Comment on "Effect of entanglement on the decay dynamics of a pair of H(2p) atoms due to spontaneous emission"

Tanabe et al (Phys. Rev. A {\bf 82} 040101(R) 2010) have experimentally demonstrated that the emission properties of unstable atoms in entangled and product states are different. The authors define an apparent decay time as a fitting parameter which falls below the lifetime of the single atom for entangled pairs. We argue that their results about coincidence time spectra are correct, but those concerning lifetimes cannot be considered conclusive because they assume the emission of photons by the two atoms to be independent processes, a doubtful hypothesis for entangled states. We suggest an improved evaluation of the lifetimes based on a rigorous approach, which demands some modifications of the experimental procedure

Metastable superpositions of ortho- and para-Helium states

We analyze superpositions of ortho- and para-Helium states, considering the possible existence of stationary and metastable states in the system. In particular, the metastable superposition of 1s2s ortho and para states seems to be accessible to experimental scrutiny

Entanglement of unstable atoms: modifications of the emission properties

We analyse the influence of entanglement on the emission properties of atoms. To this end, first, we propose a scheme for the preparation of a pair of entangled Helium atoms, one in the ortho and the other in the para spin configuration. We discuss a realistic scenario for this process, based in the double ionization of He by intense laser fields. These states are used to analyse disentanglement and the role of entanglement in the spontaneous emission from the pair. In particular, we show that the decaying rate of an entangled atom is different from that in a product state, modifying the temporal emission distribution and lifetime of the atoms.Comment: Accepted in JP

Lithium Ionization by a Strong Laser Field

We study ab initio computations of the interaction of Lithium with a strong laser field. Numerical solutions of the time-dependent fully-correlated three-particle Schroedinger equation restricted to the one-dimensional soft-core approximation are presented. Our results show a clear transition from non-sequential to sequential double ionization for increasing intensities. Non sequential double ionization is found to be sensitive to the spin configuration of the ionized pair. This asymmetry, also found in experiments of photoionization of Li with synchrotron radiation, shows the evidence of the influence of the exclusion principle in the underlying rescattering mechanism

Resolving multiple rescatterings in high-order-harmonic generation

We explore the conditions for resolving high-order electronic recollisions in high-order-harmonic spectroscopy. We identify intrinsic phase mismatch and time-frequency uncertainty as the two fundamental limitations against the spectral distinguishability of these multiple rescatterings. Our numerical computations show that flat-top mid-infrared driving fields are the optimal candidates for the study of multiple recollision phenomena.This research was supported by a Marie Curie International outgoing fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007–2013), under REA Grant Agreement No. 328334. We acknowledge support from Junta de Castilla y León (Project SA116U13, UIC016) and MINECO(FIS2013-44174-P, FIS2015-71933-REDT). This work utilized the Janus supercomputer,which is supported by the US National Science Foundation (Award No. CNS-0821794) and the University of Colorado Boulder

Invalidity of the Ehrenfest theorem in the computation of high-order-harmonic generation within the strong-field approximation

[EN]It is known that the strong-field approximations commonly used in models for computing high-order-harmonic generation invalidate the Ehrenfest theorem. Therefore, the time derivative of the atomic dipole or of the dipole velocity does not correspond to the dipole acceleration. We study the consequences of this invalidation for the quantitative evaluation of high-order-harmonic spectra in hydrogen at different wavelengths and intensities. As a main result, we propose a form for the time derivative of the kinetic momentum that allows a quantitatively accurate computation of the acceleration spectra using the dipole-velocity matrix elementWe acknowledge support from the Spanish Ministerio de Economıa y Competitividad through the Consolider program SAUUL (Grant No. CSD2007-0013) and Research Project No. FIS2009-09522 and also from the Centro de Laseres Pulsados (CLPU), Salamanca

Off-axis compensation of attosecond pulse chirp

By performing calculations on the generation of attosecond pulses by intense laser fields, we demonstrate that the pulse temporal width is sensitive to the angle of emission. We show that this effect results from the interference of the short and long electronic trajectory contributions, responsible for high-order harmonic generation. In particular, we find that the shortest pulses are emitted off-axis, which are substantially shorter than those emitted on-axis, where single trajectory contributions dominate.We acknowledge support from Spanish Ministerio de Ciencia e Innovación through the Consolider Program SAUUL (CSD2007-00013) and Research project FIS2009-09522, from Junta de Castilla y León through the Program for Groups of Excellence (GR27) and grant agreement ORDEN EDU/1708/2008, and from the EC's Seventh Framework Programme (LASERLAB-EUROPE, grant agreement 228334). LP acknowledges UK EPSRC-CAF grant EP/J002348/1 for supporting this research. We also acknowledge support from the Centro de Láseres Pulsados, CLPU, Salamanca, Spai

Comment on ‘On the dipole, velocity and acceleration forms in high-order harmonic generation from a single atom or molecule’

Authors acknowledge support from Spanish Ministerio de Ciencia e Innovacio ́n through the Consolider Program SAUUL (CSD2007-00013) and research project FIS2009-09522, from Junta de Castilla y Leo ́n through the Program for Groups of Excellence (GR27) and from the EC’s Seventh Framework Programme (LASERLAB-EUROPE, grant agreement no 228334)We show that the main conclusion of Baggesen and Madsen (2011 J. Phys. B: At. Mol. Opt. Phys. 44 115601) is not general but only valid in one dimension

Preparación de prácticas de laboratorio de las asignaturas "óptica cuántica" y "óptica avanzada" impartidas en los másteres en "física y tecnología de los láseres" y "física"

Memoria ID-0131. Ayudas de la Universidad de Salamanca para la innovación docente, curso 2013-2014

Theory of high-order harmonic generation for gapless graphene

[EN]We study the high-harmonic spectrum emitted by a single-layer graphene, irradiated by an ultrashort intense infrared laser pulse. We show the emergence of the typical non-perturbative spectral features, harmonic plateau and cut-off, for mid-infrared driving fields, at fluences below the damage threshold. In contrast to previous works, using THz drivings, we demonstrate that the harmonic cut-off frequency saturates with the intensity. Our results are derived from the numerical integration of the time-dependent Schrödinger equation using a nearest neighbor tight-binding description of graphene. We also develop a saddle-point analysis that reveals a mechanism for harmonic emission in graphene different from that reported in atoms, molecules and finite gap solids. In graphene, the first step is initiated by the non-diabatic crossing of the valence band electron trajectories through the Dirac points, instead of tunneling ionization/excitation. We include a complete identification of the trajectories contributing to any particular high harmonic and reproduce the harmonic cut-off scaling with the driving intensity.We acknowledge fruitful discussions with I J Sola, H Crespo, E Pinsanty, J Biegert, J M Pérez-Iglesias, R Rengel, M J Martín and C Hernández García. We acknowledge support from Junta de Castilla y León (Project SA046U16) and MINECO (FIS2013-44174-P, FIS2016-75652-P) and European Union (FEDER). AP acknowledges support from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 70256