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  • Bohr + Schrödinger = Students Win | JILA-PFC
    different excited states Credit Chris Malley A solid understanding of the structure and behavior of atoms is important for understanding the physical world from the basic building blocks of nature to the inner workings of modern technology However education researchers have expressed different opinions regarding the best way to teach students the ins and outs of atoms In particular some have even recommended doing away with teaching the older and simpler Bohr model asserting that it inhibits students ability to understand the quantum nature of electrons in atoms This claim recently prompted research associate Sam McKagan and her colleagues CU Assistant Professor Kathy Perkins and Fellow Carl Wieman to not only test this claim but also to develop curriculum on models of the atom including the models proposed by Niels Bohr in 1913 and Erwin Schrödinger in 1926 The researchers found that teaching the Bohr model which describes electrons as point particles orbiting the nucleus at fixed distances did not prevent CU physics students from grasping the more complex Schrödinger model The latter model views electrons as clouds of probability The key was using the Bohr model in a curriculum designed to develop students skills in scientific model building If the Bohr model was presented in an historical context with ample opportunity to compare and contrast it with other models of the atom students were more likely to eventually adopt Schrödinger s view of the atom than if the models were presented in isolation simply as different sets of equations and rules Indeed CU students found it much more difficult to understand the reasons why new models were necessary than they did to grasp the key features of the different models For this reason McKagan and her colleagues made explicit connections between the models for students and presented the historical

    Original URL path: http://jila-pfc.colorado.edu/highlights/bohr-schr%C3%B6dinger-students-win (2016-04-29)
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  • Reflection Grisms | JILA-PFC
    on one face of a prism Properly designed they can compensate for dispersion with efficiencies of up to 90 from hundreds of meters of glass fiber path and withstand high average powers They are also easy to align Reflection grisms are a major improvement over traditional transmission grisms whose low efficiency made them less than ideal for dispersion compensation and pulse compression With high efficiency reflection grisms it s a whole new ballgame With them laser light first diffracts from the grating and then propagates through the prism It then refracts out of the prism at an air glass interface tilted at a large angle with respect to the grating Precise design of this exit angle is responsible for the high diffraction efficiency of the new grisms High efficiency is a key reason why grisms make good pulse compressors It means they can be sized and aligned to precisely oppose the pulse stretching and asymmetric distortions that naturally occur when a laser pulse travels through a dispersive material such as glass Inserted into a laser s optical path pulse compressors employing reflection grisms ensure that a laser pulse reaching its destination looks just like the high quality pulse leaving the laser fully compressed with high peak power Research Associate Emily Gibson Graduate Student David Gaudiosi Fellow Margaret Murnane and colleagues from the Colorado School of Mines Cornell University and Horiba Jobin Yvon Inc worked with Jimenez and Kapteyn to develop the new high efficiency pulse compressors The new technology was applied to downchirped pulse amplification a relatively new technique for producing very high average power amplified laser pulses The original technique stretched the pulses with gratings amplified them in a Ti sapphire crystal and compressed them by sending them through 1 2 m of glass The new grism based technique

    Original URL path: http://jila-pfc.colorado.edu/highlights/reflection-grisms (2016-04-29)
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  • Echoes of Hidden Worlds | JILA-PFC
    are a good model for long range atomic interactions that take place in a dense vapor There atoms are close enough together that excitation from an ultrafast laser can travel from one atom to another The wave functions of excited atoms spread out in a dense vapor like exciton wave functions stretch across multiple lattice sites in semiconductors Both dense atomic vapors and semiconductor excitons can be probed with echo peak shift spectroscopy In this technique three laser pulses incident from three different directions are sent sequentially into a sample The first pulse excites the atoms or excitons causing oscillations at the resonance frequency The second pulse interacts with this excitation to form a transient grating a periodic spatial variation in the level of excitation The third pulse is diffracted by the grating and measured Interactions with other atoms excitons or with the environment cause the oscillations to rapidly get out of phase However if the pulses are ordered correctly the oscillating atoms excitons can come back into phase after the third pulse forming an echo in the diffracted beam However if oscillation frequencies change because of atomic excitonic interactions or variations in the local environment the echo signal is lost By varying the time delay between the first two pulses the researchers can precisely determine the degree to which the original frequencies are retained This measurement gives the maximum echo signal the echo peak shift When measured as a function of the time delay between the 2nd and 3rd pulses the delay of the echo signal decays to zero as the frequencies of the oscillating atoms excitons are randomized Using this measurement the researchers were able to understand the interactions that lead to loss of coherence in the two systems They were also able to estimate the duration of

