Dipole-Dipole Frequency Shifts in Multilevel Atoms.

Dipole-dipole interactions lead to frequency shifts that are expected to limit the performance of next-generation atomic clocks. In this work, we compute dipolar frequency shifts accounting for the intrinsic atomic multilevel structure in standard Ramsey spectroscopy. When interrogating the transitions featuring the smallest Clebsch-Gordan coefficients, we find that a simplified two-level treatment becomes inappropriate, even in the presence of large Zeeman shifts. For these cases, we show a net suppression of dipolar frequency shifts and the emergence of dominant nonclassical effects for experimentally relevant parameters. Our findings are pertinent to current generations of optical lattice and optical tweezer clocks, opening a way to further increase their current accuracy, and thus their potential to probe fundamental and many-body physics.

Dark States of Multilevel Fermionic Atoms in Doubly Filled Optical Lattices.

We propose to use fermionic atoms with degenerate ground and excited internal levels (F_{g}→F_{e}), loaded into the motional ground state of an optical lattice with two atoms per lattice site, to realize dark states with no radiative decay. The physical mechanism behind the dark states is an interplay of Pauli blocking and multilevel dipolar interactions. The dark states are independent of lattice geometry, can support an extensive number of excitations, and can be coherently prepared using a Raman scheme taking advantage of the quantum Zeno effect. These attributes make them appealing for atomic clocks, quantum memories, and quantum information on decoherence free subspaces.

Relaxation of an Isolated Dipolar-Interacting Rydberg Quantum Spin System.

How do isolated quantum systems approach an equilibrium state? We experimentally and theoretically address this question for a prototypical spin system formed by ultracold atoms prepared in two Rydberg states with different orbital angular momenta. By coupling these states with a resonant microwave driving, we realize a dipolar XY spin-1/2 model in an external field. Starting from a spin-polarized state, we suddenly switch on the external field and monitor the subsequent many-body dynamics. Our key observation is density dependent relaxation of the total magnetization much faster than typical decoherence rates. To determine the processes governing this relaxation, we employ different theoretical approaches that treat quantum effects on initial conditions and dynamical laws separately. This allows us to identify an intrinsically quantum component to the relaxation attributed to primordial quantum fluctuations.

rTMS modulates reciprocal inhibition in patients with traumatic spinal cord injury.

STUDY DESIGN: Randomized, double-blind, crossover, sham-controlled trial. OBJECTIVES: Repetitive transcranial magnetic stimulation (rTMS) over the primary motor cortex (M1) leads to a significant reduction of spasticity in subjects with spinal cord injury (SCI), but the physiological basis of this effect is still not well understood. The purpose of this study was to evaluate the disynaptic reciprocal Ia inhibition of soleus motoneurons in SCI patients. SETTING: Department of Neurology, Merano, Italy and TMS Laboratory, Paracelsus Medical University, Salzburg, Austria. METHODS: Nine subjects with incomplete cervical or thoracic SCI received 5 days of daily sessions of real or sham rTMS applied over the contralateral M1. We compared the reciprocal inhibition, the Modified Ashworth Scale and the Spinal Cord Injury Assessment Tool for Spasticity at baseline, after the last session and 1 week later in the real rTMS and sham stimulation groups. RESULTS: We found that real rTMS significantly reduced lower limb spasticity and restored the impaired excitability in the disynaptic reciprocal inhibitory pathway. CONCLUSIONS: In a small proof-of-concept study, rTMS strengthened descending projections between the motor cortex and inhibitory spinal interneuronal circuits. This reversed a defect in reciprocal inhibition after SCI, and reduced leg spasticity.

Central motor and sensory conduction in patients with hepatic myelopathy.

STUDY DESIGN: Experimental neurophysiological study. OBJECTIVES: The hepatic myelopathy (HM) is characterized by progressive weakness and spasticity of the lower extremities, while there are only a few reports of sensory impairment. However, sensory function has been poorly explored in HM. We believe that an electrophysiological assessment of dorsal columns by somatosensory evoked potentials (SEPs) and of cortico-spinal lateral tracts by motor evoked potentials (MEPs) should be of considerable value in the functional evaluation of the spinal cord involvement in patients with HM. SETTING: Salzburg (Austria) and Merano (Italy). METHODS: Eight patients diagnosed with HM were studied with MEPs and SEPs. Neurological examination revealed different degrees of cortico-spinal tract involvement in all patients and sensory abnormalities in three patients. RESULTS: Central motor conduction to lower limb muscles was abnormal in all patients, while central sensory conduction was abnormal in seven out of the eight patients. Both central motor and sensory conduction to upper limbs are normal in all patients. CONCLUSION: The main finding is that electrophysiological evidence of central sensory involvement is present in a very high percentage of patients with HM, and that the threshold for electrophysiological abnormalities is below that for clinical manifestations. Therefore, central sensory and motor conduction studies are sensitive methods for detecting, localizing and monitoring spinal cord damage in HM.

Flash evaporation of compounds with a pulsed-discharge CO(2) laser.

Pulsed laser radiation at 10.6 microm has been used to evaporate a number of compounds chosen for their difficulty of evaporation with conventional techniques. An average laser power of 8 W, pulsed at 50 Hz with a 1-msec duration, has been found sufficient to obtain useful evaporation rates for Al(2)O(3) and other highly refractory materials. For easily decomposed materials such as CdS and ZnS, comparison of cw and pulsed evaporation through measurement of optical and photoconductive film properties consistently indicates higher stoichiometry in pulsed evaporation, as well as higher refractive index. The dynamics of film growth during a single laser pulse has been studied by means of a unique thickness monitor.