452 research outputs found
Targeting qubit states using open-loop control
We present an open-loop (bang-bang) scheme which drives an open two-level
quantum system to any target state, while maintaining quantum coherence
throughout the process. The control is illustrated by a realistic simulation
for both adiabatic and thermal decoherence. In the thermal decoherence regime,
the control achieved by the proposed scheme is qualitatively similar, at the
ensemble level, to the control realized by the quantum feedback scheme of Wang,
Wiseman, and Milburn [Phys. Rev. A 64, #063810 (2001)] for the spontaneous
emission of a two-level atom. The performance of the open-loop scheme compares
favorably against the quantum feedback scheme with respect to robustness,
target fidelity and transition times.Comment: 27 pages, 7 figure
Discovery of ultra-fast outflows in a sample of Broad Line Radio Galaxies observed with Suzaku
We present the results of a uniform and systematic search for blue-shifted Fe
K absorption lines in the X-ray spectra of five bright Broad-Line Radio
Galaxies (BLRGs) observed with Suzaku. We detect, for the first time at X-rays
in radio-loud AGN, several absorption lines at energies greater than 7 keV in
three out of five sources, namely 3C 111, 3C 120 and 3C 390.3. The lines are
detected with high significance according to both the F-test and extensive
Monte Carlo simulations. Their likely interpretation as blue-shifted Fe XXV and
Fe XXVI K-shell resonance lines implies an origin from highly ionized gas
outflowing with mildly relativistic velocities, in the range 0.04-0.15c. A fit
with specific photo-ionization models gives ionization parameters in the range
log_xi~4-5.6 and column densities of N_H~10^22-10^23 cm^-2. These
characteristics are very similar to those of the Ultra-Fast Outflows (UFOs)
previously observed in radio-quiet AGN. Their estimated location within
~0.01-0.3pc from the central super-massive black hole suggests a likely origin
related with accretion disk winds/outflows. Depending on the absorber covering
fraction, the mass outflow rate of these UFOs can be comparable to the
accretion rate and their kinetic power can correspond to a significant fraction
of the bolometric luminosity and is comparable to their typical jet power.
Therefore, these UFOs can play a significant role in the expected feedback from
the AGN on the surrounding environment and can give us further clues on the
relation between the accretion disk and the formation of winds/jets in both
radio-quiet and radio-loud AGN.Comment: Accepted for publication in The Astrophysical Journal; corrected
reference
Quantum State Reconstruction of a Bose-Einstein Condensate
We propose a tomographic scheme to reconstruct the quantum state of a
Bose-Einstein condensate, exploiting the radiation field as a probe and
considering the atomic internal degrees of freedom. The density matrix in the
number state basis can be directly retrieved from the atom counting
probabilities.Comment: 11 pages, LaTeX file, no figures, to appear in Europhysics Letter
Optical and electrical behavior of synthetic melanin thin films spray-coated
AbstractWe investigated the optical and the electrical conductivity properties of synthetic melanin thin films spray-coated on glass. These films showed a broadband monotonic increase of the absorption coefficient, decreasing the wavelength in the Visible-NIR range. Conductivity as a function of the temperature evidenced a semiconductor like character and a hysteretic behaviour after thermal annealing up to 475 K. Thermal activation energies extrapolated by resistance curves have been explained by using the framework of a band-model as for an amorphous semiconductor
Pulse Control of Decoherence with Population Decay
The pulse control of decoherence in a qubit interacting with a quantum
environment is studied with focus on a general case where decoherence is
induced by both pure dephasing and population decay. To observe how the
decoherence is suppressed by periodic pi pulses, we present a simple method to
calculate the time evolution of a qubit under arbitrary pulse sequences
consisting of bit-flips and/or phase-flips. We examine the effectiveness of the
two typical sequences: bb sequence consisting of only bit-flips, and bp
sequence consisting of both bit- and phase-flips. It is shown that the
effectiveness of the pulse sequences depends on a relative strength of the two
decoherence processes especially when a pulse interval is slightly shorter than
qubit-environment correlation times. In the short-interval limit, however, the
bp sequence is always more effective than, or at least as effective as, the bb
sequence.Comment: 11 pages, 7 figure
Optomechanical sideband cooling of a thin membrane within a cavity
We present an experimental study of dynamical back-action cooling of the
fundamental vibrational mode of a thin semitransparent membrane placed within a
high-finesse optical cavity. We study how the radiation pressure interaction
modifies the mechanical response of the vibrational mode, and the experimental
results are in agreement with a Langevin equation description of the coupled
dynamics. The experiments are carried out in the resolved sideband regime, and
we have observed cooling by a factor 350 We have also observed the mechanical
