This prize is awarded annually by the Physics Department to a graduate student whose original research includes beguiling imagery. Nominations should be solicited by the beginning of April, and the decision should be made by mid-May.
(advisor: Cheng Chin)
Dr. Lei Feng was a graduate student at the University of Chicago (2013~2019) working in Cheng Chin's research group on experimental atomic physics. He graduated in 2019 with a Ph.D. in Physics and is current a postdoctoral scholar at the University of Maryland.
What is Bose firework?
"Bose fireworks" refers to a sudden burst of thin atomic jets escaping from a driven Bose-Einstein condensate, which was discovered in 2017 by the Chin group. The emission occurs when the condensate is subject to modulation of atomic interactions. The jet structure results from inelastic collisions between atoms, which is induced and amplified by the modulation. The following video shows the beautiful "fireworks" emission as the jets escape from the condensate:
Bose Fireworks emission with atomic interactions modulated at 2.5 KHz
Dr. Lei Feng discovered that there is much more to the pattern than meets the eye. He employed novel algorithmns to analyze the emission and identified an unexpected "turtle" pattern in the correlation. His analysis and findings are reported in two publications:
Lei and coworkers observed in 2018 that excited atoms experience multiple collisions and display an intriguing correlations resulting from the high-harmonic geration of matterwaves. The physics behind high-harmonic generation is illustrated below:
The first and high-harmonic generation of matter-wave jets
Lei developed a pattern recognition algorithm to reveal the angular pattern in the seemingly random jet emission structure because the hidden pattern is randomly oriented with different strengths in each experiment. To align them, Lei rotated 209 images of jet structure acquired from independent experiments to maximize the angular variance of the averaged image. The "Turtle" pattern shows the complex correlation between different momentum modes:
Finding the hidden turtle
The turtle pattern obtained from the algorithm reflects all the possible correlations between emitted jets. We can think of that at whatever angle a "turtle head" appears, there will be "tail" and "legs" that appear at the corresponding angles relative the "head". This angular occurrence gives us clues which microscopic collisional processes produces such jet structure. This work is published in "Correlations in high harmonic generation of matter-wave jets reveals by pattern recognition", Lei Feng, Jiazhong Hu, Logan Clark, Cheng Chin, Science 363, 521 (2019).
2. Phase correlation in jets
Lei and coworkers also found that the emitted atoms give a thermal-like Boltzmann distribution. This relates the jets emission to the intriguing Unruh radiation discussed in the context of gravitational physics that an accelerating observer perceives the Minkowskii vacuum in an inertial frame as thermal source. To show that the emission is coherent in nature. Lei developed the scheme to interfere two waves of matterwave jets. The interference of the matterwave confirned the phase coherence of the "thermal emission". This works confirms the mind-boggling prediction that a thermal source to the local observers can in fact be a pure macroscopic quantum state. The work is published in "Quantum Simulation of Unruh Radiation", Jiazhong Hu, Lei Feng, Zhendong and Cheng Chin in Nature Physics 15, 785 (2019).
Interference of jets
(advisor: William Irvine)
Bottom-Up Approach to Turbulence
Vorticity is a fundamental building block of turbulence. We seek to generate turbulence by bringing eddies together one at a time. We fire a collection of vortex rings, generated at the eight corners of an approximately cubic flow chamber. They collide at the center of the chamber. Below show the footages of the collisions at two frequencies (Left: 1Hz, Right: 5Hz), visualized by the hydrogen bubbles. At the low frequency, the vortex rings collide, reconnect, and leave the region where collisions take place. On the other hand, the successive collisions result in a steady blob of turbulence.
For quantitative analysis, fluid dynamicists infer the flows by tracking the passive particles in the chamber. Below illustrate their trajectories around the vortex rings before they collide. The more yellow the path is, the faster the particle is.
A steady blob of turbulence emerges when we fire the vortex rings frequently. This state can be sustained as long as the vortex rings are constantly injected to the blob.
The time-averaged energy field, obtained by a similar technique, highlights the structure of the blob sitting in a relatively quiescent environment.
We developed algorithms to probe the turbulent nature of the flows locally, and they all indicate the flows inside the blob are turbulent. One important variable in a turbulence theory is energy dissipation rate which determines the degree of turbulence. We computed the quantity inside the blob, and found the balance between the injected power by vortex rings and the dissipated power inside the blob. This means that we have great control of turbulence by altering the properties of the vortex rings such as its strength, size, and shape.
For example, the blob size is largely determined by the ring radius. When the ring radius is halved, the blob also shrinks.
Turbulent blobs generated by vortex rings with different sizes
We have focused on how turbulent flows are confined inside the blob, and how we can tune the turbulent intensity by altering the vortex ring properties. This work is currently in preparation for submission.