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  • 101.
    Larson, Jonas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Stenholm, Stig
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Validity of adiabaticity in cavity QED2006Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 73, nr 3, s. 033805-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper deals with the concept of adiabaticity for fully quantum mechanical cavity QED models. The physically interesting cases of Gaussian and standing wave shapes of the cavity mode are considered. An analytical approximate measure for adiabaticity is given and compared with numerical wave packet simulations. Good agreement is obtained where the approximations are expected to be valid. Usually for cavity QED systems, the large atom-field detuning case is considered as the adiabatic limit. We, however, show that adiabaticity is also valid, for the Gaussian mode shape, in the opposite limit. Effective semiclassical time-dependent models, which do not take into account the shape of the wave packet, are derived. Corrections to such an effective theory, which are purely quantum mechanical, are discussed. It is shown that many of the results presented can be applied to time-dependent two-level systems.

  • 102.
    Larson, Åsa
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Orel, Ann
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Wave-packet study of the products formed in dissociative recombination of HeH+2005Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 72, nr 3, s. 032701-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a theoretical study of the atomic fragment distributions formed in dissociative recombination of HeH+ at low energies (< 2 eV) using wave packets propagating on the potential-energy curves of HeH, coupled by nonadiabatic and rotational interactions. We show that at zero electron collision energy, dissociation into H(n=2)+He will completely dominate over dissociation into the ground-state fragments. When the n=3 channel become energetically open at E=0.334 eV a rapid switchover to this channel will occur. This phenomenon has previously been observed experimentally, but here we present a theoretical model showing these jumps in the branching ratios. We present an explanation for the observed sudden change in branching ratios and we also discuss how the branching ratios depend on the nonadiabatic couplings used in the model.

  • 103. Lesanovsky, I.
    et al.
    Schumm, T.
    Hofferberth, S.
    Andersson, L. Mauritz
    Kruger, P.
    Schmiedmayer, J.
    Adiabatic radio-frequency potentials for the coherent manipulation of matter waves2006Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 73, nr 3Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Adiabatic dressed state potentials are created when magnetic substates of trapped atoms are coupled by a radio-frequency field. We discuss their theoretical foundations and point out fundamental advantages over potentials purely based on static fields. The enhanced flexibility enables one to implement numerous configurations, including double wells, Mach-Zehnder, and Sagnac interferometers which even allows for internal state-dependent atom manipulation. These can be realized using simple and highly integrated wire geometries on atom chips.

  • 104. Levamaki, H.
    et al.
    Nagy, A.
    Kokko, K.
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Alternative to the Kohn-Sham equations: The Pauli potential differential equation2015Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 92, nr 6, artikkel-id 062502Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A recently developed theoretical framework of performing self-consistent orbital-free (OF) density functional theory (DFT) calculations at Kohn-Sham DFT level accuracy is tested in practice. The framework is valid for spherically symmetric systems. Numerical results for the Beryllium atom are presented and compared to accurate Kohn-Sham data. These calculations make use of a differential equation that we have developed for the so called Pauli potential, a key quantity in OF-DFT. The Pauli potential differential equation and the OF Euler equation form a system of two coupled differential equations, which have to be solved simultaneously within the DFT self-consistent loop.

  • 105. Levola, Helena
    et al.
    Itala, Eero
    Schlesier, Kim
    Kooser, Kuno
    Laine, Sanna
    Laksman, Joakim
    Ha, Dang Trinh
    Rachlew, Elisabeth
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Atom- och molekylfysik.
    Tarkanovskaja, Marta
    Tanzer, Katrin
    Kukk, Edwin
    Ionization-site effects on the photofragmentation of chloro- and bromoacetic acid molecules2015Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 92, nr 6, artikkel-id 063409Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Fragmentation of gas-phase chloro-and bromoacetic acid samples, particularly its dependency on the atomic site of the initial core ionization, was studied in photoelectron-photoion-photoion coincidence (PEPIPICO) measurements. The fragmentation was investigated after ionizing carbon 1s and bromine 3d or chlorine 2p core orbitals. It was observed that the samples had many similar fragmentation pathways and that their relative weights depended strongly on the initial ionization site. Additional Auger PEPIPICO measurements revealed a clear dependence of fragment pair intensities on the kinetic energy of the emitted Auger electrons. The modeled and measured Auger electron spectra indicated that the average internal energy of the molecule was larger following the carbon 1s core-hole decay than after the decay of the halogen core hole. This difference in the internal energies was found to be the source of the site-dependent photofragmentation behavior.

  • 106. Levämäki, H.
    et al.
    Nagy, A.
    Kokko, K.
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Uppsala University, Sweden; Wigner Research Centre for Physics, Hungary.
    Cusp relation for the Pauli potential2014Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 90, nr 6, s. 062515-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In orbital-free density functional theory, only a Schrodinger-like equation has to be solved for the square root of the electron density. In this equation, however, there is an extra potential in addition to the Kohn-Sham potential, the so-called Pauli potential. Cusp relations are now presented for this Pauli potential for spherically symmetric systems.

  • 107. Lindroth, E.
    et al.
    Wallenius, Janne
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Jonsell, S.
    Decay rates of excited muonic molecular ions2003Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 68, nr 3Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Muonic molecular ions in excited states have been predicted to form in collisions between excited muonic atoms and hydrogen molecules. We have calculated radiative and Coulombic decay rates for ppmu(*) and ddmu(*) molecular states located below the 2s threshold, using the complex rotation method. The x-ray spectrum from the radiative decay is shown to exhibit several maxima, corresponding to the vibrational motion of the decaying molecule. The branching ratio of the radiative decay mode was calculated to be less than 15% for ppmu(*), while a radiative yield of more than 80% is predicted for the decay of ddmu(*). Our results have a significant impact on the analysis of the muon catalyzed fusion cycle as well as on the interpretation of exotic hydrogen spectroscopy.

