We investigate the atom-optical analog of degenerate four-wave mixing by colliding two Bose-Einstein condensates of metastable helium. The momentum distribution of the scattered atoms is measured in three dimensions. A simple analogy with photon phase matching conditions suggests a spherical final distribution. We find, however, that it is an ellipsoid with radii smaller than the initial collision momenta. Numerical and analytical calculations agree with this and reveal the interplay between many-body effects, mean-field interaction, and the anisotropy of the source condensate. The field of atom optics has developed to the point that one can now speak of the beginning of ''quantum-atom optics'' [1] in which atoms are manipulated in ways similar to photons and in which quantum fluctuations and entanglement play an important role. The demonstration of atom pair production [2,3], either from the dissociation of ultracold molecules, a process analogous to parametric downconversion [4-6], or from collisions of Bose-Einstein condensates (BECs) [7][8][9][10], analogous to four-wave mixing (FWM) [11][12][13][14][15][16][17][18][19][20][21], holds considerable promise for generating atomic squeezed states and demonstrating nonlocal Einstein-Podolsky-Rosen (EPR) correlations [4,5,22,23]. In both these systems, atom-atom interactions play the role of the nonlinear medium that allows conversion processes. Atoms are not, however, exactly like photons, and in spite of their formal similarity, the processes of pair production of photons and of atoms exhibit some interesting and even surprising differences that must be understood in order for the quantum-atom optics field to advance. In this work, we discuss one such effect.In optical FWM or parametric down-conversion [24], energy conservation requires that the sum of the energies of the outgoing photons be fixed by the energy of the input photon(s). Phase matching requirements impose constraints on the directions and values of the individual photon momenta. A simple case is degenerate, spontaneous FWM (i.e., two input photons of equal energy) in an isotropic medium, for which energy conservation and phase matching require that the momenta of the output photons lie on a spherical shell whose radius is that of the momenta of the input photons.We have performed the atom-optical analog of degenerate FWM in colliding BECs while paying careful attention to the momenta of the outgoing atoms. We find that unlike the optical case, the output momenta do not lie on a sphere, but rather on an ellipsoid with short radius smaller than the input momentum. This behavior is due to a subtle combination of atom-atom interactions, which impose an energy cost for pair production, and the anisotropy of the condensates, which affects the scattered atoms as they leave the interaction region.Although an analogous effect could exist in optics, optical nonlinearities are typically so small that the effect is negligible. However, in the process of high-harmonic generation in intense laser fields, a simil...
We describe the fabrication and construction of a setup for creating lattices of magnetic microtraps for ultracold atoms on an atom chip. The lattice is defined by lithographic patterning of a permanent magnetic film. Patterned magnetic-film atom chips enable a large variety of trapping geometries over a wide range of length scales. We demonstrate an atom chip with a lattice constant of 10 μm, suitable for experiments in quantum information science employing the interaction between atoms in highly excited Rydberg energy levels. The active trapping region contains lattice regions with square and hexagonal symmetry, with the two regions joined at an interface. A structure of macroscopic wires, cutout of a silver foil, was mounted under the atom chip in order to load ultracold 87 Rb atoms into the microtraps. We demonstrate loading of atoms into the square and hexagonal lattice sections simultaneously and show resolved imaging of individual lattice sites. Magnetic-film lattices on atom chips provide a versatile platform for experiments with ultracold atoms, in particular for quantum information science and quantum simulation. © 2014 AIP Publishing LLC.
We study light transport in phosphor plates of white light-emitting diodes (LEDs). We measure the broadband diffuse transmission through phosphor plates of varying YAG:Ce(3+) density. We distinguish the spectral ranges where absorption, scattering, and re-emission dominate. Using diffusion theory, we derive the transport and absorption mean free paths from first principles. We find that both transport and absorption mean free paths are on the order of the plate thickness. This means that phosphors in commercial LEDs operate well within an intriguing albedo range around 0.7. We discuss how salient parameters that can be derived from first principles control the optical properties of a white LED.
Electrochemical conditioning via chronopotentiometry (CP) and cyclic voltammetry (CV) is essential for the activation of oxygen evolution reaction (OER) electrocatalysts. While many reports have activated OER electrocatalysts using either CP or CV, the inherent differences between these two electrochemical conditioning methods for the activation of OER electrocatalytic materials have yet to be explored. Here, we investigate the effects of CP and CV electrochemical conditioning on a Ni-based OER precatalyst and substrate in Fe-purified and Fe-unpurified KOH electrolytes by employing (i) Ni foil, (ii) NiSe precatalyst films with different thicknesses on the fluorine-doped tin oxide glass substrate, and (iii) NiSe precatalyst films on Ni foil substrates. It was found that CV electrochemical conditioning can result in a higher degree of in situ oxidation and Fe incorporation for Ni-based precatalysts and substrates compared to CP electrochemical conditioning. In turn, this brought about different material properties (e.g., in situ oxidized layer thickness, composition, crystallinity, and morphology) and electrochemical characteristics (e.g., active surface area, electron transport limitation, and intrinsic activity) of Ni-based electrocatalysts, thereby not only affecting their OER activity but also complicating the interpretation of the origin of OER activity. This study identifies the distinct effects of CP and CV electrochemical conditioning on Ni-based OER electrocatalysts and provides insight into the choice of the electrochemical conditioning method to better investigate OER electrocatalysts.
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