Quantum information processing in the wall of cytoskeletal microtubules

Microtubules (MT) are composed of 13 protofilaments, each of which is a series of two-state tubulin dimers. In the MT wall, these dimers can be pictured as "lattice" sites similar to crystal lattices. Based on the pseudo-spin model, two different location states of the mobile electron in each dimer are proposed. Accordingly, the MT wall is described as an anisotropic two-dimensional (2D) pseudo-spin system considering a periodic triangular "lattice". Because three different "spin-spin" interactions in each cell exist periodically in the whole MT wall, the system may be shown to be an array of three types of two-pseudo-spin-state dimers. For the above-mentioned condition, the processing of quantum information is presented by using the scheme developed by Lloyd.

Quantum computation based on a quantum switch in cavity QED

A feasible scheme for constructing quantum logic gates is proposed on the basis of quantum switches in cavity QED. It is shown that the light field which is fed into the cavity due to the passage of an atom in a certain state can be used to manipulate the conditioned quantum logical gate. In our scheme, the quantum information is encoded in the states of Rydberg atoms and the cavity mode is not used as logical qubits or as a communicating "bus"; thus, the effect of atomic spontaneous emission can be neglected and the strict requirements for the cavity can be relaxed.

Effects of relative phase in an open ladder system without incoherent pumping

We studied effects of the relative phase between the probe and driving fields on the absorption and dispersion properties in an open three-level ladder system with spontaneously generated coherence but without incoherent pumping. It is shown that by the phase controlling, switching from absorption to lasing without inversion (LWI) and enhancing remarkablely LWI gain can be realized; large index of refraction with zero absorption and the electromagnetically induced transparency can be obtained. We also find that varying the atomic injection and exit rates has a considerable influence on the phase dependent-absorption property of the probe field, existent of the atomic injection and exit rates gives the necessary condition of the realization of LWI, getting LWI is impossible in the corresponding closed system without incoherent pumping. We studied effects of the relative phase between the probe and driving fields on the absorption and dispersion properties in an open three-level ladder system with spontaneously generated coherence but without incoherent pumping. It is shown that by the phase controlling, switching from absorption to lasing without inversion (LWI) and enhancing remarkablely LWI gain can be realized; large index of refraction with zero absorption and the electromagnetically induced transparency can be obtained. We also find that varying the atomic injection and exit rates has a considerable influence on the phase dependent-absorption property of the probe field, existent of the atomic injection and exit rates gives the necessary condition of the realization of LWI, getting LWI is impossible in the corresponding closed system without incoherent pumping.

Two-colour laser-induced electron dynamics in two-level quantum system

To attempt to control the quantum state of a physical system with a femtosecond two-colour laser field, a model for the two-level system is analysed as a first step. We investigate the coherent control of the two-colour laser pulses propagating in a two-level medium. Based on calculating the influence of the laser field with various laser parameters on the electron dynamics, it is found the electronic state can be changed up and down by choosing the appropriate laser pulses and the coherent control of the two-colour laser pulses can substantially modify the behaviour of the electronic dynamics: a quicker change of two states can be produced even for small pulse duration. Moreover, the oscillatory structures around the resonant frequency and the propagation features of the laser pulses depend sensitively on the relative phase of the two-colour laser pulses. Finally, the influence of a finite lifetime of the upper level is discussed in brief.

