As it goes online, the MAGIS-100 experiment at the Department of Energy’s Fermi National Accelerator Laboratory and its successors will explore the nature of gravitational waves and look for certain types of matter dark like waves. But first, the researchers needed to figure out something pretty basic: how to get good pictures of the atomic clouds at the center of their experiment.
Researchers at the Department of Energy’s SLAC National Accelerator Laboratory realized that the mission would probably be the ultimate exercise in ultra-low light photography.
But a SLAC team that included Stanford graduate students Sanha Cheong and Murtaza Safdari, SLAC Professor Ariel Schwartzman, and SLAC scientists Michael Kagan, Sean Gasiorowski, Maxime Vandegar, and Joseph Frish found a simple way to do it: mirror. By arranging the mirrors in a dome-like configuration around an object, they can reflect more light toward the camera and capture multiple faces of an object simultaneously.
And, the team reports in the Journal of Instrumentation, there’s an added benefit. Because the camera now gathers views of an object captured from various angles, the system is an example of “light field imaging,” which records not only light intensity but also direction of light rays. As a result, mirror systems can help researchers build three-dimensional models of an object, such as a cloud of atoms.
“We are improving images in experiments like the MAGIS-100 to the latest imaging model with this system,” says Safdari.
An unusual photo challenge
The 100-meter matter-wave atomic weight interferometer, or MAGIS-100, is a new type of experiment being installed in a vertical shaft at DOE’s Fermi National Accelerator Laboratory. Known as an atomic interferometer, it will exploit quantum phenomena to detect the traveling waves of ultralight dark matter and free-falling strontium atoms.
The experimenters would release clouds of strontium atoms in a vacuum tube running the length of the axis, then shine laser light at the free-falling clouds. Each strontium atom behaves like a wave, and the laser light sends each of these atomic waves into a superposition of quantum states, one of which continues on its original path for a while when the other wave is pushed much higher.
When combined, the waves create an interference pattern in strontium atomic waves, similar to the complex ripple pattern that occurs after sliding over a rock in a pond. This form of interference is sensitive to anything that changes the relative distances between quantum wave pairs or the internal properties of the atoms, which can be affected by the presence of dark matter.
To see the interference patterns, the researchers will literally photograph a cloud of strontium atoms, which comes with a number of challenges. The strontium clouds themselves are very small, only about a millimeter across, and the details researchers need to see are about a tenth of a millimeter across. The camera itself had to be placed outside a chamber and peered through a window at a relatively long distance to see the strontium clouds inside.
But the real problem is the light. To illuminate the strontium clouds, the experimenters would shine a laser at the clouds. However, if the laser light is too strong, it can destroy the details that scientists want to see. Without enough brightness, the light from the clouds will be too dim for the camera to see.
“You will only collect as much light as it falls on the lens,” says Safdari.
One idea is to use a wide or open aperture to let more light into the camera, but there is a trade-off: Wide apertures create what photographers call a narrow depth of field, where only focus a narrow part of the image.
Another possibility is to place more cameras around a cloud of strontium atoms. This could collect more alternative light, but it would require more windows or, alternatively, mounting the cameras inside the chamber and not much space in there for a bunch of cameras.
The solution emerged during a brainstorming session in the lab, says Schwartzman. As they were brainstorming ideas around, staff scientist Joe Frisch came up with the idea for mirrors.
“What you can do is reflect the light going from the cloud back to the camera lens,” says Cheong. As a result, a camera can collect not only more light but more of an object from different angles, each showing up on the raw image as a separate point in the background. black. The team realized that that collection of distinct images meant that they had devised a form of ‘light field imaging’ that could reconstruct a three-dimensional model of the atomic cloud, not just a two-dimensional image.
3D printing ideas
With support from a Research and Development grant directed by the Laboratory, Cheong and Safdari took the mirror idea and worked with it, designing a series of small, portable mirrors directs light from around a cloud of atoms towards the camera. Using some algebra and ray tracing software developed by Kagan and Vandegar, the team calculated just the right positions and angles to allow the mirror to keep various images of the cloud in focus on camera. The team has also developed computer vision and artificial intelligence algorithms to use 2D images to perform 3D reconstructions.
This is the kind of thing that seems obvious, but it takes a lot of thought to achieve, says Schwartzman. “When we first came up with this, we thought, ‘Guys have done this before,'” he said, but in fact it was novel enough that the team patented it. this device.
To test this idea, Cheong and Safdari made a mock-up with a 3D printed scaffold holding the mirrors, then built a microscopic 3D printed fluorescent object that emits the word “DOE” when viewed from the mirrors different angles. They photographed the object with their mirror dome and showed that they could, in fact, gather light from a number of different angles and keep all the images in focus. Furthermore, their 3D reconstruction was so precise that it revealed a small flaw in the fabrication of the “DOE” object – an arm of the letter “E” that was slightly bent downwards.
The next step, the researchers say, is to build a new version to test the idea in a smaller atomic interferometer at Stanford, which will produce the first 3D images of clouds of atoms. . That version of the mirror dome will be located outside the chamber containing the atom cloud, so if those tests are successful, the team will build a stainless steel version of the mirror truss that is suitable for vacuum conditions inside the atomic interferometer.
Schwartzman said the ideas that Cheong, Safdari and the rest of the team developed could be useful beyond physics experiments. “It’s a novelty device,” he said. Our application is an atomic interferometer, but it could be useful in other applications, such as industrial small-body fabrication quality control.
The research was supported by the Department of Energy, Laboratory Directed Research and Development Program. The MAGIS-100 is supported by the Gordon and Betty Moore Foundation and the DOE Office of Science.