5,259 research outputs found
Design of asynchronous supervisors
One of the main drawbacks while implementing the interaction between a plant
and a supervisor, synthesised by the supervisory control theory of
\citeauthor{RW:1987}, is the inexact synchronisation. \citeauthor{balemiphdt}
was the first to consider this problem, and the solutions given in his PhD
thesis were in the domain of automata theory. Our goal is to address the issue
of inexact synchronisation in a process algebra setting, because we get
concepts like modularity and abstraction for free, which are useful to further
analyze the synthesised system. In this paper, we propose four methods to check
a closed loop system in an asynchronous setting such that it is branching
bisimilar to the modified (asynchronous) closed loop system. We modify a given
closed loop system by introducing buffers either in the plant models, the
supervisor models, or the output channels of both supervisor and plant models,
or in the input channels of both supervisor and plant models. A notion of
desynchronisable closed loop system is introduced, which is a class of
synchronous closed loop systems such that they are branching bisimilar to their
corresponding asynchronous versions. Finally we study different case studies in
an asynchronous setting and then try to summarise the observations (or
conditions) which will be helpful in order to formulate a theory of
desynchronisable closed loop systems
Channelization architecture for wide-band slow light in atomic vapors
We propose a ``channelization'' architecture to achieve wide-band
electromagnetically induced transparency (EIT) and ultra-slow light propagation
in atomic Rb-87 vapors. EIT and slow light are achieved by shining a strong,
resonant ``pump'' laser on the atomic medium, which allows slow and
unattenuated propagation of a weaker ``signal'' beam, but only when a
two-photon resonance condition is satisfied. Our wideband architecture is
accomplished by dispersing a wideband signal spatially, transverse to the
propagation direction, prior to entering the atomic cell. When particular
Zeeman sub-levels are used in the EIT system, then one can introduce a magnetic
field with a linear gradient such that the two-photon resonance condition is
satisfied for each individual frequency component. Because slow light is a
group velocity effect, utilizing differential phase shifts across the spectrum
of a light pulse, one must then introduce a slight mismatch from perfect
resonance to induce a delay. We present a model which accounts for diffusion of
the atoms in the varying magnetic field as well as interaction with levels
outside the ideal three-level system on which EIT is based. We find the maximum
delay-bandwidth product decreases with bandwidth, and that delay-bandwidth
product ~1 should be achievable with bandwidth ~50 MHz (~5 ns delay). This is a
large improvement over the ~1 MHz bandwidths in conventional slow light systems
and could be of use in signal processing applications.Comment: Published in SPIE Proceedings, Photonics West 2005 (San Jose, CA,
Jan. 22-27, 2005
Gas phase thermometry of hot turbulent jets using laser induced phosphorescence
This article is made available through the Brunel Open Access Publishing Fund. Copyright @ 2013 OSAThe temperature distributions of heated turbulent jets of air were determined using two dimensional (planar) laser induced phosphorescence. The jets were heated to specific temperature increments, ranging from 300 – 850 K and several Reynolds numbers were investigated at each temperature. The spectral ratio technique was used in conjunction with thermographic phosphors BAM and YAG:Dy, individually. Single shot and time averaged results are presented as two dimensional stacked images of turbulent jets. YAG:Dy did not produce a high enough signal for single shot measurements. The results allowed for a direct comparison between BAM and YAG:Dy, revealing that BAM is more suitable for relatively lower temperature, fast and turbulent regimes and that YAG:Dy is more suited to relatively higher temperature, steady flow situations
Measurement and shaping of biphoton spectral wavefunctions
In this work we present a simple method to reconstruct the complex spectral
wavefunction of a biphoton, and hence gain complete information about the
spectral and temporal properties of a photon pair. The technique, which relies
on quantum interference, is applicable to biphoton states produced with a
monochromatic pump when a shift of the pump frequency produces a shift in the
relative frequencies contributing to the biphoton. We demonstrate an example of
such a situation in type-II parametric down-conversion (SPDC) allowing
arbitrary paraxial spatial pump and detection modes. Moreover, our test cases
demonstrate the possibility to shape the spectral wavefunction. This is
achieved by choosing the spatial mode of the pump and of the detection modes,
and takes advantage of spatiotemporal correlations.Comment: Supplementary information also available. Comments and feedback
appreciated. Compared to the previous version, here we have made the
following changes: -corrected a typo in the text between Eq. (11) and (12)
-corrected a typo in the references -added reference
Indistinguishability and Interference in the Coherent Control of Atomic and Molecular Processes
The subtle and fundamental issue of indistinguishability and interference
between independent pathways to the same target state is examined in the
context of coherent control of atomic and molecular processes, with emphasis
placed on possible "which-way" information due to quantum entanglement
established in the quantum dynamics. Because quantum interference between
independent pathways to the same target state occurs only when the independent
pathways are indistinguishable, it is first shown that creating useful
coherence (as defined in the paper) between nondegenerate states of a molecule
for subsequent quantum interference manipulation cannot be achieved by
collisions between atoms or molecules that are prepared in momentum and energy
eigenstates. Coherence can, however, be transferred from light fields to atoms
or molecules. Using a particular coherent control scenario, it is shown that
this coherence transfer and the subsequent coherent phase control can be
readily realized by the most classical states of light, i.e., coherent states
of light. It is further demonstrated that quantum states of light may suppress
the extent of phase-sensitive coherent control by leaking out some which-way
information while "incoherent interference control" scenarios proposed in the
literature have automatically ensured the indistinguishability of multiple
excitation pathways. The possibility of quantum coherence in photodissociation
product states is also understood in terms of the disentanglement between
photodissociation fragments. Results offer deeper insights into quantum
coherence generation in atomic and molecular processes.Comment: 26 pages, based on one Chapter from first author's Ph.D thesis in
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