  • 108.
    Liu, Jicai
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Sun, Yu-Ping
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Wang, Chuankui
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Ågren, Hans
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Gelmukhanov, Faris
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Auger effect in the presence of strong x-ray pulses2010Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, ISSN 1050-2947, Vol. 81, nr 4, s. 043412-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study the role of propagation of strong x-ray free-electron laser pulses on the Auger effect. When the system is exposed to a strong x-ray pulse the stimulated emission starts to compete with the Auger decay. As an illustration we present numerical results for Ar gas with the frequency of the incident x-ray pulse tuned in the 2p(3/2)-4s resonance. It is shown that the pulse propagation is accompanied by two channels of amplified spontaneous emission, 4s-2p(3/2) and 3s-2p(3/2), which reshape the pulse when the system is inverted. The population inversion is quenched for longer propagation distances where lasing without inversion enhances the Stokes component. The results of simulations show that the propagation of the strong x-ray pulses affect intensively the Auger branching ratio.

  • 109.
    Liu, Ji-Cai
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Velkov, Yasen
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Rinkevicius, Zilvinas
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Ågren, Hans
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Gel`mukhanov, Faris
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Symmetry-forbidden x-ray Raman scattering induced by a strong infrared-laser field2008Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 77, nr 4, s. 043405-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Resonant inelastic x-ray scattering accompanied by core-hole hopping induced by a strong infrared-laser field is studied for the nitrogen molecule. This process involves a strong laser-field-induced promotion of ungerade core holes created by a weak x-ray pulse to a gerade core level, which opens symmetry-forbidden scattering channels and gives rise to new features in the x-ray scattering spectrum. The core-hole hopping within the short lifetime of the core-excited state required for observation of the described process can be achieved at moderate intensities of the infrared field (similar to 10(12) W/cm(2)) because of the large transition dipole moment between the relevant core levels. The dynamics of resonant inelastic x-ray scattering assisted by change of core-hole parity is studied in detail versus the intensity, detuning, phase, and duration of the incident infrared-laser and x-ray pulses.

  • 110.
    Liu, Ji-Cai
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Wang, Chuan-Kui
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Gelmukhanov, Faris
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Dynamics of multilevel molecules and pulse propagation beyond rotating wave approximation near two-photon resonance2007Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 76, nr 4, artikkel-id 043422Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Dynamics of multilevel molecules and pulse propagation is studied near the two-photon resonance. We have found a strict solution of this problem beyond the rotating wave approximation. Our analytical solution is in close agreement with the strict numerical solution for the 4,4(')-bis(dimethylamino) stilbene molecule. The compensation of the dynamical Stark shift is studied for fixed-in-space molecules. It is shown that the orientational disorder does not allow complete compensation of the dynamical Stark shift.

  • 111.
    Liu, Ji-Cai
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Wang, Chuan-Kui
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Gelmukhanov, Faris
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Optical limiting of short laser pulses2007Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 76, nr 053804Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The dynamics of pulse propagation accompanied by harmonic generation, stimulated Raman scattering, amplified spontaneous emission, and superfluorescence is studied near the two-photon resonance. We explore the optical limiting of intense and short laser pulses. The numerical solutions of the coupled Bloch and Maxwell's equations for the 4,4(')-bis(dimethylamino) stilbene molecule are compared with the two-photon area theorem. It is shown that the area theorem explains qualitatively the major dynamical properties of pulse propagation even if the propagation is accompanied by the generation of new fields. In agreement with the area theorem, we see that the conventional dependence of the transmittance on the propagation depth is not valid for intense pulses.

  • 112.
    Ljunggren, Daniel
    et al.
    KTH, Tidigare Institutioner                               , Mikroelektronik och informationsteknik, IMIT.
    Bourennane, Mohamed
    KTH, Tidigare Institutioner                               , Mikroelektronik och informationsteknik, IMIT.
    Karlsson, Anders
    KTH, Tidigare Institutioner                               , Elektronik.
    Authority-based user authentication in quantum key distribution2000Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 62, nr 2, s. 022305-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

     We propose secure protocols for user authenticated quantum key distribution on jammable public channels between two parties, Alice and Bob. Via an arbitrator, Trent, these protocols provide data integrity and mutual identification of the messenger and recipient. The first three are based on single-photon generation and detection. The first and second require (initially) an unjammable channel between the arbitrator and each party. The third requires one broadcast from the arbitrator, disclosing what type of deterministic modification of the states sent through the quantum channel was done by him. The fourth and fifth protocols are based on two-particle entanglement with a preselection of nonorthogonal superpositions of Bell states. These two protocols also require one broadcast from the arbitrator disclosing the type of entangled state in each sending.

  • 113.
    Ljunggren, Daniel
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Tengner, Maria
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Optimal focusing for maximal collection of entangled narrow-band photon pairs into single-mode fibers2005Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 72, nr 6, s. 062301-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a theoretical and experimental investigation of the emission characteristics and the flux of photon pairs generated by spontaneous parametric downconversion in quasi-phase matched bulk crystals for the use in quantum communication sources. We show that, by careful design, one can attain well defined modes close to the fundamental mode of optical fibers and obtain high coupling efficiencies also for bulk crystals, these being more easily aligned than crystal waveguides. We distinguish between singles coupling, gamma(s) and gamma(i), conditional coincidence, mu(i vertical bar s), and pair coupling, gamma(c), and show how each of these parameters can be maximized by varying the focusing of the pump mode and the fiber-matched modes using standard optical elements. Specifically we analyze a periodically poled KTP-crystal pumped by a 532 nm laser creating photon pairs at 810 nm and 1550 nm. Numerical calculations lead to coupling efficiencies above 93% at optimal focusing, which is found by the geometrical relation L/z(R) to be approximate to 1 to 2 for the pump mode and approximate to 2 to 3 for the fiber-modes, where L is the crystal length and z(R) is the Rayleigh-range of the mode-profile. These results are independent on L. By showing that the single-mode bandwidth decreases proportional to 1/L, we can therefore design the source to produce and couple narrow bandwidth photon pairs well into the fibers. Smaller bandwidth means both less chromatic dispersion for long propagation distances in fibers, and that telecom Bragg gratings can be utilized to compensate for broadened photon packets-a vital problem for time-multiplexed qubits. Longer crystals also yield an increase in fiber photon flux proportional to root L, and so, assuming correct focusing, we can only see advantages using long crystals.

