We study electron-impact-induced dissociation of small van der Waals–bound argon complexes at a projectile energy of 120 eV. Kinetic-energy-release (KER) spectra of the ${\text{Ar}}_2$ and ${\text{Ar}}_3$ parent species for the final charge states $2{\text{Ar}}^{+}, {\text{Ar}}^{+}+{\text{Ar}}^{2+}$, and ${\text{Ar}}_2^{+}+{\text{Ar}}^{+}$ and electron energies have been measured together with angular distributions of fragment ions. They are used to identify dissociation mechanisms such as interatomic Coulombic decay (ICD).

We study solutions of the one-dimensional Gross-Pitaevskii equation to better understand dynamical instabilities occurring in flowing atomic condensates. Whereas transonic stationary flows can be fully described in simple terms, time-dependent flows exhibit a wide variety of behaviors. When the sound speed is crossed once, we observe that flows analogous to black holes obey something similar to the so-called no hair theorem since their late time profile is stationary and uniquely fixed by parameters entering the Hamiltonian and conserved quantities. For flows analogous to white holes, at late time one finds a macroscopic undulation in the supersonic side which has either a fixed amplitude or a widely varying one, signaling a quasiperiodic emission of solitons on the subsonic side. When considering flows which cross the sound speed twice, we observe various scenarios which can be understood from the above behaviors and from the hierarchy of the growth rates of the dynamical instabilities characterizing such flows.

We report a detailed study of effective magnetically levitated loading of cold atoms in a crossed dipole trap: an appropriate magnetic field gradient precisely compensates for the destructive gravitational force of the atoms and an additional bias field simultaneously eliminates the antitrapping potential induced by the magnetic field gradient. The magnetic levitation is required for a large-volume crossed dipole trap to form a shallow but very effective loading potential, making it a promising method for loading and trapping more cold atoms. For cold cesium atoms in the $F=3, mF=3$ state prepared by three-dimensional degenerated Raman sideband cooling, a large number of atoms $\sim 3.2\times {10}^6$ have been loaded into a large-volume crossed dipole trap with the help of the magnetic levitation technique. The dependence of the number of atoms loaded and trapped in the dipole trap on the magnetic field gradient and bias field, respectively, is in good agreement with the theoretical analysis. The optimum magnetic field gradient of 31.13 G/cm matches the theoretical value of 31.3 G/cm well. This method can be used to obtain more cold atoms or a large number of Bose-Einstein condensation atoms for many atomic species in high-field seeking states.

We study the spontaneous spin textures induced by magnetic dipole-dipole interaction in ferromagnetic spinor condensates under various trap geometries. At the mean-field level, we show the system is dual to a Dirac string gas with a negative string tension in which the ground state spin texture can be easily determined. We find that three-dimensional condensates prefer a meronlike vortex texture, quasi-one-dimensional condensates prefer the axially polarized flare texture, while condensates in quasi-two-dimensions exhibit either a meron texture or an in-plane polarized texture.

We separate Maxwell's equations for background media that allow for both electric and magnetic time dependence in a generalized Lorenz gauge. In a process analogous to the dynamical Casimir effect (DCE) we discuss how surface plasmon polaritons (SPPs) can be created out of vacuum via the time-dependent variation of a dielectric and magnetic insulator at a metal interface for TM and TE branches, respectively. We suggest how to extend currently proposed DCE experiments to set up and detect these excitations. Numerical simulations (without any approximation) indicate that vacuum-excited SPPs can be of a similar magnitude to the photon creation rate in such experiments. Potential benefits of detecting vacuum-excited SPPs, as opposed to DCE photons, are that parametric enhancement does not require a sealed cavity in the axial direction and the detection apparatus might be able to use simple phase-matching techniques. For the case of constant permeability $\mu$, TM branch SPPs and photons do not suffer from detuning and attenuation like TE photons.

Real-time tracking of atomic two-electron coherent dynamics is investigated through excitation of autoionizing wave packets in the ionization continuum of xenon. Extreme-ultraviolet high-order harmonics of ultrashort Ti:Sapphire laser pulses start the wave packets in energy ranges with a dense covering by two-valence-electron and single-inner-valence-electron excited-state resonances. The 15th harmonic covers resonances of type $5s5p^{6}nl$ with principle quantum number $n$ ranging from $n\approx 14$ up to $n\approx 21$ and one resonance of type $5s^{2}5p^{4}nl{n}^{\prime }{l}^{\prime }$ with fixed $n,n^{\prime }$ and orbital angular momenta $l,l^{\prime }$. In parallel the 17th harmonic starts a wave packet beyond the Xe $5s5p^6$ ionization threshold where exclusively resonances contribute with two-valence electrons being excited. The evolution in time of these wave packets is probed by inducing a continuum-continuum transition with the Ti:Sapphire laser pulses at variable delay times and detecting the photoelectrons after this two-photon transition. The dependence of the experimental data on the pump-probe delay time can be approximated by a two-photon transition probability derived on the basis of Fano's theory.

