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  • richardmitnick 8:58 am on April 9, 2015 Permalink | Reply
    Tags: Atomic Physics, , , ,   

    From XFEL: “European XFEL scientists look deep into the atom” 

    XFEL bloc

    European XFEL

    09 April 2015
    No Writer Credit

    Digging into the “Giant Resonance”, scientists find hints of new quantum physics

    A cooperation between theoretical and experimental physicists has uncovered previously unknown quantum states inside atoms. The results, described in a paper published today in the journal Nature Communications, allow a better understanding of some aspects of electron behaviour in atoms, which in turn could lead to better insights into technologically relevant materials.

    In this study, scientists from European XFEL and the Center for Free Electron Laser Science (CFEL) at DESY examined the unknown quantum states in atoms of the noble gas xenon using DESY’s X-ray laser FLASH.

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    DESY FLASH
    DESY FLASH

    The bright X-ray light of free-electron lasers such as FLASH and the European XFEL allowed the scientists to observe these states for the first time. (CFEL is a joint venture between DESY, the University of Hamburg, and the Max Planck Society.)

    Atoms can develop an electrical charge by losing or gaining one or more electrons. This process, called ionization, was thought to be fairly simple. As an electron departs, it can briefly “hang” between the different locations of electrons in the atom, also called “shells”. In the world of quantum mechanics, this brief pause—lasting less than a femtosecond, or a quadrillionth of a second—is enough to be measured as what is called a “resonance”.

    “In a resonance, the electrons are ‘talking’ to each other”, says Michael Meyer, a leading scientist at European XFEL. This conversation of sorts can be picked up on a spectrograph, and, in most atoms, it shows up in a very narrow energy range.

    Yet for the past half century, scientists also have noted a strange resonance in atoms of the noble gas xenon and some rare earth elements. In contrast to other resonances, it covers a very broad energy range. This became known as the “giant dipole resonance”. “There were no good tools to investigate the giant dipole resonance more deeply”, says Meyer. “But extreme-ultraviolet and X-ray FELs present an opportunity to re-examine xenon’s strange property.” Such facilities have the possibility of studying nonlinear processes, or phenomena that are not a direct result of a single interaction—in the case of photoionization the disappearance of one photon, with its energy being transferred to the electron that can thus escape the atom. The extraordinary intensity of FELs makes non-linear processes observable—in this case, a process whereby two photons disappear, simultaneously transferring their energy to the escaping electron.

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    Graphical representation of a 4d electron orbital in atomic xenon. Antonia Karamatskou / DESY

    This has become evident when Tommaso Mazza, a scientist in Meyer’s group at European XFEL, and others investigated the ionization of xenon atoms under intense FEL radiation at DESY’s FLASH. In parallel, DESY scientist Robin Santra, the leader of the CFEL theory group, and a student in his group, Antonia Karamatskou, thought there was something more to the giant dipole resonance. They worked off of a forty-year old suggestion that had been largely ignored: that xenon actually had not one but two resonances, and that earlier spectrographic techniques could not distinguish between them. In contrast, X-ray FELs can target very specific energies in the electronic structure of the atom using just two individual particles of light, enabling scientists to see both resonances more clearly.

    Santra and Karamatskou made calculations describing the energies of the resonances. The data from the experiments performed at FLASH by Tommaso Mazza and others match Santra’s and Karamatskou’s predictions. This is the first evidence of the giant dipole resonance being composed of two other resonances.

    Both Santra and Meyer think that there is far more to the behaviour of electrons within atoms in general than has been previously understood. The result point to not yet fully understood aspects of how atoms function.

    “We don’t even yet understand why there might be a second resonance”, Santra says. “Many people think simple atomic physics is figured out, but as this collaboration has shown, there is a lot of hidden stuff out there!”

    Also, the experiment has shown that FELs can be highly sophisticated tools for studying quantum physics. Santra says he expects the European XFEL, which is due to open to users in 2017, to expand these possibilities even further.

    The research collaboration between Meyer and Santra was initiated and supported by the Collaborative Research Center at the University of Hamburg, SFB 925.

    “Sensitivity of nonlinear photoionization to resonance substructure in collective excitation”; T. Mazza, A. Karamatskou, M. Ilchen, S. Bakhtiarzadeh, A.J. Rafipoor, P. O’Keeffe, T.J. Kelly, N. Walsh, J.T. Costello, M. Meyer & R. Santra; Nature Communications, 2015; DOI: 10.1038/ncomms7799

    See the full article here.

    Please help promote STEM in your local schools.

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    XFEL Campus

    The Hamburg area will soon boast a research facility of superlatives: The European XFEL will generate ultrashort X-ray flashes—27 000 times per second and with a brilliance that is a billion times higher than that of the best conventional X-ray radiation sources.

    The outstanding characteristics of the facility are unique worldwide. Starting in 2017, it will open up completely new research opportunities for scientists and industrial users.