  • 114.
    Ljunggren, Daniel
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Tengner, Maria
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Marsden, Philip
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Pelton, Matthew
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Theory and experiment of entanglement in a quasi-phase-matched two-crystal source2006Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Phys. Rev. A, ISSN 1050-2947, Vol. 73, nr 3, s. 032326-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report results regarding a source of polarization entangled photon pairs created by the process of spontaneous parametric downconversion in two orthogonally oriented, periodically poled, bulk KTiOPO4 crystals. The source emits light colinearly at the nondegenerate wavelengths of 810 and 1550 nm, and is optimized for a single-mode optical fiber collection and long-distance quantum communication. The configuration favors long crystals, which promote a high photon-pair production rate at a narrow bandwidth, together with a high pair probability in fibers. The quality of entanglement is limited by chromatic dispersion, which we analyze by determining the output state. We find that such a decoherence effect is strongly material dependent, providing for long crystals an upper bound on the visibility of the coincidence fringes of 41% for KTiOPO4, and zero for LiNbO3. The best obtained raw visibility, when canceling decoherence with an extra piece of crystal, was 91 +/- 0.2%, including background counts. We confirm by a violation of the CHSH-inequality (S=2.679 +/- 0.004 at 55 s(-1/2) standard deviations) and by complete quantum state tomography that the fibers carry high-quality entangled pairs at a maximum rate of 55x10(3) s(-1) THz(-1) mW(-1).

  • 115.
    Lundh, Emil
    KTH, Tidigare Institutioner, Fysik.
    Calculation of collective modes for the Bose-Hubbard model with confinement2004Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 70, nr 6, s. 061602-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The collective excitations in the Bose-Hubbard model in a trap are studied by means of numerical diagonalization in one dimension. The strength function is calculated for monopole and dipole perturbations, and moments of the strength function are utilized in order to obtain information about the collective behavior under external forces. In the superfluid regime, the spectrum is found to be exhausted by one single frequency, while in systems that contain a Mott insulating plateau, several frequencies are excited. An explanation of recent experimental findings in terms of a Mott plateau is suggested.

  • 116.
    Lundh, Emil
    KTH, Tidigare Institutioner, Fysik.
    Dipole and monopole modes in the Bose-Hubbard model in a trap2004Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 70, nr 3, s. 033610-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The lowest-lying collective modes of a trapped Bose gas in an optical lattice are studied in the Bose-Hubbard model. An exact diagonalization of the Hamiltonian is performed in a one-dimensional five-particle system in order to find the lowest few eigenstates. The dipole and breathing character of the eigenstates is confirmed in the limit where the tunneling dominates the dynamics, but under Mott-like conditions the excitations do not correspond to oscillatory modes.

  • 117.
    Lundholm, Douglas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Matematik (Inst.), Matematik (Avd.).
    Solovej, Jan Philip
    University of Copenhagen.
    Local exclusion principle for identical particles obeying intermediate and fractional statistics2013Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 88, nr 6, s. 062106-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A local exclusion principle is observed for identical particles obeying intermediate and fractional exchange statistics in one and two dimensions, leading to bounds for the kinetic energy in terms of the density. This has implications for models of Lieb-Liniger and Calogero-Sutherland type and implies a nontrivial lower bound for the energy of the anyon gas whenever the statistics parameter is an odd numerator fraction. We discuss whether this is actually a necessary requirement.

  • 118. Magnuson, M
    et al.
    Guo, J
    Sathe, C
    Rubensson, E
    Nordgren, J
    Glans, P
    Yang, L
    Salek, Pawel
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Ågren, Hans
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Competition between decay and dissociation of core-excited carbonyl sulfide studied by x-ray scattering1999Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 59, nr 6, s. 4281-4287Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We show evidence of dissociation during resonant inelastic soft x-ray scattering Carbon and oxygen K-shell and sulfur L-shell resonant and nonresonant x-ray emission spectra were measured using monochromatic synchrotron radiation for excitation and ionization. After sulfur L-2,L-3-->pi*, sigma* excitation, atomic lines are observed in the emission spectra as a consequence of competition between de-excitation and dissociation. In contrast thr carbon and oxygen spectra show weaker line-shape variations and no atomic Lines. The spectra are compared to results from ab initio calculations. The discussion of the dissociation paths is based on calculated potential energy surfaces and atomic transition energies.

  • 119.
    Makinen, A.
    et al.
    Department of Physics, University of Oulu, Finland.
    Niskanen, Johannes
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Aksela, H.
    Department of Physics, University of Oulu, Finland.
    Relative photoionization cross section of Cr atoms in the valence region2012Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 85, nr 5, s. 053411-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, we report a measurement of a single photoionization cross section of atomic Cr as a function of photon energy between 10 and 35 eV. The result is compared to a semiempirical model employing orbital-based ionization cross-section calculations from the literature. While the wide energy splitting of the singly ionized valence states explains the behavior of the ionization cross section in the lower-energy end, reasons for the mismatch between the calculations and the experiment in the higher-energy end are discussed.