Effects on the proton production from explosions of hydrogen clusters subjected to intense femtosecond laser fields

The dependence of the maximum and average energies of protons, which were produced in the interaction of an intense laser pulse (similar to 1 x 10(16) W cm(-2), 65 fs) with hydrogen clusters in a gas jet backed up to 80 bar at liquid nitrogen temperature (similar to 80 K), on the backing pressure has been studied. The general trend of the proton energy dependence on the square of the average cluster radius, which is determined by a calibrated Rayleigh scattering measurement, is similar to that described by theory under the single size approximation. Calculations are made to fit the experimental results under a simplified model by taking into account both a log-normal cluster size distribution and the laser intensity attenuation in the interaction volume. A very good agreement between the experimental proton energy spectra and the calculations is obtained in the high- energy part of the proton energy distributions, but a discrepancy of the fits is revealed in the low-energy part at higher backing pressures which are associated with denser flows. A possible mechanism which would be responsible for this discrepancy is discussed. Finally, from the fits, a variation of the cluster size distributions was revealed to be dependent on the gas backing pressure as well as on the evolving time of the gas flow of clusters.

Tunneling-induced large cross-phase modulation in an asymmetric quantum well

We propose an asymmetric double AlGaAs/GaAs quantum well structure with a common continuum to generate a large cross-phase modulation (XPM). It is found, owing to resonant tunneling, that a large XPM can be achieved with vanishing linear and two-photon absorptions. (c) 2007 Optical Society of America.

Generation of attosecond pulses in a system with permanent dipole moment

The generation of attosecond pulses in a two-level system with permanent dipole moment is investigated. It is shown due to the presence of permanent dipole moments, that the plateau of the high-order harmonic generation spectrum can be extended to X-ray range. Moreover, attosecond pulses with higher intensity can be synthesized by using both even and odd harmonics because of their quantum interference. (c) 2006 Elsevier B.V. All rights reserved.

Electromagnetically induced negative refractive index in a V-type four-level atomic system

We propose a scheme for realizing negative refractive index in a V-type four-level atomic system. It is shown that the negative refractive index can be achieved in a wide frequency band based on the effect of quantum coherence. It is also found that the frequency band of negative refractive index and the absorption property of left-handed material are manipulated by the pump and control fields. Furthermore, left-handed material with reduced absorption is possible by choosing appropriate parameters. (c) 2006 Elsevier B.V. All rights reserved.

Quantum path control in few-optical-cycle regime

We theoretically show that selection of a single quantum path in high-order harmonics generation can be realized in a few-optical-cycle regime with two-color schemes. We also demonstrate, in theory as well, the generation of spectrally smooth and ultrabroad extreme ultraviolet supercontinuum in argon gas which can produce single similar to 79 as pulses with currently available ultrafast laser sources. Our finding can be beneficial for generating isolated sub-100 as extreme ultraviolet pulses.

Generation of strong quasistatic magnetic fields in interactions of ultraintense and short laser pulses with overdense plasma targets

An analytical fluid model is proposed for the generation of strong quasistatic magnetic fields during normal incidence of a short ultraintense Gaussian laser pulse with a finite spot size on an overdense plasma. The steepening of the electron density profile in the originally homogeneous overdense plasma and the formation of electron cavitation as the electrons are pushed inward by the laser are included self-consistently. It is shown that the appearance of the cavitation plays an important role in the generation of quasistatic magnetic fields: the strong plasma inhomogeneities caused by the formation of the electron cavitation lead to the generation of a strong axial quasistatic magnetic field B-z. In the overdense regime, the generated quasistatic magnetic field increases with increasing laser intensity, while it decreases with increasing plasma density. It is also found that, in a moderately overdense plasma, highly intense laser pulses can generate magnetic fields similar to 100 MG and greater due to the transverse linear mode conversion process.

Polaron effects on the optical absorptions in asymmetrical semi-parabolic quantum wells

The linear and nonlinear optical absorptions considering the weak-coupling electron-LO-phonon interaction in asymmetrical semiparabolic quantum wells are theoretically investigated. The numerical results for the typical GaAs/AlxGa1-xAs material show that the factors of Al content x, the relaxation time and the photon energy have great influence on the optical absorption coefficients. Moreover, the theoretical values of the optical absorptions are more than a factor of 2-3 higher than the one in the structure without considering the electron-LO-phonon interaction by calculating. (C) 2007 Elsevier B.V. All rights reserved.