277 research outputs found
Statistics of Velocity from Spectral Data: Modified Velocity Centroids
We address the problem of studying interstellar turbulence using spectral
line data. We find a criterion when the velocity centroids may provide
trustworthy velocity statistics. To enhance the scope of centroids
applications, we construct a measure that we term ``modified velocity
centroids'' (MVCs) and derive an analytical solution that relates the 2D
spectra of the modified centroids with the underlying 3D velocity spectrum. We
test our results using synthetic maps constructed with data obtained through
simulations of compressible magnetohydrodynamical (MHD) turbulence. We show
that the modified velocity centroids (MVCs) are complementary to the the
Velocity Channel Analysis (VCA) technique. Employed together, they make
determining of the velocity spectral index more reliable and for wider variety
of astrophysical situations.Comment: 4 pages, 1 figure, Accepted for publication in ApJ Letters. minor
change
Detailed studies of non-linear magneto-optical resonances at D1 excitation of Rb-85 and Rb-87 for partially resolved hyperfine F-levels
Experimental signals of non-linear magneto-optical resonances at D1
excitation of natural rubidium in a vapor cell have been obtained and described
with experimental accuracy by a detailed theoretical model based on the optical
Bloch equations. The D1 transition of rubidium is a challenging system to
analyze theoretically because it contains transitions that are only partially
resolved under Doppler broadening. The theoretical model took into account all
nearby transitions, the coherence properties of the exciting laser radiation,
and the mixing of magnetic sublevels in an external magnetic field and also
included averaging over the Doppler profile. Great care was taken to obtain
accurate experimental signals and avoid systematic errors. The experimental
signals were reproduced very well at each hyperfine transition and over a wide
range of laser power densities, beam diameters, and laser detunings from the
exact transition frequency. The bright resonance expected at the F_g=1 -->
F_e=2 transition of Rb-87 has been observed. A bright resonance was observed at
the F_g=2 --> F_e=3 transition of Rb-85, but displaced from the exact position
of the transition due to the influence of the nearby F_g=2 --> F_e=2
transition, which is a dark resonance whose contrast is almost two orders of
magnitude larger than the contrast of the bright resonance at the F_g=2 -->
F_e=3 transition. Even in this very delicate situation, the theoretical model
described in detail the experimental signals at different laser detunings.Comment: 11 pages, 9 figure
Observing the spin of a free electron
Long ago, Bohr, Pauli, and Mott argued that it is not, in principle, possible to measure the spin components of a free electron. One can try to use a Stern-Gerlach type of device, but the finite size of the beam results in an uncertainty of the splitting force that is comparable with the gradient force. The result is that no definite spin measurement can be made. Recently there has been a revival of interest in this problem, and we will present our own analysis and quantum-mechanical wave-packet calculations which suggest that a spin measurement is possible for a careful choice of initial conditions
Determination of the phase of an electromagnetic field via incoherent detection of fluorescence
We show that the phase of a field can be determined by incoherent detection
of the population of one state of a two-level system if the Rabi frequency is
comparable to the Bohr frequency so that the rotating wave approximation is
inappropriate. This implies that a process employing the measurement of
population is not a square-law detector in this limit. We discuss how the
sensitivity of the degree of excitation to the phase of the field may pose
severe constraints on precise rotations of quantum bits involving low-frequency
transitions. We present a scheme for observing this effect in an atomic beam,
despite the spread in the interaction time.Comment: 4 pages, 2 fig
Double Occupancy Errors in Quantum Computing Operations: Corrections to Adiabaticity
We study the corrections to adiabatic dynamics of two coupled quantum dot
spin-qubits, each dot singly occupied with an electron, in the context of a
quantum computing operation. Tunneling causes double occupancy at the
conclusion of an operation and constitutes a processing error. We model the
gate operation with an effective two-level system, where non-adiabatic
transitions correspond to double occupancy. The model is integrable and
possesses three independent parameters. We confirm the accuracy of Dykhne's
formula, a nonperturbative estimate of transitions, and discuss physically
intuitive conditions for its validity. Our semiclassical results are in
excellent agreement with numerical simulations of the exact time evolution. A
similar approach applies to two-level systems in different contexts
Velocity Modification of Power Spectrum from Absorbing Medium
Quantitative description of the statistics of intensity fluctuations within
spectral line data cubes introduced in our earlier work is extended to the
absorbing media. A possibility of extracting 3D velocity and density statistics
from both integrated line intensity as well as from the individual channel maps
is analyzed. We find that absorption enables the velocity effects to be seen
even if the spectral line is integrated over frequencies. This regime that is
frequently employed in observations is characterized by a non-trivial relation
between the spectral index of velocities and the spectral index of intensity
fluctuations. For instance when density is dominated by fluctuations at large
scales, i.e. when correlations scale as r^{-\gamma}, \gamma<0, the intensity
fluctuations exhibit a universal spectrum of fluctuations ~K^{-3} over a range
of scales. When small scale fluctuations of density contain most of the energy,
i.e. when correlations scale as r^{-\gamma}, \gamma>0, the resulting spectrum
of the integrated lines depends on the scaling of the underlying density and
scales as K^{-3+\gamma}. We show that if we take the spectral line slices that
are sufficiently thin we recover our earlier results for thin slice data
without absorption. As the result we extend the Velocity Channel Analysis (VCA)
technique to optically thick lines enabling studies of turbulence in molecular
clouds. In addition, the developed mathematical machinery enables a
quantitative approach to solving other problems that involved statistical
description of turbulence within emitting and absorbing gas.Comment: 51 page, 3 figures. Accepted to Astrophysical Journa
Molecular Emission Line Formation in Prestellar Cores
We investigate general aspects of molecular line formation under conditions
which are typical of prestellar cores. Focusing on simple linear molecules, we
study formation of their rotational lines by radiative transfer simulations. We
present a thermalization diagram to show the effects of collisions and
radiation on the level excitation. We construct a detailed scheme (contribution
chart) to illustrate the formation of emission line profiles. This chart can be
used as an efficient tool to identify which parts of the cloud contribute to a
specific line profile. We show how molecular line characteristics for uniform
model clouds depend on hydrogen density, molecular column density, and kinetic
temperature. The results are presented in a 2D plane to illustrate cooperative
effects of the physical factors. We also use a core model with a non-uniform
density distribution and chemical stratification to study the effects of cloud
contraction and rotation on spectral line maps. We discuss the main issues that
should be taken into account when dealing with interpretation and simulation of
observed molecular lines.Comment: Accepted for publication in Ap
Frictional quantum decoherence
The dynamics associated with a measurement-based master equation for quantum
Brownian motion are investigated. A scheme for obtaining time evolution from
general initial conditions is derived. This is applied to analyze dissipation
and decoherence in the evolution of both a Gaussian and a Schr\"{o}dinger cat
initial state. Dependence on the diffusive terms present in the master equation
is discussed with reference to both the coordinate and momentum
representations.Comment: 18 pages, 7 figure
Canonical Quantum Teleportation
Canonically conjugated observables such as position-momentum and phase-number
are found to play a 3-fold role in the drama of the quantum teleportation.
Firstly, the common eigenstate of two commuting canonical observables like
phase-difference and number-sum provides the quantum channel between two
systems. Secondly, a similar pair of canonical observables from another two
systems is measured in the Bell operator measurements.Comment: revtex,4 pages,e-mail:[email protected] www.itp.ac.cn/~suncp; The
Institute of Theoretical Physics, Academia Sinica, Beijing 100080, P.R. Chin
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