  • 120. Malossi, N.
    et al.
    Damkjaer, S.
    Hansen, P. L.
    Jacobsen, L. B.
    Kindt, L.
    Sauge, Sebastien
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Fotonik och optik.
    Thomsen, J. W.
    Cruz, F. C.
    Allegrini, M.
    Arimondo, E.
    Two-photon cooling of magnesium atoms2005Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 72, nr 5Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A two-photon mechanism for cooling atoms below the Doppler temperature is analyzed. We consider the magnesium ladder system (3s(2))S-1(0)->(3s3p)P-1(1) at 285.2 nm followed by the (3s3p)P-1(1)->(3s3d)D-1(2) transition at 880.7 nm. For the ladder system quantum coherence effects may become important. Combined with the basic two-level Doppler cooling process this allows for reduction of the atomic sample temperature by more than a factor of 10 over a broad frequency range. First experimental evidence for the two-photon cooling process is presented and compared to model calculations. Agreement between theory and experiment is excellent. In addition, by properly choosing the Rabi frequencies of the two optical transitions a velocity independent atomic dark state is observed.

  • 121. Meyer, M. L.
    et al.
    Sreejith, Ganesh Jaya
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Viefers, S.
    Rotational properties of two-component Bose gases in the lowest Landau level2014Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 89, nr 4, s. 043625-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study the rotational (yrast) spectra of dilute two-component atomic Bose gases in the low angular momentum regime, assuming equal interspecies and intraspecies interaction. Our analysis employs the composite fermion (CF) approach including a pseudospin degree of freedom. While the CF approach is not a priori expected to work well in this angular momentum regime, we show that composite fermion diagonalization gives remarkably accurate approximations to low-energy states in the spectra. For angular momenta 0 < L < M ( where N and M denote the numbers of particles of the two species, and M >= N), we find that the CF states span the full Hilbert space and provide a convenient set of basis states which, by construction, are eigenstates of the symmetries of the Hamiltonian. Within this CF basis, we identify a subset of the basis states with the lowest A-level kinetic energy. Diagonalization within this significantly smaller subspace constitutes a major computational simplification and provides very close approximations to ground states and a number of low-lying states within each pseudospin and angular momentum channel.

  • 122. Mikhailov, Ivan A.
    et al.
    Kokoouline, Viatcheslav
    Larson, Åsa
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Tonzani, Stefano
    Greene, Chris H.
    Renner-Teller effects in HCO+ dissociative recombination2006Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 74, nr 3, s. 032707-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A theoretical description of the dissociative recombination process for the HCO+ ion suggests that the nonadiabatic Renner-Teller coupling between electronic and vibrational degrees of freedom plays an important role. This finding is consistent with a recent study of this process for another closed-shell molecule, the H-3(+) ion, where Jahn-Teller coupling was shown to generate a relatively high rate. The cross section obtained here for the dissociative recombination of HCO+ exhibits encouraging agreement with a merged-beam experiment.

  • 123.
    Niskanen, Johannes
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Andersson, E.
    Eland, J. H. D.
    Linusson, P.
    Hedin, L.
    Karlsson, L.
    Feifel, R.
    Vahtras, Olav
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Symmetry breaking in core-valence double photoionization of SO22012Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 85, nr 2, s. 023408-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Core-valence double photoionization electron spectra of the SO2 molecule involving the S 2p and O 1s inner shells have been measured using a time-of-flight multiparticle coincidence technique. The experimental spectra are compared with quantum-chemical calculations based on density functional theory by which several core-valence dicationic states are identified. Assignments conform with a picture where the formation of a O 1s-valence dicationic state is associated with a physical, "pseudo-Jahn-Teller," symmetry breaking and core-hole localization. It is shown that while density functional theory gives very good transition energies in the symmetry-broken case, it gives a poor representation in the symmetry-restricted case, and an incomplete account of the Hartree-Fock localization energy.

  • 124.
    Niskanen, Johannes
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Carravetta, V.
    Vahtras, Olav
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Ågren, Hans
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Aksela, H.
    Andersson, E.
    Hedin, L.
    Linusson, P.
    Eland, J. H. D.
    Karlsson, L.
    Rubensson, J. -E
    Feifel, R.
    Experimental and theoretical study of core-valence double photoionization of OCS2010Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 82, nr 4, s. 043436-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    O 1s, C 1s, and S 2p core-valence double ionization electron spectra of the OCS molecule have been obtained experimentally by a time-of-flight photoelectron-photoelectron coincidence spectroscopy technique. In order to analyze and assign the spectral features observed, we present a protocol for computing core-valence ionization energies of such systems. The protocol is based on a restricted active space multiconfigurational self-consistent field (MCSCF) methodology with a freeze-relax procedure to guarantee a correct core-valence state root index without variational collapse. Corrections for extended dynamical correlation and core-core correlation, respectively, are made by multiconfigurational perturbation theory and by uncontracted basis set Moller-Plesset theory. Envisioning applications to larger molecules, a spin-restricted open-shell density functional method is also applied for the lowest core-valence energies. Furthermore, cross sections through a scheme for computing multiatom Auger transitions generating core-valence holes are presented. We find that the procedure outlined is capable of deriving the energy onset of core-valence ionization within a fraction of an eV and that assignments can be made of the most salient spectral features.

  • 125.
    Niskanen, Johannes
    et al.
    KTH, Skolan för bioteknologi (BIO).
    Urpelainen, S.
    Aksela, S.
    Aksela, H.
    Vahtras, O.
    Carravetta, V.
    Ågren, Hans
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Valence photoionization of the LiCl monomer and dimer2010Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 81, nr 4, s. 043401-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper reports a study of valence photoionization of the LiCl monomer and dimer. The behavior of the photoionization partial cross section for molecular valence orbitals was measured as a function of photon energy between 15 and 35 eV. A square-integrable-function method was used to model the ionization partial cross section in both the LiCl monomer and dimer.

