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  • Stalking the X-Ray Frequency Comb | JILA-PFC
    frequency combs in the extreme ultraviolet EUV and soft X ray regions of the electromagnetic spectrum Schibli s new fiber based infrared frequency comb is a high peak power high precision and superstable device It is not only one of the most stable frequency combs ever built but also boasts an average power at a record level of 10 W These two attributes will be critical for creating phase stable frequency combs of radiation whose wavelengths are shorter than optical frequencies i e in the vacuum ultraviolet VUV and EUV wavelengths To create coherent comblike EUV radiation the fiber frequency comb s output of a few hundred thousand colors comb lines is routed into a passive broadband enhancement cavity There custom designed mirrors which are highly reflective bounce the light back and forth through the cavity This process enhances pulse energies by as much as a thousandfold while maintaining the shape of the pulses The peak intensities of the enhanced pulses are great enough to rip electrons out of xenon atoms fed into the cavity via a gas jet When the electrons recollide with the xenon atoms coherent EUV radiation is emitted The key challenge at this point has been to get the very short wavelength pulses out of the cavity before they hit the mirrors which appear black to short wavelength radiation and cannot reflect or transmit it Yost recently solved this problem by suggesting that the researchers etch a diffraction grating directly in the surface of one of the mirrors The grating routes the short wavelength radiation out of the cavity while continuing to reflect the fiber laser s light pulses around the cavity Yost s insight was critical for continued progress toward creating an X ray comb generator He saved our bacon Schibli noted With a way

    Original URL path: http://jila-pfc.colorado.edu/highlights/stalking-x-ray-frequency-comb (2016-04-29)
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  • Splash 2 | JILA-PFC
    liquid perfluorinated polyether or PFPE actually gets stickier as it gets hotter according to a new study by graduate student Brad Perkins and Fellow David Nesbitt This behavior contrasts with solid surfaces which usually get stickier when they get colder The discovery is part of a long term project to study the behavior of Carbon dioxide CO 2 molecules colliding with a liquid surface in a vacuum chamber Perkins says this research is motivated by a desire to better understand how gases enter a liquid why gases stick to a liquid surface before they can dissolve in it and how gases like oxygen and CO 2 are transported across membranes like those in our lungs In the new study Perkins investigated what happened when fast cold 20 K CO 2 molecules collided with PFPE surfaces whose temperatures ranged from 232 to 323 K As in earlier studies see JILA Light Matter Fall 2005 some of the molecules splashed directly off the liquid surface while others skipped across it stuck around for a bit and finally were reflected off it back into the vacuum Interestingly a larger fraction of the molecules stuck to the surface when surface temperatures were warmer And the molecules that directly splashed off the liquid surface were much hotter than expected 600 K for the coldest liquid to 840 K for the warmest Apparently that warm sticky surface really packs a punch Recently Perkins started watching exactly what happens when the hot CO 2 molecules splash off a room temperature PFPE surface He found that most of them cartwheel off the surface end over end the majority tumbling forward with just a few going backward This is a lot like what you get bouncing a tennis ball off a racket at a glancing angle which both rotationally

    Original URL path: http://jila-pfc.colorado.edu/highlights/splash-2 (2016-04-29)
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  • The Gravity of the Situation | JILA-PFC
    one could use the exquisite precision of modern atomic clocks to look for evidence of quantum gravity He suggested comparing different atomic clocks to see whether the ratio of their clock transition frequencies depends on Earth s distance from the Sun This distance varies because Earth travels around the Sun in an elliptical orbit The gravitational potential on Earth varies by 3 3 during a year If coupling between quantum effects and gravity exists it might manifest as sidereal variations in atomic clock transition frequency ratios high precision clock comparisons would either reveal this coupling or place a limit on its strength Plus a long term record of the ratio of different clock frequencies could provide evidence that fundamental constants may in fact be inconstant albeit at very tiny levels detectable only with extremely precise measurements Boulder was the right place to test Flaumbaum s idea In early 2007 Tara Fortier Jason Stalnaker and their colleagues at NIST reported measurements over a six year period comparing the frequency ratio of the optical clock transition of a single mercury ion with the microwave transition in 133 Cs cesium used to define the second No variations in the fundamental constants related to Earth s position in its orbit were detected Likewise no variation of the frequency ratio with time was evident allowing them to constrain any change of the fine structure constant to less than 1 3 x 10 16 yr In a similar analysis Neil Ashby and his NIST colleagues also found no evidence of a yearly variation In 2007 Fellow Jun Ye s group was also planning comparisons of its strontium Sr lattice atomic clock with NIST s Cs fountain time standard and an optical atomic clock based on calcium Ca Graduate student Sebastian Blatt took on the project of

