Cerebral CO2-reactivity was tested by transcranial Doppler sonography (Doppler CO2 test) in 232 patients. Time averaged flow velocity in the middle cerebral artery at the 40 mm Hg blood pCO2 level was taken as a reference point, and the relative increase of flow in hypercapnia of 46.5 mm Hg pCO2 was defined as "Normalized Autoregulatory Response" (NAR). A total of 82 patients with no evidence of cerebrovascular disease gave "normal" values for NAR (23.2 +/- 5.2 SD). In 150 patients with 233 stenoses and occlusions of the internal carotid artery NAR was significantly decreased in higher-grade stenoses (P = 0.01 for 80% diameter reduction, P less than 10(-6) for 90% or more). In such stenoses, patients with NAR less than 14 had suffered more frequently (P less than 0.01) from ipsilateral transient ischemic attacks and/or stroke during the previous 6 months than patients with "normal" NAR. Preoperative NAR less than 14 always improved to "normal" values following carotid surgery, while preoperative NAR greater than 19 remained unchanged (60 cases). The transcranial Doppler CO2 test is thought to be a reliable noninvasive method to detect hemodynamically critical carotid stenoses and occlusions. This may be of interest in selecting patients for superficial temporal artery-middle cerebral artery bypass and carotid surgery. For practical use 4 categories of NAR are suggested.
We show that quantum localization occurs in the center-of-mass motion of an ion stored in a Paul trap and interacting with a standing laser field. The present experimental state of the art makes the observation of this phenomenon feasible.Comment: 8 pages, 4 PostScript figure
We investigate the classical and quantum dynamics of atoms moving in a phase-modulated standing light field. In both cases the width of the momentum distribution exhibits characteristic oscillations as a function of the modulation amplitude. We argue that at the maxima of these oscillations the system is chaotic, whereas in the valleys it is almost regular. Quantum localization appears only in the chaotic regime. We connect our analysis with a recent experiment [F. Moore et al. , Phys. Rev. Lett. 73, 2974(1994].PACS numbers: 05.45.+b, 42.50.Lc, 42.50.Vk Most recently the description of cold atoms [1] in the framework of atom optics [2] has opened [3 -5] a new avenue in the search for fingerprints of classical chaos in quantum systems: A strongly detuned atom in a standing light field moves like a particle in a spatially periodic potential [6,7]. An atom in a phase-modulated light field additionally experiences a time dependent force [3]. Classically, the resulting motion can be chaotic [8]. But how does classical chaos manifest itself in the quantum dynamics of the atom and how to reach the quantum domain'l The landmark experiment [4,5] by the Austin group answers these questions: The measured momentum transfer of atoms in a phase-modulated light field shows the characteristic signature of quantum chaos: dynamical localization. Moreover, the appropriate choice of the experimental parameters such as the wave number of the standing light field or the modulation frequency allows one to step from regimes which ask for a classical description to regimes which ask for a quantum description.In the present paper we predict a new effect in this playground of atom optics and quantum chaos: The width of the atomic momentum distribution shows characteristic oscillations as a function of the modulation depth A.These oscillations appear in the classical as well as in the quantum description. In the domains of A where maxima appear the classical system is chaotic, whereas in the valleys the motion is almost regular. In the quantum case the peaks are lower, that is the quantum mechanical momentum distribution is narrower than the classical one. Moreover, it is an exponential rather than a Gaussian a signature of quantum localization [9,10]. The motion of a strongly detuned atom of mass M, position x, and momentum p along a standing light field of wave number k, is described by the Hamiltonian H = P /2M -Vpcos(2kx), where Vp is the height of the periodic potential determined by the coupling of the atom to the light field [6]. The motion of the atom in a phase-modulated light field, as created, for example, by an oscillating mirror, is described [3] by the Hamiltonian H = P /2M -Vp cos[2kx -csin(tot)], where A denotes -+ p --a sin(x -Asinr) P(x, p;r) = 0 (2) l97 Bx Bp and periodic boundary conditions P(x + L, p; r) = P(x, p; r), where L is a multiple of 2~. The solution of Eq. (2) satisfying the initial condition P(xp, pp', r = 0) = Po(xp, pp) has the form P(x, p;r) = dxo dpp 6[x g(r; xp, pp)] X 6[p -7r(r;xp, pp)] && Po(xo, po)...
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