323,104 research outputs found
Control of decoherence
No full textWe discuss three control strategies aimed at countering the effects of decoherence: the first hinges on frequent projective measurements, the second on frequent unitary "kicks" ("bang-bang" pulses) and the third on a strong continuous coupling. Decoherence is suppressed if the frequency N of the measurements/kicks is large enough or if the coupling K is sufficiently strong: in all these cases, the Hilbert space of the system splits into invariant subspaces, among which any transition is hindered. However, if N or K are large, but not extremely large, all these control procedures accelerate decoherence
Deviation from exponential law and Van Hove's “lambda^2 t” limit
No full textThe deviations from a purely exponential behavior in a decay process are analyzed in relation to Van Hove’s “lambda^2 t” limiting procedure. Our attention is focused on the effects that arise when the coupling constant is small but nonvanishing. We first consider a simple model (two-level atom in interaction with the electromagnetic field), then gradually extend our analysis to a more general framework. We estimate all deviations from exponential behavior at leading orders in the coupling constant
On noise-induced superselection rules
No full textThe dynamical properties of a quantum system can be profoundly influenced by its environment. Usually, the environment provokes decoherence and its action on the system can often be schematized by adding a noise term in the Hamiltonian. However, other scenarios are possible: we show that by increasing the strength of the noise, the Hilbert space of the system gradually splits into invariant subspaces, among which transitions become increasingly difficult. The phenomenon is equivalent to the formation of the quantum Zeno subspaces. We explore the possibility that noise can prevent , rather than provoke decoherence
SAR Interferometry and Tomography: Theory and Applications
No full textSynthetic Aperture Radar (SAR) is among of the most used remote sensing systems for Earth observation and has wide application in security in both marine and terrestrial environments. The last decade has been a period of extraordinary development of SAR systems with an impressive growth in the number of launch and operational deployment of spaceborne SAR remote sensing systems. Enabling an extensive range of new applications is the advent of several very high resolution spaceborne SARs, such as TerraSAR-X/Tandem-X and the COSMO-SKYMED constellation. Very fine details of Earth surface are provided on a regular basis by data acquired and processed by those sensors. A significant contribution to the desire to field such systems has been the development of coherent processing techniques, in particular interferometry, that have dominated SAR applications since their first demonstration in the late 70's and early 80's. Evidence of the importance and versatility of radar interferometry is its application to such diverse area as the monitoring of volcanoes, earthquakes, landslides, ice sheet motion and anthropogenic sources such as ground pumping of water and oil. Development of innovative processing techniques, like permanent scatterer interferometry, polarimetric-interferometry and tomography have expanded the number of applications and data sets that can be successfully exploited. For example, permanent scatterer interferometry and tomography have revolutionized what can be done by SARs in urban environments. In this article we aim to provide a description of the some of the major developments in SAR interferometry and SAR tomography with particular emphasis on the digital signal processing aspects. We will illustrate SAR tomography using urban and infrastructures applications although it has other applications such as in forest and ice structure. Examples of applications of interferometry and tomography are provided to demonstrate the practical usefulness of the technological advances occurring on both the SAR system and data processing. With respect to other published tutorial on interferometry, we focus on the development of multibaseline/multipass coherent processing approach from a signal processing perspective with the aim to provide to readers a comprehensive description of the topics demanding to the reference bibliography deeper investigations
Quantum Zeno and inverse quantum Zeno effects
No full textThe evolution of a quantum mechanical system can be profoundly modified
by the action of an external agent, such as a detection apparatus or a
field. If quantum measurements are performed on the system, its
evolution can be hindered or enhanced, depending on the physical
features of the system itself and the interaction.
The quantum Zeno effect consists in the hindrance of the evolution, the
inverse quantum Zeno effect in its enhancement. We shall discuss both
these effects by considering some examples in quantum optics and quantum
electrodynamics and shall critically analyze the notion of quantum
measurement: the Zeno effects take place both if the measurements are
``pulsed" and almost instantaneous or ``continuous" and of long
duration.
We also discuss the profound differences between oscillating systems,
whose Poincare' time is finite, and unstable ones.
For the latter, the Zeno effects are much more interesting and
transitions become possible between a Zeno and an inverse Zeno regime
Unstable systems and quantum Zeno phenomena in quantum field theory
No full textWe analyze the Zeno phenomenon in quantum field theory. The decay of an unstable system can be modified by changing the time interval between successive measurements (or by varying the coupling to an external system that plays the role of measuring apparatus). We speak of quantum Zeno effect if the decay is slowed and of inverse quantum Zeno (or Heraclitus) effect if it is accelerated. The analysis of the transition between these two regimes requires close scrutiny of the features of the interaction Hamiltonian. We took in detail at quantum field theoretical models of the Lee type
A solvable model for quantum mechanical dissipation
No full textWe study the Coleman-Hepp or AgBr Hamiltonian, concentrating our attention on a weak-coupling, macroscopic Limit that yields dissipative effects. We analyze the role played by Van Hove's "lambda(2)T" limit
Temporal behavior and quantum Zeno time of an excited state of the hydrogen atom
No full textThe quantum “Zeno” time of the 2P-1S transition of the hydrogen atom is computed and found to be approximately 3.59 × 10−15 s (the lifetime is approximately 1.595 × 10−9 s). The temporal behavior of this system is analyzed in a quantum field theoretical framework and compared to the exponential decay law
Dynamical imperfections in quantum computers
No full textWe study the effects of dynamical imperfections in quantum computers. By considering an explicit example, we identify different regimes ranging from the low-frequency case, where the imperfections can be considered as static but with renormalized parameters, to the high-frequency fluctuations, where the effects of imperfections are completely wiped out. We generalize our results by proving a theorem on the dynamical evolution of a system in the presence of dynamical perturbations
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