  • 126.
    Niskanen, Johannes
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Urpelainen, S.
    Jankala, K.
    Schulz, J.
    Heinasmaki, S.
    Fritzsche, S.
    Kabachnik, N. M.
    Aksela, S.
    Aksela, H.
    Photoelectron angular distribution and linear magnetic dichroism in the 4p photoemission from Rb atoms2010Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 81, nr 1, s. 013406-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The angular distribution of photoelectrons and linear magnetic dichroism in the angular distribution for the 4p photoemission from Rb atoms in the ground state, oriented by laser pumping, were measured in the photon energy range from 50 to 100 eV. The experimental results are compared with the multiconfiguration Dirac-Fock (MCDF) calculations. We show that the zero-crossing of the dichroism as a function of energy is connected with the Cooper minimum in the cross section. This fact can be used for an accurate determination of the position of the Cooper minimum.

  • 127. Pachos, Jiannis K.
    et al.
    Alba, Emilio
    Lahtinen, Ville
    KTH.
    Garcia-Ripoll, Juan J.
    Seeing Majorana fermions in time-of-flight images of staggered spinless fermions coupled by s-wave pairing2013Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 88, nr 1, s. 013622-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Chern number., as a topological invariant that identifies the winding of the ground state in the particle-hole space, is a definitive theoretical signature that determines whether a given superconducting system can support Majorana zero modes. Here we show that such a winding can be faithfully identified for any superconducting system (p wave or s wave with spin-orbit coupling) through a set of time-of-flight measurements, making it a diagnostic tool also in actual cold-atom experiments. As an application, we customize the measurement scheme for a chiral topological model of spinless fermions. The proposed model only requires the experimentally accessible s-wave pairing and staggered tunneling that mimics spin-orbit coupling. By adiabatically connecting this model to Kitaev's honeycomb lattice model, we show that it gives rise to nu = +/- 1 phases, where vortices bind Majorana fermions, and nu = +/- 2 phases that emerge as the unique collective state of such vortices. Hence, the preparation of these phases and the detection of their Chern numbers provide an unambiguous signature for the presence of Majorana modes. Finally, we demonstrate that our detection procedure is resilient against most inaccuracies in experimental control parameters as well as finite temperature.

  • 128. Piilo, J.
    et al.
    Lundh, Emil
    KTH, Tidigare Institutioner, Fysik.
    Suominen, K. A.
    Radiative collisional heating at the Doppler limit for laser-cooled magnesium atoms2004Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 70, nr 1Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report Monte Carlo wave function simulation results on cold collisions between magnesium atoms in a strong red-detuned laser field. This is the normal situation, e.g., in magneto-optical traps (MOT). The Doppler limit heating rate due to radiative collisions is calculated for Mg-24 atoms in an MOT based on the S-1(0)-P-1(1) atomic laser cooling transition. We find that radiative heating does not seem to affect the Doppler limit in this case. We also describe a channeling mechanism due to the missing Q branch in the excitation scheme, which could lead to a suppression of inelastic collisions, and find that this mechanism is not present in our simulation results due to the multistate character of the excitation process.

  • 129.
    Pinheiro, Fernanda
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden; Universität zu Köln, Germany.
    Larson, Jonas
    Disordered cold atoms in different symmetry classes2015Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 92, nr 2, artikkel-id 023612Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We consider an experimentally realizable model of noninteracting but randomly coupled atoms in a two-dimensional optical lattice. By choosing appropriate real or complex-valued random fields and species-dependent energy offsets, this system can be used to analyze effects of disorder in four different symmetry classes: the chiral BDI and AIII and the nonchiral A and AI. These chiral classes are known to support a metallic phase at zero energy, which here, due to the inevitable finite size of the system, should also persist in a neighborhood of nonzero energies. As we discuss, this is of particular interest for experiments involving quenches. Away from the center of the spectrum, we find that excitations appear as domain walls in the cases with time-reversal symmetry or as vortices in the cases where time-reversal symmetry is absent. Therefore, a quench in a system with uniform density would lead to the formation of either vortices or domain walls depending on the symmetry class. For the nonchiral models in classes A and AI, a population imbalance between the two atomic species naturally occurs. In these cases, one of the two species is seen to favor a more uniform density. We also study the onset of localization as the disorder strength is increased for the different classes, and by deriving an effective model for the nonchiral cases we show how their eigenstates remain extended for larger values of the coupling with the disorder when compared to the nonchiral ones.

  • 130. Plimak, L. I.
    et al.
    Ivanov, Misha
    Aiello, A.
    Stenholm, S.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Kvantelektronik och -optik, QEO.
    Quantum theory of an electromagnetic observer: Classically behaving macroscopic systems and the emergence of the classical world in quantum electrodynamics2015Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 92, nr 2, artikkel-id 022122Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Quantum electrodynamics under conditions of distinguishability of interactingmatter entities, and of controlled actions and back-actions between them, is considered. Such "mesoscopic quantum electrodynamics" is shown to share its dynamical structure with the classical stochastic electrodynamics. In formal terms, we demonstrate that all general relations of the mesoscopic quantum electrodynamics may be recast in a form lacking Planck's constant. Mesoscopic quantum electrodynamics is therefore subject to "doing quantum electrodynamics while thinking classically," allowing one to substitute essentially classical considerations for quantum ones without any loss in generality. Implications of these results for the quantum measurement theory are discussed.