     
  • richardmitnick 9:18 am on June 27, 2014 Permalink | Reply
    Tags: Atomic Physics, , , ,   

    From Berkeley Lab: “Not Much Force: Berkeley Researchers Detect Smallest Force Ever Measured” 


    Berkeley Lab

    June 26, 2014
    Lynn Yarris

    What is believed to be the smallest force ever measured has been detected by researchers with the Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley. Using a combination of lasers and a unique optical trapping system that provides a cloud of ultracold atoms, the researchers measured a force of approximately 42 yoctonewtons. A yoctonewton is one septillionth of a newton and there are approximately 3 x 1023 yoctonewtons in one ounce of force.

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    Mechanical oscillators translate an applied force into measureable mechanical motion. The Standard Quantum Limit is imposed by the Heisenberg uncertainty principle, in which the measurement itself perturbs the motion of the oscillator, a phenomenon known as “quantum back-action.” (Image by Kevin Gutowski)

    “We applied an external force to the center-of-mass motion of an ultracold atom cloud in a high-finesse optical cavity and measured the resulting motion optically,” says Dan Stamper-Kurn, a physicist who holds joint appointments with Berkeley Lab’s Materials Sciences Division and the UC Berkeley Physics Department. “When the driving force was resonant with the cloud’s oscillation frequency, we achieved a sensitivity that is consistent with theoretical predictions and only a factor of four above the Standard Quantum Limit, the most sensitive measurement that can be made.”

    Stamper-Kurn is the corresponding author of a paper in Science that describes these results. The paper is titled Optically measuring force near the standard quantum limit. Co-authors are Sydney Schreppler, Nicolas Spethmann, Nathan Brahms, Thierry Botter and Maryrose Barrios.

    If you want to confirm the existence of gravitational waves, space-time ripples predicted by Albert Einstein in his theory of general relativity, or want to determine to what extent the law of gravity on the macroscopic scale, as described by Sir Isaac Newton, continues to apply at the microscopic scale, you need to detect and measure forces and motions that are almost incomprehensively tiny. For example, at the Laser Interferometer Gravitational-Wave Observatory (LIGO), scientists are attempting to record motions as small as one thousandth the diameter of a proton.

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    From left, Sydney Schreppler, Dan Stamper-Kurn and Nicolas Spethmann were part of a team that detected the smallest force ever measured using a unique optical trapping system that provides ultracold atoms. (Photo by Roy Kaltschmidt)

    At the heart of all ultrasensitive detectors of force are mechanical oscillators, systems for translating an applied force into measureable mechanical motion. As measurements of force and motion reach quantum levels in sensitivity, however, they bump up against a barrier imposed by the Heisenberg uncertainty principle, in which the measurement itself perturbs the motion of the oscillator, a phenomenon known as “quantum back-action.” This barrier is called the Standard Quantum Limit (SQL). Over the past couple of decades, a wide array of strategies have been deployed to minimize quantum back-action and get ever closer to the SQL, but the best of these techniques fell short by six to eight orders of magnitude.

    “We measured force with a sensitivity that is the closest ever to the SQL,” says Sydney Schreppler, a member of the Stamper-Kurn research group and lead author of the Science paper. “We were able to achieve this sensitivity because our mechanical oscillator is composed of only 1,200 atoms.”

    In the experimental set-up used by Schreppler, Stamper-Kurn and their colleagues, the mechanical oscillator element is a gas of rubidium atoms optically trapped and chilled to nearly absolute zero. The optical trap consists of two standing-wave light fields with wavelengths of 860 and 840 nanometers that produce equal and opposite axial forces on the atoms. Center-of-mass motion is induced in the gas by modulating the amplitude of the 840 nanometer light field. The response is measured using a probe beam with a wavelength of 780 nanometers.

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    To measure force, a cloud of atoms (gray oval) are trapped in an optical cavity created by two standing-wave light fields, ODT A and ODT B. The amplitude of ODT B is varied to create a force that is optomechanically transduced onto the phase of a probe light for measurement. No image credit.

    “When we apply an external force to our oscillator it is like hitting a pendulum with a bat then measuring the reaction,” says Schreppler. “A key to our sensitivity and approaching the SQL is our ability to decouple the rubidium atoms from their environment and maintain their cold temperature. The laser light we use to trap our atoms isolates them from external environmental noise but does not heat them, so they can remain cold and still enough to allow us to approach the limits of sensitivity when we apply a force.”

    Schreppler says it should be possible to get even closer to the SQL for force sensitivity through a combination of colder atoms and improved optical detection efficiency. She also says there are back-action evading techniques that can be taken by performing non-standard measurements. For now, the experimental approach demonstrated in this study provides a means by which scientists trying to detect gravitational waves can compare the limits of their detection abilities to the predicted amplitude and frequency of gravitational waves. For those seeking to determine whether Newtonian gravity applies to the quantum world, they now have a way to test their theories. The enhanced force-sensitivity in this experiment could also point the way to improved atomic force microscopy.

    “A scientific paper in 1980 predicted that the SQL might be reached within five years,” Schreppler says. “It took about 30 years longer than predicted, but we now have an experimental set-up capable both of reaching very close to the SQL and of showing the onset of different kinds of obscuring noise away from that SQL.”

    This research was supported by the Air Force Office of Scientific Research and the National Science Foundation.

    See the full article here.

    A U.S. Department of Energy National Laboratory Operated by the University of California

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