We re-examine the properties of the states produced by nondegenerate coherently stimulated parametric down-conversion wherein the signal and idler modes are seeded with coherent states of light and where the nonlinear crystal is driven by a strong classical field as described by the parametric approximation. The states produced are the two-mode squeezed coherent states defined with a specific ordering of operators, namely, the displacement operators of the two modes acting on the double vacuum state followed by the action of the two-mode squeeze operator representing the down-conversion process. Though mathematically equivalent to the reverse ordering of operators, but with different displacement parameters, the ordering we consider is closely related to what could most easily be implemented in the laboratory. The statistical properties of the state are studied with an emphasis on how they, and its average photon number, are affected by the various controllable phases, namely, those of the classical pump field of the two input coherent states. We then consider the multiphoton interference effects that arise if the two beams are overlapped on a 50:50 beam splitter, investigating the role of the phases in controlling the statistical properties of the output states. Finally, we study the prospects for the application of the states to quantum-optical interferometry to obtain sensitivities in phase-shift measurements beyond the standard quantum limit.

We develop a general Hamiltonian treatment of spontaneous four-wave mixing in a microring resonator side-coupled to a channel waveguide. The effect of scattering losses in the ring is included, as well as parasitic nonlinear effects including self- and cross-phase modulation. A procedure for computing the output of such a system for arbitrary parameters and pump states is presented. For the limit of weak pumping an expression for the joint spectral intensity of generated photon pairs, as well as the singles-to-coincidences ratio, is derived.

In a recent article [Phys. Rev. A 88, 012708 (2013)], Tian et al. claimed coherent interference among the various negative ion resonant states involved in the dissociative electron attachment (DEA) to carbon monoxide (CO) by investigating ${\mathrm{O}}^{-}$ angular distribution using the anion velocity time-sliced map imaging technique. However, our recent detailed study [Phys. Chem. Chem. Phys. 17, 7130 (2015)] on DEA to CO using the identical technique shows that the above claim can be questioned.

We respond to Nag and Nandi's Comment on our paper [Phys. Rev. A 88, 012708 (2013)], in which the arguments arise from the data analyses of the angular distributions of O¯ produced in the dissociative electron attachments to CO at 10.5 or 10.6 eV although the same technique, anion-velocity time-sliced map imaging, was used. The forward distribution in the O¯ image observed by Nag and Nandi arises from the low momentum resolution and distortion in their ion velocity imaging measurements. These artifacts are due to their arbitrary elongation of the ion flight tube and the uncertainty in the size control of the reaction volume.

In quantum systems, one usually seeks to minimize dephasing noise and disorder. The efficiency of transport in a quantum system is usually degraded by the presence of noise and disorder. However, it has been shown that the combination of the two can lead to significantly more efficiency than each by itself. Here, we consider how the addition of nonlocal noise, in the form of incoherent hopping, affects the transport efficiency. We show that incoherent hopping introduces additional local extrema in the efficiency function and investigate how the transport dynamics crosses over from a quantum random walk to a classical random walk.

In measurement-based quantum computation (MBQC), elementary quantum operations can be more parallelized than the quantum circuit model by employing a larger Hilbert space of graph states used as the resource. Thus MBQC can be regarded as a method of quantum computation where the temporal resource described by the depth of quantum operations can be reduced compared to the quantum circuit model by using the extra spatial resource described by graph states. To analyze the trade-off relationship of the spatial and temporal resources, we consider a method to obtain quantum circuit decompositions of general unitary transformations represented by MBQC on graph states with a certain underlying geometry called generalized flow. We present a method to translate any MBQC with generalized flow into quantum circuits without extra spatial resource. We also show an explicit way to unravel acausal gates without postselection that appear in the quantum circuit decomposition derived by a translation method [V. Danos and E. Kashefi, Phys. Rev. A 74, 052310 (2006)] and that represents an effect of the reduction of the temporal resource in MBQC. Finally, by considering a way to deterministically simulate these acausal gates, we investigate a general framework to analyze the trade-off between the spatial and temporal resources for quantum computation.

