JQI Papers at March APS
Roughly 40 papers are being given by speakers from the Joint Quantum Institute (*). Here are summaries of a few of them.
Theory of Positronium BEC’s (future Gamma Laser)
Yi-Hsieh Wang, paper J40.9
Positronium, abbreviated Ps, is a strange kind of “atom” made of a briefly-coupled couple consisting of an electron and its antiparticle the positron. A gas of Ps atoms can be made by a shooting positron pulse (the positrons coming from the decay of radioactive atoms) into a silicon crystal. There some of the positrons will attach to electrons, forming Ps atoms. If the density can be high enough and the temperature cold enough a Ps Bose Einstein condensate is expected to form. Ps-BECs haven’t been observed so far, but Ps-Ps interactions and Ps-molecules have been detected in experiments.
Like other atoms, Ps can exist in various energy levels. The singlet state (para-Ps) has a lifetime of only 0.1 ns. Another species, ortho-Ps, lasts for 100 ns. After these brief intervals, the electron and positron will typically annihilate each other, creating either three gamma rays (o-Ps decays) or two gamma rays (p-Ps decays).
An important reason for creating a Ps BEC would be to create a gamma laser. Because the Ps atoms in a BEC are coherent, the laser action (Ps self-annihilating into gamma rays) would also be coherent. The JQI theoretical research in this area is to study how best to initiate and maintain gamma laser action. It turns out that an efficient means of starting laser action in a Ps BEC is to convert the relatively long-lived o-Ps atoms into the short-lived p-Ps (by turning a magnetic field) all at once. However, this process can be hampered by some of the p-Ps turning into o-Ps through scattering.
Haitan Xu, paper G41.10
JQI scientists use the temperature-dependent properties of a ring resonator to provide highly-sensitive thermometry on a microchip. A ring resonator consists of a straight waveguide transferring light into a ring-shaped waveguide (with a radius of about 10 microns) placed tangentially next to it. Only light of a special range of wavelengths will propagate and build up in the ring (analogous to the sound waves that build up in a whispering gallery). This transfer process depends on the geometry of the ring, the wavelength of the light, and the temperature. The JQI sensor can measure local temperature within an area of less than 10-9 m2. Its sensitivity is about 40 micro-K at 1 Hz. In addition, it has the potential to measure temperatures over a range of about 1000 K.
1932: It was a very good year
Charles Clark, paper N32.3
In this one year physics discoveries were made that would garner six Nobel Prizes. James Chadwick discovered neutrons, one of the two major constituents of normal nuclei. Harold Urey discovered deuterium, the first isotope of a known element. Werner Heisenberg suggested the concept of isospin, according to which neutrons and protons were two aspects of an underlying nucleon. Herbert Anderson discovered positrons, the first known example of anti-matter, greatly strengthening Paul Dirac’s theory of quantum fields. Cockcroft and Walton triggered the first disintegration of nuclei by particles accelerated by high voltages, and Lawrence and Livingston showed that the cyclotron could make high energy particles without high voltages.
The substance of Charles Clark’s paper appears also in the March 2013 issue of Physics Today.
Hysteresis in an atomic SQUID
Papers F41.01, 02, 03, 04, 05
Atomtronics hopes to do with neutral atoms what electronics does with electrons and photonics does with photons, namely, process information and make sensitive measurement using sophisticated quantum phenomena. Scientists want to produce with atoms analogs of many of the devices used in electronics, such as transistors and sensors.
Gretchen Campbell and her colleagues have for the first time made an atom equivalent of a superconducting quantum interference device (SQUID). In conventional SQUIDs a supercurrent of electrons flow around a circuit which is interrupted by a “weak link,” representing a sort of impediment to the flow. A magnetic flux passing through the circuit will show up as a voltage across the circuit. In this way very tiny magnetic fields can be measured.
In the JQI device, it is neutral atoms that flow around a circular path in a superfluid manner. The atoms, at ultracold temperatures, are part of a Bose Einstein condensate (BEC). The analog of a weak barrier in this case is a beam of laser light shone through the BEC. This has the result of thinning out the density of atoms at that point. Just as in the regular SQUID voltage changes can measure tiny magnetic fields, so here a tiny change in the current of BEC atoms can measure tiny rotations.
Scientists from Campbell’s lab will also report observations of the BEC system undergoing hysteresis, a process in which forcing a material to attain a certain condition does not necessarily revert to its original state when the force is removed. In conventional ferromagnets, for example, a material can be magnetized by an external force field. But when the field is withdrawn, the magnetization of the material will not immediately drop back to its original state. In the JQI atomtronic case, the analogous thing happens: there is a lag effect between the rotation rate of the applied laser beam and the observed atom current, the area of the hysteresis curve can be controlled by varying the intensity of the laser beam. By studying the hysteresis curve the group helps to better understand how the behavior of their atomtronic “weak link” compares to junctions superconducting systems.
Reference: Wright et al., http://arxiv.org/abs/1208.3608
Mapping Plasmonic antennas
Chad Ropp, paper M20.7
One possible way of sustaining Moore’s law---the doubling of computing power over 18 months or so---is the use of plasmonic waves, waves excited in a metal surface when it is struck by light. The ensuing wave has a shorter wavelength than the incident light, allowing more light energy to be crammed into a smaller volume. But before plasmonics can be exploited, theses special waves need to be studied before components for prospective devices, the analog of electronic or photonic devices, can be built.
JQI research in this area brings together three marvelous physics research fields: microfluidics, quantum dots, and plasmonics to probe and study optical nanostructures with spatial accuracy as fine as 12 nm. Microfluidics, a relatively new science all by itself, features the movement of nanoliter volumes of fluids through channels defined on microchips, analogous to the conducting paths strung across microprocessors for carrying electrical currents. Quantum dots, nanometer-sized semiconductor balls, are tailored to possess a specified set of allowed energy states; in effect the dots are artificial atoms that can be moved around.
In the JQI experiment the 10-nm-wide dots (the important cadmium-selenide layer is only 3 nm thick) float in a fluid whose flow can be controlled by varying an applied voltage. The dots are drawn up close to the nanowire as if they were mines next to a submarine. The silver wire is 4 microns long and 100 nm wide, lights up.
The dot is there precisely to excite the wire. The dot is fluorescence machine---in a loose sense a nanoscopic lightbulb. Striking it with green laser light, it quickly re-emits red light (one photon at a time), and it is this radiation which excites waves in the nearby wire, which acts like an antenna. But the interaction is a two-way street; the dot’s emissions will vary depending on where along the length of the wire it is; the end of the wire (like any pointy lightning rod on a barn) is where electrical fields are highest and this attracts the most emission from the dot.
A CCD camera captures light coming from the dots and from the wire. The camera qualities, the optical properties of the dot, the careful positioning of the dot, and the shape and purity of the nanowire combine to provide an image of the electric field intensity of the nanowire with 12-nm accuracy. The intensity map shows that the input red light from the quantum dot (wavelength of 620 nm) has effectively been transformed into a plasmonic wavelength of 320 nm.
In an actual device, the quantum dot could be replaced by a bio-particle which could be identified through the nanowire’s observed effect on particle’s emissions. Or the dot-wire duo could be combined in various configurations as plasmonic equivalents of electronic circuit components. Other uses for this kind of nanowire setup might exploit the high energy density in the plasmonic state to support nonlinear effects. This could enable the nanowire-dot combination to operate as an optical transistor.
Reference: Ropp et al., Nature Communications, 5 February 2013.
(*)The Joint Quantum Institute is operated jointly by the National Institute of Standards and Technology in Gaithersburg, MD and the University of Maryland in College Park.