  • 131. Popov, A K
    et al.
    Kimberg, Victor
    George, T F
    Adiabatic passage and dissociation controlled by interference of two laser-induced continuum structures2003Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 68, nr 3, artikkel-id 033407Artikkel i tidsskrift (Fagfellevurdert)
  • 132. Popov, A. K.
    et al.
    Kimberg, Viktor V.
    KTH, Tidigare Institutioner, Bioteknologi. Institute of Physics, Russian Academy of Sciences, Russian.
    George, T. F.
    Large enhancement of fully resonant sum-frequency generation through quantum control via continuum states2004Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 69, nr 4, s. 043816-1Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The enhancement of resonant sum-frequency generation through quantum control, which employs the continuum states, was discussed. The scheme employs all-resonant coupling and trade-off optimization of the constructive and destructive quantum interference effects in the higher-order and lower-order polarizations controlled by the overlap of two autoionizinglike laser-induced continuum structure. It was found that the analytical solution of coupled density-matrix equations were implemented for analysis of the solution of Maxwell's equation describing four-wave mixing. The results show that the feasibility of the nearly complete conversion of low-frequency radiation to sum frequency radiation was shown for the case of quasiresonant coupling.

  • 133.
    Porta Mana, Piero Giovanni Luca
    KTH, Tidigare Institutioner, Mikroelektronik och informationsteknik, IMIT.
    Consistency of the Shannon entropy in quantum experiments2004Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 69, nr 6, s. 062108-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The consistency of the Shannon entropy, when applied to outcomes of quantum experiments, is analyzed. It is shown that the Shannon entropy is fully consistent and its properties are never violated in quantum settings, but attention must be paid to logical and experimental contexts. This last remark is shown to apply regardless of the quantum or classical nature of the experiments.

  • 134.
    Rachlew (Källne), Elisabeth
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Double ionisation of noble gases.II Formation by photon impact.1976Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 13, s. 1748-1755Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Using synchrotron radiation and a new two-grating monochromator to remove higher-order components, the relative intensities of double-photoionization transitions have been measured in helium, neon, and argon between 70 and 200 eV by ion spectrometry. Extensive tests of a new ion spectrometer were undertaken to eliminate any charge discrimination for the ions. These new results resolve discrepancies existing between previous experimental data and favor, in general, the earliest results of

    Carlson for photon energies higher than 100 eV. For helium and neon they are in good qualitative agreement

    with theoretical calculations that include electron-electron correlations. In particular they agree very well with

    the many-body perturbation-theory approach of Chang and Poe in the case of neon.

  • 135.
    Rius Riu, Jaime
    et al.
    KTH, Tidigare Institutioner                               , Fysik.
    Melero Garcia, Emilio
    KTH, Tidigare Institutioner                               , Fysik.
    Alvarez Ruiz, Jesus
    KTH, Tidigare Institutioner                               , Fysik.
    Erman, Peter
    KTH, Tidigare Institutioner                               , Fysik.
    Hatherly, Paul
    Rachlew, Elisabeth
    KTH, Tidigare Institutioner                               , Fysik.
    Stankiewicz, Marek
    KTH, Tidigare Institutioner                               , Fysik.
    Core-excitation-induced dissociation in CD4 after participator Auger decay2003Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 68, nr 2Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The fragmentation of the CD4 molecule after selective ionization of the 1t(2) and 2a(1) electrons with photons from 70 to 290 eV has been studied with the energy-resolved electron-ion coincidence technique. The mass spectra acquired in coincidence with 1t(2) electrons reveal CD4+, CD3+, and CD2+ fragments, depending on the excitation energy used. The production of CD3+ is strongly enhanced after C 1s excitation to different core excited states, with respect to the production observed after direct ionization of the 1t(2) orbital. This enhancement is correlated with the changes of the molecular geometry when it relaxes from the core-excited state.

  • 136. Romero, J. L.
    et al.
    Björk, Gunnar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och tillämpad fysik, MAP.
    Klimov, A. B.
    Sanchez-Soto, L. L.
    Structure of the sets of mutually unbiased bases for N qubits2005Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 72, nr 6Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    For a system of N qubits, living in a Hilbert space of dimension d=2(N), it is known that there exists d+1 mutually unbiased bases. Different construction algorithms exist, and it is remarkable that different methods lead to sets of bases with different properties as far as separability is concerned. Here we derive four sets of nine bases for three qubits, and show how they are unitarily related. We also briefly discuss the four-qubit case, give the entanglement structure of 16 sets of bases, and show some of them and their interrelations, as examples. The extension of the method to the general case of N qubits is outlined.

  • 137.
    Roos, Johanna
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Larson, Åsa
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    E. Orel, Ann
    Department of Applied Science, University of California.
    Dissociative recombination of HF+2008Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 78, nr 2Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The direct mechanism of dissociative recombination of HF+ is studied using both time-dependent and time-independent methods, where the dynamics on 30 resonant states is explored. The relevant electronic states are calculated ab initio, by combining electron scattering calculations with multireference configuration interaction structure calculations. For collision energies between 0.04 and 10 eV, we obtain qualitative agreement with experiment. At 1.9 eV there is a sharp threshold in both the experimental and theoretical cross sections that can be explained by the opening of new asymptotic limits. The measured cross section below 0.04 eV is not reproduced due to the neglect of the electronic couplings between the neutral states. We examine the validity of the local approximation for treating autoionization from the resonant states included in this study.

  • 138.
    Roos, Johanna
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Larson, Åsa
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    E. Orel, Ann
    Electron collisions with H3+: ion-pair formation2007Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 76, nr 4Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The formation of the ion pair H-2(+)+ H- in electron recombination with H-3(+) is studied. The diabatic potentials and electronic couplings are extracted from ab initio electron scattering calculations as well as quantum chemistry calculations. In order to describe this reaction we include six coupled electronic states and propagate wave packets in two dimensions using the multiconfiguration time-dependent Hartree method. Also, the cross section for ion-pair formation in electron recombination with D-3(+) is calculated. The cross section for this isotopomer is found to be about a factor of 3 smaller than the cross section for H-3(+).