Universal blind quantum computation (UBQC) is a new secure quantum computing protocol which allows a user Alice who does not have any sophisticated quantum technology to delegate her computing to a server Bob without leaking any privacy. Using the features of UBQC, we propose a protocol to achieve symmetrically private information retrieval, which allows a quantum limited Alice to query an item from Bob with a fully fledged quantum computer; meanwhile, the privacy of both parties is preserved. The security of our protocol is based on the assumption that malicious Alice has no quantum computer, which avoids the impossibility proof of Lo. For the honest Alice, she is almost classical and only requires minimal quantum resources to carry out the proposed protocol. Therefore, she does not need any expensive laboratory which can maintain the coherence of complicated quantum experimental setups.

We predict several effects associated with the optical response of systems prepared in a nonequilibrium state by impulsive optical excitations. The linear response depends on the phase of the electric field even if the initial nonequilibrium state has only populations, no coherences. Initial quantum coherences induce additional phase dependence, which also shows new resonances in nonlinear wave mixing. In systems strongly driven by an external optical field, the field frequency generates a phase-dependent probe absorption. This gives further control to manipulate the relative contribution to the linear signal due to initial populations and coherences.

The dynamics of a two-state system whose energies undergo a real crossing at some instant of time is studied. At this instant, both the coupling and the detuning vanish simultaneously, which leads to an exact degeneracy of the eigenenergies of the system. It is found that the dynamics of the system is primarily determined by the manner in which the degeneracy occurs. This interesting behavior is reminiscent of a symmetry-breaking process, since the totally symmetric situation occurring at the crossing is significantly altered by infinitesimal quantities, which remove the degeneracy, with very important dynamical implications from there on. A very simple analytical formula is derived, which is found to describe the population changes very accurately.

In order to study the interesting interplay between localized and dispersive orbital states in a system of strongly interacting ultracold atoms in an optical lattice, we investigate the possibility to coherently couple the lowest two Bloch bands by means of resonant periodic forcing. For bosons in one dimension we show that a strongly interacting Floquet system can be realized, where at every lattice site two (and only two) near-degenerate orbital states are relevant, whose tunneling matrix elements differ in sign and magnitude. By smoothly tuning both states into resonance, the system is predicted to undergo an orbital-driven Mott-insulator-to-superfluid transition. As a consequence of kinetic frustration, this transition can be either continuous or first order, depending on parameters such as lattice depth and filling.

Shannon entropy and Fisher information calculated from one-particle density distributions and von Neumann and linear entropies (the latter two as measures of entanglement) computed from the reduced one-particle density matrix are analyzed for the ${}^{1,3}S^e,{}^{1,3}P^o$, and ${}^{1,3}D^e$ Rydberg series of He doubly excited states below the second ionization threshold. In contrast with the Shannon entropy, we find that both the Fisher information and entanglement measures are able to discriminate low-energy resonances pertaining to different ${}_2(K,T)_{n_2}^A$ series according to the Herrick-Sinanoğlu-Lin classification. Contrary to bound states, which show a clear and unique asymptotic value for both Fisher information and entanglement measures in their Rydberg series $1sn\ell$ for $n\to \infty$ (which implies a loss of spatial entanglement), the variety of behaviors and asymptotic values of entanglement above the noninteracting limit value in the Rydberg series of doubly excited states ${}_2(K,T)_{n_2}^A$ indicates a signature of the intrinsic complexity and remnant entanglement in these high-lying resonances even with infinite excitation $n_2\to \infty$, for which all known attempts of resonance classifications fail in helium.

The ionization modulation of the hydrogen atom is investigated theoretically by the numerical solution to the time-dependent Schrödinger equation and the strong-field-approximation method. Considering a pair of identical but time-delayed one-cycle infrared laser pulses, we find that the total ionization yield is modulated by the time delay between the two pulses. We show that the modulation is mainly due to the interference of electron wave packets in the continuum states respectively generated by each of the two pulses. In this tunneling regime, we identify some similarities to and differences from the usual quantum control of electron transition that is in the perturbation regime. Finally, we also extend our study of ionization modulation to the one-multicycle-laser-pulse case with different wavelengths in the tunneling regime.

Analytical expressions are derived for sums of matrix elements and their squared moduli over many-body states with given total spin—the states built from spin and spatial wave functions belonging to multidimensional irreducible representations of the symmetric group, unless the total spin has the maximal allowed value. For spin-dependent one-body interactions with external fields and spin-independent two-body ones between the particles, the sum dependence on the many-body states is given by universal factors, which are independent of the interaction details and Hamiltonians of noninteracting particles. The sum rules are applied to perturbative analysis of energy spectra.