BEGIN:VCALENDAR VERSION:2.0 CALSCALE:GREGORIAN PRODID:UW-Madison-Physics-Events BEGIN:VEVENT SEQUENCE:1 UID:UW-Physics-Event-4853 DTSTART:20180802T190000Z DTEND:20180802T200000Z DTSTAMP:20260308T142317Z LAST-MODIFIED:20180709T132731Z LOCATION:5310\, Chamberlin Hall SUMMARY:From atomic clocks to cryo-EM: Pushing measurement limits in time and space\, Atomic Physics Seminar\, Dr. Sara Campbell\, Postdoc in Holger Müller's group\, UC Berkeley DESCRIPTION:What sets the boundaries of what humans can perceive? Fro m time and frequency standards\, to molecular biology\, the limits of what we can measure depend on two related factors:
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\n1. H ow much information we get in a single measurement
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\n2. How well we can combine and average these measurements.
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\nI wi ll tell two stories from these two different fields of metrology.
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\nThe first story is about a new atomic clock design aimed at m easuring time to the 19th decimal place. By using a Fermi-degenerate gas in a three-dimensional optical lattice\, we controlled all quantum degrees of freedom of our atomic frequency references and suppressed atomic interactions. This allowed us to increase our single-shot freq uency sensitivity by both extending the atom-light coherence time and by using more atoms to reduce the quantum projection noise. This new technology enabled new records in clock stability and the correspondin g improvements in our ability to evaluate and stabilize systematic shi fts.
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\nThe second story is about pushing the limits of cryo genic transmission electron microscopy (cryo-EM). Cryo-EM is rapidly usurping x-ray crystallography for determining protein structure\, as it allows the visualization of molecules in their native environments\ , without the need for crystallization. Information from the nearly-t ransparent specimen manifests as a small phase shift on the electron w avefunction\, which goes undetected unless the microscope is intention ally defocused. Defocusing compromises resolution and still results i n low contrast at low spatial frequencies. Reaching atomic resolution requires using low-frequency information to align ~100\,000 2D projec tions of randomly-oriented particles before averaging. The need for s ufficient low-frequency information has limited the scope of cryo-EM t o large macromolecular complexes. Zernike phase contrast converts pha se to amplitude by applying a 90 degree phase shift to the unscattered electron beam\, but has yet to be widely implemented\, as all previou s phase plate designs degrade under the charged electron beam. Laser- based electron optics offer stable\, tunable operation\, without mater ial objects in the electron beam path. We phase shift the unscattered electron wavefunction via the ponderomotive force of a tightly-focuse d laser in a near-concentric buildup cavity\, which reaches 100 GW/cm^ 2 continuous intensity.
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\nThis aims to be a light\, introdu ctory talk with many pictures of my cat and Fourier transforms of my c at. Pickles has a lot of 1/f noise. URL:https://www.physics.wisc.edu/events/?id=4853 END:VEVENT END:VCALENDAR