    Original URL path: http://jila-pfc.colorado.edu/highlights/gravity-situation (2016-04-29)
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  • Clock Talk | JILA-PFC
    strontium Sr lattice atomic clock to be just shy of that of the nation s primary time and frequency standard the NIST F1 cesium Cs fountain clock Graduate students Marty Boyd and Andrew Ludlow led the effort to improve the clock s accuracy But then the clock team had to spend another year proving that its imporved clock would neither gain nor lose a second in more than 200 million years thus surpassing the NIST F1 Cs fountain clock In the race to create the world s best optical atomic clock every detail needs to be ironed out After Boyd and his colleagues tested their clock at JILA and showed that it could reach an accuracy level approaching that of the Cs fountain clock they used a fiber optic link to compare the clock s performance with a hydrogen maser calibrated to the NIST F1 However their ability to assess the accuracy of their clock was hindered by the performance limits of the calibrated hydrogen maser Meanwhile graduate student Seth Foreman and his colleagues had been busy improving the transmission of optical clock signals across the fiber optic network between NIST and JILA By mid 2007 Foreman s time transfer team had improved the noise cancellation of the network by a hundredfold This meant that the network was capable of precisely and accurately transferring optical atomic clock signals between JILA and NIST This development allowed the clock team to devise a system to compare the performance of the Sr lattice clock with a NIST optical clock that is based on neutral calcium Ca atoms The combined clock system shown schematically in the figure here allowed the researchers to make very precise comparisons between the signals from each clock Both atomic clocks drove the phases of their respective optical frequency combs

    Original URL path: http://jila-pfc.colorado.edu/highlights/clock-talk (2016-04-29)
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  • Every Breath You Take | JILA-PFC
    cystic fibrosis or cancer Thanks to graduate student Mike Thorpe and his colleagues in Fellow Jun Ye s group medical practitioners may one day be able to identify these disease markers with a low cost noninvasive breath test The new laser based breath test is an offshoot of Thorpe s research on cavity enhanced direct optical frequency comb spectroscopy a molecular fingerprinting technique reported in Science two years ago See JILA Light Matter Spring 2006 Optical comb spectroscopy is powerful enough to sort through all the molecules in human breath and sensitive enough to find traces of chemicals that mark the onset of disease For example excess methylamine might indicate liver or kidney disease ammonia on the breathe could signal the early stages of kidney failure traces of acetone would suggest the onset of diabetes and higher than normal nitric oxide levels in combination with carbonyl sulfide carbon monoxide and hydrogen peroxide would point straight to asthma The new breath test would even make it impossible for smokers to hide their unhealthy habit from their doctors Thorpe and his fellow researchers tested their new technique by having student volunteers breathe into an optical cavity which is a space between two standing mirrors The cavity was designed to bounce pulsed laser light back and forth enough times to travel several kilometers before the light exited the cavity This design allowed the laser light to not only strike every molecule introduced into the cavity by the students but also to create sufficient light matter interaction time to be exquisitely sensitive to trace amounts of different substances The researchers compared the laser light coming out of the cavity with the light that went in This comparison allowed them to determine the exact frequencies of light that were absorbed and by how much This

