A mesoscopic superposition of quantum states involving radiation fields with classically distinct phases was created and its progressive decoherence observed. The experiment involved Rydberg atoms interacting one at a time with a few photon coherent field trapped in a high Q microwave cavity. The mesoscopic superposition was the equivalent of an "atom 1 measuring apparatus" system in which the "meter" was pointing simultaneously towards two different directions -a "Schrödinger cat." The decoherence phenomenon transforming this superposition into a statistical mixture was observed while it unfolded, providing a direct insight into a process at the heart of quantum measurement. [S0031-9007(96)01848-0] The transition between the microscopic and macroscopic worlds is a fundamental issue in quantum measurement theory [1]. In an ideal model of measurement, the coupling between a macroscopic apparatus ("meter") and a microscopic system ("atom") results in their entanglement and produces a quantum superposition state of the "meter 1 atom" system. Such a superposition is however never observed. Schrödinger has illustrated vividly this problem, replacing the meter by a "cat" [2] and considering the dramatic superposition of dead and alive animal "states." Although such a striking image can only be a metaphor, quantum superpositions involving "meter states" are often called "Schrödinger cats." Following von Neumann [3], it is postulated that an irreversible reduction process takes the quantum superposition into a statistical mixture in a "preferred" basis, corresponding to the eigenvalues of the observable measured by the meter. From then on, the information contents in the system can be described classically. The nature of this reduction has been much debated, with recent theories stressing the role of quantum decoherence [4,5]. According to these approaches, the meter coordinate is always coupled to a large reservoir of microscopic variables inducing a fast dissipation of macroscopic coherences.The simplest model of a quantum measurement involves a two-level atom (e, g) coupled to a quantum oscillator (meter or cat). An oscillator in a coherent state [6] is indeed defined by a c number a, represented by a vector in phase space (jaj p n where n is the mean number of oscillator quanta). Quantum fluctuations make the tip of this vector uncertain, with a circular gaussian distribution of radius unity [ Fig. 1(a)]. Consider the ideal measurement where the "atom-meter" interaction entangles the phase of the oscillator (6f) to the state of the atom, leading to the combined stateWhen the "distance" D 2 p n sin f between the meter states is larger than 1, a Schrödinger cat is obtained [ Fig. 1(b)].Decoherence is modeled by coupling the oscillator to a reservoir, which damps its energy in a characteristic time T r . When D ¿ 1, decoherence is found to occur within a time scale 2T r ͞D 2 [7,8]. This result illustrates the basic feature of the quantum to classical transition [4]. Mesoscopic superpositions made of a few quanta are ex...
Pairs of atoms have been prepared in an entangled state of the Einstein-Podolsky-Rosen ( EPR) type. They were produced by the exchange of a single photon between the atoms in a high Q cavity. The atoms, entangled in a superposition involving two different circular Rydberg states, were separated by a distance of the order of 1 cm. At variance with most previous EPR experiments, this one involves massive particles. It can be generalized to three or more atoms and opens the way to new tests of nonlocality in mesoscopic quantum systems.[S0031-9007 (97)03502-3] PACS numbers: 03.65. -w, 32.80. -t, 42.50. -pOne of the most puzzling aspects of quantum mechanics, its nonseparability, is illustrated vividly by the famous Einstein-Podolsky-Rosen (EPR) paradox [1]. A pair of particles flying apart from each other is predicted by quantum mechanics to yield measurement results incompatible with our intuitive conceptions about locality and reality. Such a nonclassical behavior is expected from any system made of two parts whose wave function cannot be written, in any basis, as a direct product of independent substates. The system parts are then said to be entangled. The study of entanglement has been given a firm conceptual ground by Bell who derived inequalities that Nature should obey if locality and reality were respected and which are violated by quantum mechanics [2]. Many experiments since Bell's paper have demonstrated violations of these inequalities and have vindicated quantum theory [3][4][5][6][7].In most EPR experiments so far [3,4,6,7], pairs of photons flying apart are created in a correlated state by a radiative process (spontaneous emission cascade in an atom or down-conversion in a nonlinear medium). Entangled protons have also been studied in an early experiment [5]. All these studies have dealt with very simple elementary particle systems, in which the entanglement mechanism is imposed by spontaneous processes.Entangling more complex systems in a controlled way is a challenging goal, which has been discussed in many recent proposals. The generation of EPR pairs of massive atoms instead of massless photons has been considered [8][9][10][11]. Ideas to generalize entanglement to larger numbers of particles have also been analyzed [8,10,12].The "manipulation" of entanglement is another important aspect of the new EPR experiment proposals. The idea is to apply a set of well-controlled interactions to the particles of the system in order to bring them into a "tailored" entangled state. In this context, the physics of entanglement meets the theory of quantum information processing. Teleportation of quantum states could in principle be achieved [13] as well as quantum cryptography [14]. Simple quantum computation steps could also be carried out. Particles can then be viewed as carriers of quantum bits of information and the realization of "engineered" entanglement is closely related to the building of gates acting on these bits [15].We describe here an experiment in which we have entangled two initially independent a...
