Abstract: The efficiency of a nonlinear optical process is equal to the product of the transition rate and interaction time. If the interaction time can be maximized, it is possible to achieve high efficiency even below single-photon level. Here, we report the experimental demonstration that two light pulses were made motionless and interacted with each other through a medium. To demonstrate the enhancement of optical nonlinear efficiency, we used the process of one optical pulse switched by another. A light pulse was stopped as the atomic excitation  and another is stopped in the form of the electromagnetic wave . The stored atomic excitation is switched by the stationary electromagnetic wave . Because the two light pulses are motionless, a very long interaction time of 7 ms was achieved. This interaction time makes our system analogous to the scheme of trapping light pulses by an optical cavity with a very high Q factor of approximately 10^10. Our result shows that motionless light pulses can activate switching at 0.56 photons per atomic absorption cross section . The great potential of the scheme is that the switching efficiency is not limited to the present result but can be further improved by increasing the optical density of the medium. This work advances the technology of low-light-level nonlinear optics and quantum information manipulation utilizing photons.
 Y. H. Chen, M. J. Lee, I. C. Wang, S. Du, Y. F. Chen, Y. C. Chen, & I. A. Yu, PRL 110, 083601 (2013).
 Y. W. Lin, W. T. Liao, T. Peters, H. C. Chou, J. S. Wang, H. W. Cho, P. C. Kuan, & I. A. Yu, PRL 102, 213601 (2009).
 Y. F. Chen, C. Y. Wang, S. H. Wang, & I. A. Yu, PRL 96, 043603 (2006).
 Y. H. Chen, M. J. Lee, W. Hung, Y. C. Chen, Y. F. Chen, & I. A. Yu, PRL 108, 173603 (2012).
Abstract: The gravitational interaction has two unique features: universal and attractive. These fundamental requirements for any gravity theory are intimately associated with energy and its positivity. From the gravitational response, because it is universal, one can uniquely detect the presence of any physical energy-momentum, even if it is associated with some kind of otherwise non-interacting source, i.e., ``dark.'' Note that energy and momentum can be exchanged between the gravitational field and its sources, and this happens locally, yet, curiously, the energy of gravitating systems---and hence the energy of all physical systems---is essentially elusive; for fundamental reasons it cannot be localized. It is simply not possible to find a proper energy-momentum density; instead there are only various quasi-local (i.e., associated with a closed 2-surface) expressions, which, moreover, are inherently reference frame dependent. We have found that the Hamiltonian approach tames both of these perplexing classical ambiguities. We identified one quasi-local Hamiltonian boundary expression for Einstein's gravity theory, general relativity, which is physically distinguished; furthermore we have a procedure which identifies the ``best'' quasi-local reference frame.
Speaker: Prof. Ching Cheng, Department of Physics, National Cheng Kung University Hosted by 葉崇傑 研究員 (Research Fellow, Sungkit Yip) Time: 14:00
-15:00 Place: The auditorium on 1st floor, Institute of Physics Title: Structural, electronic and magnetic properties of cubic spinel systems: density functional studies
Abstract: In this talk, results of our recent studies on the structural, electronic, and magnetic properties of a few cubic spinel systems using density functional based methods will be presented. These materials are considerably complex in structures and electronic as well as magnetic properties. Despite of the numerous experimental studies and the long well-known applications of some of these systems, the corresponding theoretical studies are scarce. The issues to be addressed include: the magnetic and structural properties in the geometrically frustrated systems of ZnFe2O4 and CdFe2O4, the origin for the experimentally observed quadruple enhanced magnetization in both insulating and conductive state of NiFe2O4, the single-valence state for the mixed spinel MnFe2O4, and the electrical transport through hopping by the mixed valencies in NiMn2O4.