    Original URL path: http://jila-pfc.colorado.edu/highlights/echoes-hidden-worlds (2016-04-29)
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  • A Failure to Communicate | JILA-PFC
    superatoms made from about 7000 garden variety rubidium Rb atoms into neighboring BECs This communication keeps the array coherent i e the phases of all condensates remain locked to each other But something interesting happens when the tiny superatoms stop communicating among themselves Vortices form And how many appear depends on temperature In a recent experiment Graduate Students Volker Schweikhard and Shihkuang Tung showed for the first time exactly how the process works First they learned how to precisely control the level of communication among the BECs in their lattice array by controlling the intensity of the light field that defines the array Then as this field grew stronger they found that it got harder for atoms to emerge from a condensate and tunnel across to nearby superatoms Eventually tunneling became so weak that communication essentially stopped Once communication failed they discovered that another process begins to dominate thermal agitation At warmer temperatures agitation caused vortices to form randomly in the condensate array Warmer temperatures can destroy system wide coherence in a condensate array that isn t communicating This happens because warmer temperatures cause more uncondensed Rb atoms around the condensates to move around faster making it more likely that one of them will crash into one of the superatoms These collisions can change the phase of a BEC However if there is good communication happening the array of condensates corrects itself back into a coherent state In contrast the failure to communicate prevents energetic self correction and the developing phase changes will lead to the formation of vortices The number that form depends on temperature and on the level of communication between the condensates In fact the researchers found that vortex formation is controlled exclusively by the ratio of thermal agitation to coherence restoring communication The challenge now is

    Original URL path: http://jila-pfc.colorado.edu/highlights/failure-communicate (2016-04-29)
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  • The Second Wave | JILA-PFC
    unexplored quantum phase transitions The first wave of ultracold atom research focused on s wave pairing between atoms where the s meant the resultant molecules are not rotating In contrast p waves involve higher order pairing where the atoms do rotate around each other p wave studies promise to expand and enhance the understanding of ultracold Fermi gases gained from s wave based studies by the Jin group of the crossover from fermionic superfluidity to molecular condensates With p waves the group can attempt to create a superfluid gas that involves higher order pairing akin to superfluid liquid helium 3 He Graduate students John Gaebler and Jayson Stewart and Fellows John Bohn and Debbie Jin recently took an important step towards creating p wave paired superfluids with the creation of p wave pairs of fermionic potassium 40 K atoms The researchers used a p wave Feshbach resonance to convert the ultracold atoms into p wave K 2 molecules The p wave molecules were intrinsically different from the s wave molecules used in the crossover studies The rotation of the p wave atom pairs created a centrifugal energy barrier that would normally prevent molecule formation at ultralow temperatures However the researchers were able to use the p wave Feshbach resonance to encourage atoms to tunnel through the barrier and form quasi bound p wave molecules Once the 40 K atoms formed molecules inside the energy barrier Gaebler and his colleagues discovered they could change the magnetic field and alter the molecules characteristics When they decreased the field a quasi bound molecule would become a real molecule a k a a true bound state Molecules created this way last only a few milliseconds before they disappear most likely decaying into lower energy states undetectable in the experiment When the researchers increased the

