss the implications of our analysis for the optimized production of states with negative Wigner functions.

Some mixed states composed of only Greenberger-Horne-Zeilinger (GHZ) states can be expressed in terms of only $W$ states. This fact implies that such states have vanishing three-tangle. One of such rank-3 states, ${\Pi }_{GHZ}$, is explicitly presented in this Rapid Communication. These results are used to compute analytically the three-tangle of a rank-4 mixed state $\sigma$ composed of four GHZ states. This analysis with considering Bloch sphere $S^{16}$ of $d=4$ qudit system allows us to derive the hyperpolyhedron. It is shown that the states in this hyperpolyhedron have vanishing three-tangle. Computing the one-tangles for ${\Pi }_{GHZ}$ and $\sigma$, we prove the monogamy inequality explicitly. Making use of the fact that the three-tangle of ${\Pi }_{GHZ}$ is zero, we try to explain why the $W$ class in the whole mixed states is not of measure zero contrary to the case of pure states.

The quantum private query is a quantum cryptographic protocol to recover information from a database, preserving both user and data privacy: the user can test whether someone has retained information on which query was asked and the database provider can test the amount of information released. Here we discuss a variant of the quantum private query algorithm that admits a simple linear optical implementation: it employs the photon’s momentum (or time slot) as address qubits and its polarization as bus qubit. A proof-of-principle experimental realization is implemented.

We report a direct observation of the $D$-line doublet of Na-like ions with $Z\ge 72$. Spectra in the 1–9 nm spectral range are presented showing the widely split doublet for ${\text{Hf}}^{61+}$, ${\text{Ta}}^{62+}$, ${\text{W}}^{63+}$, and ${\text{Au}}^{68+}$. The ions were produced and confined in an electron-beam ion trap and observed with a flat-field grazing incidence spectrometer. The measured wavelengths agree well with the predictions of relativistic many-body perturbation theory with QED corrections. We also report the measured wavelengths for several transitions in Si-, Al-, and Mg-like ions of Hf, Ta, and W and in Mg-like Au.