    Original URL path: http://jila-pfc.colorado.edu/highlights/every-breath-you-take (2016-04-29)
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  • Lights, Magnets, Action! | JILA-PFC
    which are bosons that happily occupy the same quantum state at ultracold temperatures to 40K atoms which are fermions that resist occupying the same quantum state as their neighbors The loosely bound KRb products were also fermions The KRb molecules were manufactured by graduate students Josh Zirbel and Kang Kuen Ni research associate Silke Ospelkaus and Fellows Carl Wieman Jun Ye and Deborah Jin Once they had the molecules the experimentalists investigated collisions between them and each of the atoms that had produced them Their goals were both to understand the new system and to prepare for future experiments to drive the KRb molecules into more stable molecular states Zirbel and his colleagues worked with research associate and theorist José D Incao who analyzed the fundamental physics responsible for the observed collision behavior Zirbel and his colleagues observed three different types of collisions that D Incao explained in terms of the physics governing atom molecule interactions The experimentalists were able to monitor collisions in an optical trap by observing how fast molecules were lost from the trap Molecules exited the trap because they gained energy when an atom crashed into them Rb atoms collided with KRb molecules the most effectively causing a rapid loss of a large fraction of molecules from the trap D Incao s calculations explained this behavior by showing that the bosonic Rb atoms are attracted to the KRb molecules enhancing the collision rate In contrast KRb collisions with K atoms with the same spin as the K in the molecules occurred significantly less often Fewer molecules flew out of the trap In fact most of the molecules in this experiment stayed in the trap for 100 milliseconds the longest life observed so far for this type of ultracold molecule D Incao s analysis explained this collision

    Original URL path: http://jila-pfc.colorado.edu/highlights/lights-magnets-action (2016-04-29)
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  • One Ring to Rule Them All | JILA-PFC
    the blue positively charged regions and repelled by the red negatively charged regions Although C6F5H and C6F6 look very similar an anion will bind to the last remaining H atom the little blue stub in the center picture Benzene has a special ring structure that allows some of its electrons to be shared among all six carbon atoms in the ring It turns out that chemists like Fellow J Mathias Weber can adjust the charge density in the ring by exchanging hydrogen H atoms in the ring with other atoms or groups of atoms Such exchanges can change the charge pattern in the ring seen by neighboring molecules The interaction based on the charge distribution isn t the whole story however Often negatively charged atoms or molecules generally called anions can link to a carbon hydrogen CH group in a benzene molecule by hydrogen bonding Hydrogen bonding is an attractive interaction between an H atom in one molecule and a negatively charged atom such as oxygen or fluorine in another molecule It is based not only on the attraction between opposite charges but also like many chemical bonding phenomena on the partial transfer of electrons from the negatively charged bonding partner to the molecule that contains the H atom Graduate students Holger Schneider and Kristen Vogelhuber recently investigated how anions bind to benzene molecules as they successively exchange H atoms in the ring for fluorine F atoms Normal benzene C 6 H 6 is negatively charged inside the ring and positively charged on the outside The F atoms pull negative charge out of the ring As more F atoms are added the polarity of the ring changes to positively charged on the inside and negatively charged on the outside for hexafluorobenzene C 6 F 6 as shown in the figure

    Original URL path: http://jila-pfc.colorado.edu/highlights/one-ring-rule-them-all (2016-04-29)
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  • DNA: Force of Nature | JILA-PFC
    estimating tiny forces with measurements of the torque transfer between one atomic force microscope AFM tip pushing against a second AFM tip This method is very hard to do very slow and will never be able to provide the precision possible with a biomolecule Biomolecules are going to be great precision measurement tools Perkins says The current challenge for Fellow Tom Perkins research associate Yeonee Seol and their theoretical colleagues from the Universities of Colorado and Pennsylvania is to determine how to precisely measure the elasticity of relatively short strands of DNA Until they tackled the problem elasticity measurements of short 630 nm lengths of DNA had systematic error rates of up to 18 which is absolutely abysmal for any kind of precision measurement The Perkins group determined that the biggest problem was not with elasticity measurements of double stranded DNA but rather with the theoretical model describing DNA elasticity The old theory known as the worm like chain WLC model neglected to consider three important effects that influence elasticity 1 the finite length of the DNA chain 2 rotation of the bead attached to the DNA during elasticity measurements and 3 boundary conditions For example the Perkins group experimentally showed that the assumption that DNA polymers were infinitely long only works well for very large molecules In the 300 2000 nm length range used in most research this faulty assumption leads to significant errors in interpreting experimental data To correct this problem the Perkins group teamed up with its theorist colleagues to incorporate these three corrections into the WLC creating the finite worm like chain FWLC solution In so doing they reduced the systematic error rate of elasticity measurements by threefold The new FWLC reproduces experimental reality much more closely than its predecessor did By elucidating the origin of

    Original URL path: http://jila-pfc.colorado.edu/highlights/dna-force-nature (2016-04-29)
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