  • 139. Royen, P.
    et al.
    Gurell, J.
    Lundin, P.
    Norlin, Lars-Olov
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Mannervik, S.
    Monitoring the weak collisional excitation of a stored ion beam reveals the radiative decay rate of extremely long-lived metastable levels2007Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 76, nr 3, s. 030502-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A fast ion beam stored in an ion storage ring will suffer from collisional interaction with the residual gas even in extreme ultrahigh vacuum. Normally these effects are negligible or easy to correct for. For measurements of extremely long radiative lifetimes of metastable levels we have found that correction for collisional effects is very important and usually limits the accuracy. We now suggest that by monitoring the process of collisional excitation of the metastable level of interest the lifetime value could be determined directly with improved accuracy. The method is applied to available data from our recent measurement of the 5d D-2(5/2) level in Ba+, yielding a value of 32.0 +/- 2.9 s, thus squeezing the error bar by 35%.

  • 140. Saastamoinen, Kimmo
    et al.
    Turunen, Jari
    Vahimaa, Pasi
    Friberg, Ari T.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och tillämpad fysik, MAP.
    Spectrally partially coherent propagation-invariant fields2009Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 80, nr 5Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Polychromatic propagation-invariant fields (localized waves) with variable spectral coherence properties are considered. A bridge is provided between coherent propagation-invariant pulses, including Bessel pulses, X-waves and focus wave modes, and stationary polychromatic propagation-invariant fields using a difference-frequency spectral-correlation model. In the space-time domain, this assumption leads to a representation of a partially coherent field as a continuum of spectrally fully coherent, mutually independent pulses originating at different instants of time. The influence of partial spectral coherence on temporal coherence and pulse shape as well as intensity distribution and transverse spatial coherence is illustrated by considering fields with propagation-invariant Bessel-mode monochromatic components.

  • 141. Sainz, Isabel
    et al.
    Björk, Gunnar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och tillämpad fysik, MAP.
    Entanglement invariant for the double Jaynes-Cummings model2007Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 76, nr 4Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study entanglement dynamics between four qubits interacting through two isolated Jaynes-Cummings Hamiltonians, via an entanglement measure based on the wedge product. We compare the results with similar results obtained using bipartite concurrence resulting in what is referred to as entanglement sudden death. We find a natural entanglement invariant under evolution, demonstrating that entanglement spreads out over all of the system's degrees of freedom that become entangled through the interaction. We also provide an analysis of why certain initial states lose all their entanglement in a finite time, although their excitation and coherence vanish only asymptotically with time.

  • 142. Sainz, Isabel
    et al.
    Björk, Gunnar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Quantum error correction may delay, but also cause, entanglement sudden death2008Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 77, nr 5Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Dissipation may cause two initially entangled qubits to evolve into a separable state in a finite time. This behavior is called entanglement sudden death (ESD). We study to what extent quantum error correction can combat ESD. We find that in some cases quantum error correction can delay entanglement sudden death but in other cases quantum error correction may cause ESD for states that otherwise do not suffer from it. Our analysis also shows that fidelity may not be the best measure to compare the efficiency of different error correction codes since the fidelity is not directly coupled to a state's remaining entanglement.

  • 143.
    Salek, Pawel
    et al.
    KTH, Tidigare Institutioner, Bioteknologi.
    Fink, F
    Gel'mukhanov, Faris
    KTH, Tidigare Institutioner, Bioteknologi.
    Piancastelli, N
    Feifer, R
    Bassler, M
    Sorensen, L
    Miron, C
    Wang, H
    Hjelte, I
    Bjorneholm, O
    Ausmees, A
    Svensson, S
    Ågren, Hans
    KTH, Tidigare Institutioner, Bioteknologi.
    Resonant x-ray Raman scattering involving avoided crossings in the final-state potential-energy curves2000Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 62, nr 6Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The role of avoided crossings for resonant x-ray Raman scattering is explored. The strong dependence of the electronic transition amplitudes on internuclear distance at avoided crossings causes new, and even anomalous, spectral features, and can bring about a breakdown of the generally accepted participator or spectator classification of the transitions. We report, and apply the theory on, an experimental observation of the electron decay of the N 1s-->pi* core-excited level in N-2 the B(2)Sigma (+)(u) final state with peculiar, non-Franck-Condon, vibrational intensity distributions.

  • 144.
    Salek, Pawel
    et al.
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Gel'mukhanov, Faris
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Ågren, Hans
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Wave-packet dynamics of resonant x-ray Raman scattering: Excitation near the Cl L-II,L-III edge of HCl1999Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 59, nr 2, s. 1147-1159Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A theory of radiative and nonradiative x-ray Raman scattering based on nuclear wave-packet dynamics is presented. The theory is evaluated with special emphasis on the cases when the intermediate and/or final state potentials are dissociative. Different "one-step" and "two-step" time-dependent wave-packet formalisms are proposed and evaluated, giving different interpretational content and computational efficiency. An interference between the molecular background and the narrow atomiclike contributions is predicted and evaluated. Due to this interference the atomiclike spectral feature manifests itself as a peak or as a spectral hole depending on the circumstances in terms of excitation energy, spectral width of incident radiation, and the form of the interatomic potentials. The counterintuitive situation may even arise that a narrow peak is formed by increasing the spectral photon width. The duration of the resonant x-ray Raman scattering influences in a qualitatively different manner the space distributions of the wave packets in the molecular and the dissociative domains, something that is crucial for the formation of the cross section profile. It is shown that the scattering cross section is proportional to the square of the core excited wave packet and inversely proportional to the derivative of the difference between core excited and final state potentials. The atomiclike profile is shown to consist of a Lorentzian inner part and red or blue wings which give direct information about the long-range regions of the potentials; red wings for diverging potentials, and blue for converging. A technique of mapping of the space distribution of the squared core excited wave packet and the interatomic potentials is suggested. The various features are demonstrated by an ab initio computational study of the resonant Auger spectra of the HCl molecule close to the Cl L-II,L-III edge.