The quantum information carried by a two-level atom was transferred to a high-Q cavity and, after a delay, to another atom. We realized in this way a quantum memory made of a field in a superposition of 0 and 1 photon Fock states. We measured the "holding time" of this memory corresponding to the decay of the field intensity or amplitude at the single photon level. This experiment implements a step essential for quantum information processing operations. [S0031-9007(97)03701-0]
Imaging plays an important role in the diagnosis and therapeutic response evaluation of muscular diseases. However, one important limitation is its incapacity to assess the in vivo biomechanical properties of the muscles. The emerging shear wave sonoelastography technique offers a quantifiable spatial representation of the viscoelastic characteristics of skeletal muscle. Elastography is a non-invasive tool used to analyze the physiologic and biomechanical properties of muscles in healthy and pathologic conditions. However, radiologists need to familiarize themselves with the muscular biomechanical concepts and technical challenges of shear wave elastography. This review introduces the basic principles of muscle shear wave elastography, analyzes the factors that can influence measurements and provides an overview of its potential clinical applications in the field of muscular diseases.
Computational fluid dynamics (CFD) and magnetic resonance (MR) gas velocimetry were concurrently performed to study airflow in the same model of human proximal airways. Realistic in vivo-based human airway geometry was segmented from thoracic computed tomography. The three-dimensional numerical description of the airways was used for both generation of a physical airway model using rapid prototyping and mesh generation for CFD simulations. Steady laminar inspiratory experiments (Reynolds number Re = 770) were performed and velocity maps down to the fourth airway generation were extracted from a new velocity mapping technique based on MR velocimetry using hyperpolarized (3)He gas. Full two-dimensional maps of the velocity vector were measured within a few seconds. Numerical simulations were carried out with the experimental flow conditions, and the two sets of data were compared between the two modalities. Flow distributions agreed within 3%. Main and secondary flow velocity intensities were similar, as were velocity convective patterns. This work demonstrates that experimental and numerical gas velocity data can be obtained and compared in the same complex airway geometry. Experiments validated the simulation platform that integrates patient-specific airway reconstruction process from in vivo thoracic scans and velocity field calculation with CFD, hence allowing the results of this numerical tool to be used with confidence in potential clinical applications for lung characterization. Finally, this combined numerical and experimental approach of flow assessment in realistic in vivo-based human airway geometries confirmed the strong dependence of airway flow patterns on local and global geometrical factors, which could contribute to gas mixing.
Metastability exchange optical pumping of helium-3 is performed in a strong magnetic field of 1.5 T. The achieved nuclear polarizations, between 80% at 1.33 mbar and 25% at 67 mbar, show a substantial improvement at high pressures with respect to standard low-field optical pumping. The specific mechanisms of metastability exchange optical pumping at high field are investigated, advantages and intrinsic limitations are discussed. From a practical point of view, these results open the way to alternative technological solutions for polarized helium-3 applications and in particular for magnetic resonance imaging of human lungs.
Magnetic resonance elastography (MRE) is a non invasive imaging modality, which holds the promise of absolute quantification of the mechanical properties of human tissues in vivo. MRE reconstruction with algebraic inversion of the Helmholtz equation upon the curl of the shear displacement field may theoretically be flawless. However, its performances are challenged by multiple experimental parameters, especially the frequency and the amplitude of the mechanical wave, the voxel size and the signal-to-noise ratio of the MRE acquisition. A point source excitation was simulated and realistic displacement fields were analytically computed to simulate MRE data sets in an isotropic, homogeneous, linearly-elastic, and half-space infinite medium. Acquisition and reconstruction methods were challenged and the joint influence of the aforementioned parameters was studied. For a given signal-to-noise ratio, the conditions on the number of voxels per wavelength were determined for optimizing voxel-wise accuracy and precision in MRE. It was shown that, once data are acquired, the reconstruction quality could even be improved by effective interpolation or decimation so data could eventually fulfill favorable conditions for mechanical characterization of the tissue. Finally, the overall outcome, which is usually computed from the three acquired motion-encoded directions, may further be improved by appropriate averaging strategies that are based on adapted curl of shear displacement field quality-weighting.
A family of velocity-selective pulses consisting of a series of RF hard pulses followed by bipolar gradients was designed. The succession of required pulses was deduced using a k-space approach within a small tip-angle approximation. Fourier transform of the desired velocity excitation determined the flip-angle series, and the corresponding position in the generalized kspace identified the bipolar-gradient first moments. Spins from any velocity class can be selected. To illustrate this approach we designed and experimentally tested a velocity-slice selection that is analogous to standard spatial-slice selection but involves excitation of spins moving at a chosen velocity (velocity-slice center) and within a given interval (velocity-slice thickness). The assumed approximation does not limit the design to small angles, because velocity selection still holds for angles up to 90°. Velocity slices were experimentally selected, centered on velocities ranging from ؊1 m s ؊1 to 1 m s ؊1 with a velocityslice thickness of 0.4 m s ؊1 . The experimental velocity-slice profile was assessed and the flow was quantified. Magn Reson Med 55:171-176, 2006.
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