How to take a better picture of atom clouds? Mirrors — lots of mirrors — ScienceDaily

When it comes 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 search for certain types of wave-like dark matter. But first, researchers need to figure out something pretty basic: how to get good photographs of the clouds of atoms at the heart of their experiment.

Researchers at the Department of Energy’s SLAC National Accelerator Laboratory realized that this task might be the ultimate exercise in ultra-low-light photography.

But an 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: mirrors. By arranging mirrors in a domed configuration around an object, they can reflect more light back to the camera and image multiple sides of an object simultaneously.

And, the team reports in the Instrumentation review, there is an additional advantage. Since the camera now gathers views of an object taken from many different angles, the system is an example of “bright field imaging”, which not only captures the intensity of light, but also the direction in which the light rays travel. As a result, the mirror system can help researchers build a three-dimensional model of an object, such as a cloud of atoms.

“We are advancing imaging in experiments like MAGIS-100 to the new imaging paradigm with this system,” Safdari said.

An unusual photographic challenge

The 100-meter-long Matter-Wave Atomic Gradiometer Interferometric Sensor, or MAGIS-100, is a new type of experiment installed in a vertical well at the DOE’s Fermi National Accelerator Laboratory. Known as the Atomic Interferometer, it will exploit quantum phenomena to detect passing waves of ultralight dark matter and free-falling strontium atoms.

Experimenters will release clouds of strontium atoms into a vacuum tube that spans the length of the shaft, then shine laser light on the falling clouds. Each strontium atom acts 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 while the other is propelled much higher.

When recombined, the waves create an interference pattern in the strontium atom wave, similar to the complex pattern of ripples that emerges after jumping off a rock into a pond. This interference pattern is sensitive to anything that changes the relative distance between quantum wave pairs or the internal properties of atoms, which could be influenced by the presence of dark matter.

To see the interference patterns, researchers will literally take pictures of a cloud of strontium atoms, which comes with a number of challenges. The strontium clouds themselves are small, only about a millimeter in diameter, and the details researchers need to see are about a tenth of a millimeter in diameter. The camera itself must be placed outside a room and looking through a window for a relatively long distance to see the strontium clouds inside.

But the real problem is light. To illuminate the strontium clouds, the experimenters will shine lasers on the clouds. However, if the laser light is too intense, it can destroy the details scientists want to see. If it is not intense enough, the light from the clouds will be too faint for the cameras to see.

“You’ll only collect the amount of light that falls on the lens,” Safdari said, “which isn’t a lot.”

Mirrors to the rescue

One idea is to use a large aperture, or aperture, to let more light into the camera, but there’s a trade-off: a large aperture creates what photographers call a narrow depth of field, where only a slice close of the image is in focus.

Another possibility would be to position more cameras around a cloud of strontium atoms. It might collect more re-emitted light, but that would require more windows or, alternatively, mounting the cameras inside the chamber, and there’s not much space in there for a bunch of cameras.

The solution emerged, Schwartzman said, during a brainstorming session in the lab. While they were brainstorming, scientist Joe Frisch came up with the idea for mirrors.

“What you can do is reflect the light traveling away from the cloud back to the camera lens,” Cheong said. As a result, a camera can gather not only significantly more light, but also more views of an object from different angles, each of which appears in the raw photograph as a distinct dot on a black background. This collection of distinct images, the team realized, meant they had devised a form of so-called “bright-field imaging” and might be able to reconstruct a three-dimensional model of the cloud of atoms, not just an image. two-dimensional.

3D printing of an idea

With the support of a lab-led research and development grant, Cheong and Safdari took the mirror idea and continued it, designing an array of tiny mirrors capable of redirecting light from all around a cloud of atoms towards a camera. Using algebra and ray-tracing software developed by Kagan and Vandegar, the team calculated exactly the right positions and angles that would allow the mirror to keep many different images of the cloud in focus on the camera. The team also developed computer vision and artificial intelligence algorithms to use the 2D images to perform 3D reconstruction.

It’s the kind of thing that might seem obvious in retrospect, but it took a lot of thought to pull it off, Schwartzman said. “When we first came up with this we thought, ‘People must have done this before,'” he said, but in fact it’s new enough that the group has filed for a patent. on the device.

To test the idea, Cheong and Safdari made a mockup with a 3D-printed scaffold holding the mirrors, then made a micro-3D printed fluorescent object that spells out “DOE” when viewed from different angles. They took a picture of the object with their mirror dome and showed that they could, in fact, collect light from several different angles and keep all the images sharp. Additionally, their 3D reconstruction was so accurate that it revealed a small flaw in the making of the “DOE” object – an arm of the “E” slightly bent downward.

The next step, the researchers say, is to build a new version to test the idea in a smaller atom interferometer at Stanford, which would produce the first 3D images of clouds of atoms. This version of the mirror dome would sit outside the chamber containing the atom cloud, so if these tests are successful, the team would then build a stainless steel version of the mirror scaffold suitable for the vacuum conditions inside. inside an atomic interferometer.

Schwartzman said ideas developed by Cheong, Safdari and the rest of the team could be useful beyond physics experiments. “It’s a new device. Our application is atom interferometry, but it can be useful in other applications,” he said, such as quality control for making small objects in industry.

The research was funded by the Department of Energy, Laboratory-Led Research and Development Program. MAGIS-100 is supported by the Gordon and Betty Moore Foundation and the DOE Office of Science.