frequency shift associated with the quadratic term in the expansion of the
cavity mode frequency versus the effective membrane position, which is
typically negligible in other cavity optomechanical devices.Comment: 15 pages, 7 figure
Generating continuous variable quantum codewords in the near-field atomic lithography
Recently, D. Gottesman et al. [Phys. Rev. A 64, 012310 (2001)] showed how to
encode a qubit into a continuous variable quantum system. This encoding was
realized by using non-normalizable quantum codewords, which therefore can only
be approximated in any real physical setup. Here we show how a neutral atom,
falling through an optical cavity and interacting with a single mode of the
intracavity electromagnetic field, can be used to safely encode a qubit into
its external degrees of freedom. In fact, the localization induced by a
homodyne detection of the cavity field is able to project the near-field atomic
motional state into an approximate quantum codeword. The performance of this
encoding process is then analyzed by evaluating the intrinsic errors induced in
the recovery process by the approximated form of the generated codeword.Comment: 9 pages, 5 figure
Interference effects in f-deformed fields
We show how the introduction of an algeabric field deformation affects the
interference phenomena. We also give a physical interpretation of the developed
theory.Comment: 6 pages, Latex file, no figures, accepted by Physica Script
Speed limits for radiation-driven SMBH winds
Context. Ultra-fast outflows (UFOs) have become an established feature in analyses of the X-ray spectra of active galactic nuclei (AGN). According to the standard picture, they are launched at accretion disc scales with relativistic velocities, up to 0.3-0.4 times the speed of light. Their high kinetic power is enough to induce an efficient feedback on a galactic scale, possibly contributing to the co-evolution between the central supermassive black hole (SMBH) and the host galaxy. It is, therefore, of paramount importance to gain a full understanding of UFO physics and, in particular, of the forces driving their acceleration and the relation to the accretion flow from which they originate.Aims. In this paper, we investigate the impact of special relativity effects on the radiative pressure exerted onto the outflow. The radiation received by the wind decreases for increasing outflow velocity, v, implying that the standard Eddington limit argument has to be corrected according to v. Due to the limited ability of the radiation to counteract the black hole gravitational attraction, we expect to find lower typical velocities with respect to the non-relativistic scenario.Methods. We integrated the relativistic-corrected outflow equation of motion for a realistic set of starting conditions. We concentrated on a range of ionisations, column densities, and launching radii consistent with those typically estimated for UFOs. We explore a one-dimensional, spherical geometry and a three-dimensional setting with a rotating, thin accretion disc.Results. We find that the inclusion of special relativity effects leads to sizeable differences in the wind dynamics and that v is reduced up to 50% with respect to the non-relativistic treatment. We compare our results with a sample of UFOs from the literature and we find that the relativistic-corrected velocities are systematically lower than the reported ones, indicating the need for an additional mechanism, such as magnetic driving, to explain the highest velocity components. Finally, we note that these conclusions, derived for AGN winds, are generally applicable
The Suzaku view of 3C 382
We present a long Suzaku observation of 3C 382. A Swift BAT spectrum from the
58-month survey is also analyzed, together with an archival XMM-Newton EPIC
exposure. Our main result is the finding with Suzaku of a broad FeK line with a
relativistic profile consistent with emission from an accretion disk at tens of
gravitational radii from the central black hole. The XIS data indicate emission
from highly ionized iron and allow us to set tight, albeit model-dependent,
constraints on the inner and outer radii of the disk reflecting region,
r_in~10r_g and r_out~20r_g, respectively, and on the disk inclination, i~30deg.
Two ionized reflection components are possibly observed, with similar
contributions of ~10% to the total continuum. A highly ionized one, with
log_xi~3, which successfully models the relativistic line and a mildly ionized
one, with log_xi~1.5, which models the narrow Fe K line and high energy hump.
When both these components are included, there is no further requirement for an
additional black body soft excess below 2keV. The Suzaku data confirm the
presence of a warm absorber previously known from grating studies. After
accounting for all the spectral features, the intrinsic photon index of the
X-ray continuum is ~1.8 with a cutoff energy at ~200keV, consistent with
Comptonization models and excluding jet-related emission up to these energies.
Comparison of the X-ray properties of 3C 382 and other BLRGs to Seyferts
confirms the idea that the distinction between radio-loud and radio-quiet AGN
at X-rays is blurred.Comment: Accepted for publication in Ap
- …