  • 145.
    Salek, Pawel
    et al.
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Gel'mukhanov, Faris
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Ågren, Hans
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Björneholm, O
    Svensson, S
    Generalized Franck-Condon principle for resonant photoemission1999Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 60, nr 4, s. 2786-2791Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A generalized Franck-Condon (GFC) principle for resonant x-ray Raman scattering and for resonant photoemission in particular is derived and numerically investigated. The GFC amplitudes differ from ordinary FC amplitudes by the presence of photon and photoelectron phase factors which describe the coupling-or interference-of the x-ray photons or Anger electrons with the nuclear motion. With the GFC amplitudes, a Kramers-Heisenberg relation is obtained for vibronic transitions that corrects the so-called lifetime-vibrational interference formula. For resonant photoemission in the soft-x-ray region involving typical bound potential surfaces, the generalization gives a contribution to the FC factors that can amount to 20%. For core excitation above the dissociation threshold, the GFC principle relates to Doppler effects on the ejected photoelectron both for the so-called "molecular'' and "atomic" bands. The role of the GFC principle in direct photoionization is briefly discussed.

  • 146. Samuelsson, S.
    et al.
    Björk, Gunnar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och tillämpad fysik, MAP.
    Local uncertainty relations serving as measures of entanglement in a bipartite two-level system2006Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 73, nr 1Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We comment on the recent suggestion to use a family of local uncertainty relations as a standard way of quantifying entanglement in two-qubit systems. Some statements made on the applicability of the proposed measures are overly optimistic. We exemplify how these specific measures fall short, and present a minor modification of the general theory which uses the same experimentally gathered information, but in a slightly different, better way.

  • 147. Sanchez-Soto, L. L.
    et al.
    Delgado, J.
    Klimov, A. B.
    Björk, Gunnar
    KTH, Tidigare Institutioner                               , Mikroelektronik och informationsteknik, IMIT.
    Description of entanglement in terms of quantum phase2002Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 66, nr 4Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We explore the role played by the phase in an accurate description of the entanglement of bipartite systems. We first present an appropriate polar decomposition that leads to a truly Hermitian operator for the phase of a single qubit. We also examine the positive operator-valued measures that can describe the qubit phase properties. When dealing with two qubits, the relative phase seems to be a natural variable to understand entanglement. In this spirit, we propose a measure of entanglement based on this variable.

  • 148. Sanchez-Soto, Luis L.
    et al.
    Yustas, Eulogio C.
    Björk, Gunnar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och tillämpad fysik, MAP.
    Klimov, Andrei B.
    Maximally polarized states for quantum light fields2007Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 76, nr 4Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The degree of polarization of a quantum field can be defined as its distance to an appropriate set of states. When we take unpolarized states as this reference set, the states optimizing this degree for a fixed average number of photons (N) over bar present a fairly symmetric, parabolic photon statistic, with a variance scaling as (N) over bar (2). Although no standard optical process yields such a statistic, we show that, to an excellent approximation, a highly squeezed vacuum can be taken as maximally polarized. We also consider the distance of a field to the set of its SU(2) transformed, finding that certain linear superpositions of SU(2) coherent states make this degree to be unity.

  • 149. Sathe, C.
    et al.
    Guimaraes, Freddy Fernandes
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Rubensson, J. -E
    Nordgren, J.
    Agui, A.
    Guo, J.
    Ekstrom, U.
    Norman, P.
    Gel'mukhanov, Faris
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Ågren, Hans
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Resonant L(II,III) x-ray Raman scattering from HCl2006Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 74, nr 6, s. 062512-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We have studied the spectral features of Cl L(II,III) resonant x-ray Raman scattering of HCl molecules in gas phase both experimentally and theoretically. The theory, formulated in the intermediate-coupling scheme, takes into account the spin-orbital and molecular-field splittings in the Cl 2p shells, as well as the Coulomb interaction of the core hole with unoccupied molecular orbitals. Experiment and theory display nondispersive dissociative peaks formed by decay transitions in both molecular and dissociative regions. The molecular and atomic peaks collapse in a single narrow resonance because the dissociative potentials of core-excited and final states are parallel to each other along the whole pathway of the nuclear wave packet.

  • 150. Schef, P
    et al.
    Lundin, P
    Biémont, E
    Källberg, A
    Norlin, Lars-Olov
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnfysik.
    Palmeri, P
    Royen, P
    Simonsson, A
    Mannervik, S
    Disentanglement of magnetic field mixing reveals the spontaneous M2 decay rate for a metastable level in Xe+2005Inngår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 72, nr 2, s. 020501-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We have investigated the radiative decay of the metastable level 5d(4)D(7/2) in Xe+. Theoretically we find the decay to be heavily dominated by an M2 transition and not by M1/E2 transitions. Lifetime measurements of 5d(4)D(7/2) in a storage ring are difficult since magnetic mixing of the metastable with a short-lived level quenches its population. Decay rates were determined at different magnetic field strengths (B) in order to allow a nonlinear extrapolation to B=0. The experimental lifetime of 2.4 +/- 0.8 s was in agreement with the calculated value, but much smaller than previously estimated.

1234 101 - 150 of 189
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