Events at Physics
Events on Thursday, August 2nd, 2018
- Atomic Physics Seminar
- From atomic clocks to cryo-EM: Pushing measurement limits in time and space
- Time: 2:00 pm
- Place: 5310, Chamberlin Hall
- Speaker: Dr. Sara Campbell, Postdoc in Holger Müller's group, UC Berkeley
- Abstract: What sets the boundaries of what humans can perceive? From time and frequency standards, to molecular biology, the limits of what we can measure depend on two related factors:
1. How much information we get in a single measurement
2. How well we can combine and average these measurements.
I will tell two stories from these two different fields of metrology.
The first story is about a new atomic clock design aimed at measuring 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 frequency 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 corresponding improvements in our ability to evaluate and stabilize systematic shifts.
The second story is about pushing the limits of cryogenic 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-transparent specimen manifests as a small phase shift on the electron wavefunction, which goes undetected unless the microscope is intentionally defocused. Defocusing compromises resolution and still results in low contrast at low spatial frequencies. Reaching atomic resolution requires using low-frequency information to align ~100,000 2D projections of randomly-oriented particles before averaging. The need for sufficient low-frequency information has limited the scope of cryo-EM to large macromolecular complexes. Zernike phase contrast converts phase to amplitude by applying a 90 degree phase shift to the unscattered electron beam, but has yet to be widely implemented, as all previous phase plate designs degrade under the charged electron beam. Laser-based electron optics offer stable, tunable operation, without material objects in the electron beam path. We phase shift the unscattered electron wavefunction via the ponderomotive force of a tightly-focused laser in a near-concentric buildup cavity, which reaches 100 GW/cm^2 continuous intensity.
This aims to be a light, introductory talk with many pictures of my cat and Fourier transforms of my cat. Pickles has a lot of 1/f noise.
- Host: Shimon Kolkowitz