Search

We use a rectangular pulse to freeze the possibility of estimating the coherent parameters ([theta], [phi]) of a single qubit, the local information, and the fidelity of the driven qubit. The possibility of freezing these three quantities depends on the initial state settings and the strength of the pulse. It is shown that the freezing areas increase as the detuning increases, while they decrease at smaller values of the pulse strength. Moreover, as the Fisher information reaches its minimum bounds, the local information and the fidelity are maximized. We show that, when the Fisher information is completely frozen, the pulsed local information and the fidelity are frozen. The freezing degree depends on the initial detuning and the pulse strength. Our results show that, despite the initial and final states of the system being different, the amount of local information is independent of the initial state of the system. These results may be useful in the context of quantum cryptography, teleportation, and secure communication. Key words: Fisher information, driven qubit, pulses, freezing, fidelity. Nous utilisons une impulsion rectangulaire pour figer la possibilite d'estimer les parametres de coherence ([theta], [phi]) d'un qubit simple, l'information locale et la fidelite du qubit entraine. La possibilite de figer ces trois quantites depend des reglages de l'etat initial et de l'intensite de l'impulsion. Nous montrons que l'ampleur du gel augmente si la desyntonisation croit, alors qu'elle diminue aux plus petites amplitudes de l'intensite de l'impulsion. De plus, puisque l'information de Fisher atteint ses limites minimales, l'information locale et la fidelite sont maximisees. Nous demontrons que lorsque l'information de Fisher est completement figee, l'information locale pulsee et la fidelite sont egalement figees. Le degre de gel depend de la desyntonisation initiale et de l'intensite de l'impulsion. Nos resultats montrent que, malgre le fait que les etats initial et final du systeme soient differents, la quantite d'information locale est independante de l'etat initial du systeme. Ces resultats peuvent etre utiles pour la cryptographie quantique, la teleportation et les communications securitaires. [Traduit par la Redaction] Mots-cles: information de Fisher, qubit entraine, impulsions, fidelite., 1. Introduction It is well known that quantum information tasks (e.g., quantum cryptography, quantum encoding [1, 2], and quantum computation [3]) require pure states to be implemented with high efficiencies. [...]

An iterative approach, not restricted to field strength nor to resonance conditions, is used to solve the nonautonomous Bloch equations modeling the interaction of a bichromatically driven two-level atom in the presence of an off-resonant broadband squeezed vacuum (SV) radiation reservoir. The system parameters are chosen such that the absolute difference between the iterative and (exact) numerical solutions is O([10.sup.-2]) or less. For some limiting cases, range of validity of the iterative procedure is estimated analytically. The derived analytical solutions are used to calculate the transient fluorescent spectrum. Some previously obtained results (both experimental and theoretical) in the normal vacuum case are recovered. In the SV case, with strong resonant (off-resonant) bichromatic field strength, the transient spectrum shows many symmetrical (asymmetrical) resonances of weights and locations dependent on the SV detuning parameter. PACS Nos: 32.50.+d, 42.50.p, 42.50.ct Nous utilisons une approche iterative, sans restriction sur l'intensite du champ ni les conditions de resonance, pour resoudre l'equation de Bloch non autonome qui decrit l'interaction d'un atome a deux niveaux stimule bichromatiquement avec un reservoir de radiation du vide comprime (SV) a large bande et hors resonance. Les parametres du systeme sont choisis de telle sorte que la difference absolue entre les resultats iteratifs et les resultats numeriques exacts soit de l'ordre de O([10.sup.-2]) ou moins. Pour quelques cas limites, nous estimons analytiquement le domaine de validite de la procedure iterative. Les solutions analytiques obtenues sont utilisees pour calculer le spectre de fluorescence transitoire. Nous retrouvons certains resultats deja obtenus (experimentalement et theoriquement) dans le cas du vide normal. Dans le cas du SV, avec une forte intensite de champ bichromatique resonant (hors resonance), le spectre transitoire montre plusieurs resonances symetriques (asymetriques) de poids et de position qui dependent du parametre de desyntonisation. [Traduit par la Redaction], 1. Introduction Atomic energy-level splitting is a direct response of atomic systems to an exciting field. The spectral nature of the scattered radiation due to atom ⊕ field interaction is [...]

The non-autonomous Bloch equations modelling a driven 2-level atom in the presence of an off-resonant broadband squeezed vacuum (SV) field is treated analytically. This concerns iterative solutions valid for large SV detuning parameter but for arbitrary strength of the laser field. Computational results are presented for the averaged atomic variables for various data and compared with the resonant SV field case. The iterated analytical results for nonzero SV detuning are compared with the (exact) numerical solutions of the Bloch equations, hence we have an insight about the range of other system parameters (other than the Rabi frequency) for which the iterated solutions are valid to O([10.sup.-2]) or less. The main purpose of deriving these analytical results is to calculate analytically the transient fluorescent spectrum. For an initially ground-state atom, both the SV phase and detuning parameters induce pronounced asymmetrical spectrum in the strong field case. PACS Nos: 32.50.+d, 42.50.-p, 42.50.ct Nous etudions analytiquement les equations de Bloch qui modeelisent un atome a deux niveaux soumis a un champ laser en presence d'un champ du vide (SV) comprimee a large bande et hors resonance. Ceci touche les solutions iteratives pour de grands parametres de desyntonisation du SV, mais pour une intensite arbitraire du champ laser. Les resultats calcules sont presentes pour des variables atomiques moyennes pour diffeerentes donneees et compares avec le cas du champ SV resonant. Nous comparons les resultats analytiques iteeratifs pour un SV non nul avec les solutions numeriques (exactes) des equations de Bloch, ce qui nous donne un apenju du domaine des parametres du systeme autres que la frequence de Rabi, pour lequel les solutions iteratives sont valides a une precision de 0([10.sup.-2]) ou mieux. Le but principal de ces resultats analytiques est de calculer analytiquement le spectre de fluorescence transitoire. Pour un atome initialement dans le fondamental, la phase SV et les parametres de desyntonisation peuvent tous deux conduire a un spectre fortement asymetrique en champ fort. [Traduit par la Redaction], 1. Introduction From the early days of quantum optics, theoretical studies of the interaction of radiation fields with the simplest atomic structure, namely a two-level atom, has provided valuable information [...]

Preparedness for a possible influenza pandemic caused by highly pathogenic avian influenza A subtype H5N1 has become a global priority. The spread of the virus to Europe and continued human infection in Southeast Asia have heightened pandemic concern. It remains unknown from where the pandemic strain may emerge; current attention is directed at Vietnam, Thailand, and, more recently, Indonesia and China. Here, we report that genetically and antigenically distinct sublineages of H5N1 virus have become established in poultry in different geographical regions of Southeast Asia, indicating the long-term endemicity of the virus, and the isolation of H5N1 virus from apparently healthy migratory birds in southern China. Our data show that H5N1 influenza virus, has continued to spread from its established source in southern China to other regions through transport of poultry and bird migration. The identification of regionally distinct sublineages contributes to the understanding of the mechanism for the perpetuation and spread of H5N1, providing information that is directly relevant to control of the source of infection in poultry. It points to the necessity of surveillance that is geographically broader than previously supposed and that includes H5N1 viruses of greater genetic and antigenic diversity. genetics | human | influenza A | virus evolution | avian