    Original URL path: http://jila-pfc.colorado.edu/highlights/second-wave (2016-04-29)
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  • Exploring a Cold New World | JILA-PFC
    colleagues from the University of Durham U K and the Czech Technical University determined that the collision behavior of OH greatly diminished the prospects for sympathetic cooling Bohn dubbed the proposed technique simply pathetic cooling and recommended keeping the nonspherical OH molecules out of magnetic traps altogether A comparison of sympathetic and simply pathetic cooling is shown below Fortunately all was not lost The collaboration realized it had stumbled onto a very interesting collision process In roughly one of every two collisions the dipole moment of the OH would flip its direction the rest of the time the OH would maintain its original internal state Every time an OH molecule flipped its dipole orientation it would gain sufficient energy to fly out of a trap making it easy to precisely measure the fraction of molecules changing state Thus OH looked as if it might be ideal for studying cold molecule collisions if the experimentalists could devise the right trap for it The more the scientists thought about it the more fun and exciting it seemed to explore this new research avenue They could still look for alternatives to sympathetic cooling for lowering the temperature of large numbers of OH molecules to µK levels as the Ye and Greene groups are currently exploring However they could also study cold OH OH collisions dominated by strong dipole dipole interactions as well as other phenomena subject to similar long range quantum influences It occurred to them that since strong dipole dipole interactions are sensitive to an external electric field it might be possible to use tunable electric fields to control cold chemical reactions Plus Fellow Jun Ye s NRC research associate Benjamin Lev realized that molecules like OH are viable candidates for encoding the qubits necessary for implementing quantum logic gates in quantum computers The next step in studying cold OH OH collisions was the development of a new magneto electrostatic trap in Ye s lab by graduate student Brian Sawyer and Lev with help from theorist Manuel Lara Bohn s research associate former graduate student Eric Hudson graduate student Ben Stuhl Bohn and Ye It took Sawyer Lev Hudson and Stuhl nine months to build the new apparatus The hardest part was optimizing their decelerator to produce molecules moving slowly enough to trap The new trap is shown above Trapping OH molecules began with making them in an electric discharge through water vapor followed by a supersonic expansion This process created a molecular OH packet at a temperature of 1 K traveling at 490 m s The OH packet was filtered to 130 mK and slowed to 20m s in an OH Stark decelerator developed in Ye s lab several years ago by Jason Bochinski Eric Hudson and then NRC postdoc Heather Lewandowski now an associate JILA Fellow The end of the decelerator yellow with grey electrodes in the illustration on the previous page couples the now slowly moving cold molecule packets into the magnetic trap blue which is surrounded by electrodes

    Original URL path: http://jila-pfc.colorado.edu/highlights/exploring-cold-new-world (2016-04-29)
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  • Deep Sea Diving | JILA-PFC
    University of Kentucky Seth got the ball rolling so to speak by representing a cloud of ultracold fermions with a form of elliptic geometry known as the hyperspherical model which allowed him to characterize a dilute Fermi gas as a sphere of atoms Initially the size of the gas cloud in its ground state was described by a single coordinate He determined the cloud s size from a parameter known as the scattering length which describes all the interactions between the fermions in different spin states inside the ultracold cloud With his model Seth varied the size of the gas cloud which allowed him to map the system energy for differing cloud sizes and determine an effective potential characterizing the behavior of the cloud Then he created a breathing mode for fermion clouds by squeezing them and letting them expand as shown above and then measuring the frequency of the oscillations He found that while his model did a reasonable job of describing the behavior of low density gas clouds it didn t work nearly as well for high density clouds For instance the model predicted that high density clouds would collapse down to a single point and then explode to smithereens The problem was that such Bosenova like behavior doesn t actually occur in the laboratory When a theoretical analysis doesn t reflect reality it must be modified Javier pitched in to help with a detailed analysis of the effective scattering length that shows how it depends on density In this analysis as the density increases the number of interactions experienced by any given particle increases because there are more particles nearby However the interaction experienced by that particle actually decreases because it only sees the average of the interactions of all the other nearby particles When he incorporated

    Original URL path: http://jila-pfc.colorado.edu/highlights/deep-sea-diving (2016-04-29)
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  • Warm Side of the Force | JILA-PFC
    are responsible for a variety of curious phenomena a gecko s ability to walk across ceilings the evaporation of black holes via Hawking radiation and the fact that warmer surfaces can be stickier than cold ones in micro and nanoscale electromechanical systems MEMS and NEMS The tendency of tiny parts to stick together is a consequence of the Casimir force A related attractive force called the Casimir Polder force occurs between an atom and a surface The temperature dependence of this force was recently measured for the first time by graduate students John Obrecht and Robert Wild and Fellow Eric Cornell Colleagues from the Università di Trento in Italy contributed theoretical calculations to their efforts To measure the temperature dependence of the force between an atom and a surface the researchers positioned a Bose Einstein condensate BEC of 250 000 rubidium atoms in a magnetic trap a few microns from a glass plate As they moved the BEC closer to the plate they were able to measure changes in the frequency of normal oscillations of the BEC and calculate the Casimir Polder force Doubling the temperature of the plate from 310 to 605 K resulted in a threefold increase in the force The key to their success was doubling the temperature of a glass plate while keeping the plate s surroundings near room temperature The researchers accomplished this by using a laser to selectively heat the fused silica surface This strategy allowed the researchers to experimentally measure one of two thermal processes that contribute to the Casimir Polder force These processes have opposing effects on the size of the force When the laser heated the silica surface it increased the electromagnetic waves inside the plate which mostly remained inside the glass However a few evanescent electromagnetic waves leaked out through

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