Multiple mirrors illuminate atom interferometry

atom interferometry
Various views of a 3D-printed object captured by a single camera. (Courtesy: Sanha Cheong/SLAC National Accelerator Laboratory)

A new multiple-mirror imaging technique could greatly improve the performance of atom interferometers, making them more useful in applications ranging from dark matter detection to quality control in manufacturing. By capturing incoming light from many different angles, the new technique enables scientists to collect more light than is possible using conventional imaging set-ups, boosting the system’s sensitivity.

The new technique, which was developed by researchers at the US Department of Energy’s SLAC National Accelerator Laboratory, is an example of light-field imaging, which captures not just the intensity of light, but also the direction in which light rays travel. The multiple mirrors redirect the different light views and overlap them onto an imaging sensor. This light field information can then be used to reconstruct a three-dimensional image of an object.

Gravitational searches for dark matter

One possible use for the new technique would be in the Matter-wave Atomic Gradiometer Interferometric Sensor, a 100-metre-long atom interferometer currently being installed at the Fermi National Accelerator Laboratory in Illinois, US. MAGIS-100, as it is known, will be a new tool in the ongoing search for dark matter – the mysterious substance that is thought to make up 85% of the matter in the universe but is currently only observable through its gravitational influence, which prevents large objects such as galaxies from flying apart as they rotate. The experiment will also serve as a pathfinder towards larger scale mid-band gravity wave detectors.

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In MAGIS-100, researchers will release clouds of strontium atoms in a vacuum tube and then shine laser light on the clouds to image them as they fall within the tube. Each atom acts like a wave and the laser light puts these atomic waves into a superposition of quantum states: one state in which the atom continues down its original path and another in which the light “kicks” it higher up the tube. The two waves then recombine, creating an interference pattern. The relative phase between pairs of such interference patterns created using two interferometers can be highly sensitive to the presence of gravitational waves, as well as ultra-light dark matter manifesting as classically oscillating waves.

For this technique to work, however, the laser light used to make the atoms fluoresce for imaging the final interference pattern needs to be just the right intensity. Too intense, and it will destroy the structure of the atom clouds; not intense enough, and the clouds will be too dim to be picked up by the experiment’s imaging camera (which sits outside the chamber that holds the atoms). One solution to this problem would be to use a camera with a wider aperture, but this would create a narrow depth of field in which only a small part of the image is in focus.

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Capturing more light

In the new work, the team led by Ariel Schwartzman and graduate students Murtaza Safdari and Sanha Cheong of the SLAC National Accelerator Laboratory overcame this problem by reflecting light travelling away from the cloud back into the camera lens. The camera can then gather not just more light, but also more views of an object from different angles, each of which shows up on the image as a distinct spot on a black background. A collection of such distinct images can be used to reconstruct a 3D model of the atom cloud.

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“Conventional imaging captures only as much light as the lens aperture can accept, and it necessarily loses directional information since it integrates light over the aperture of the lens,” Safdari tells Physics World. “Conventional spatially multiplexed light field imaging is also hampered by the limited lens aperture. Our system is able to benefit from the 3D information capturing ability of spatially multiplexed systems, while also capturing more light than the lens’ aperture would conventionally allow.”

Safdari adds that while the system would directly benefit imaging in atom interferometer experiments like MAGIS-100, it could also have other applications, such as parts inspection on production lines and particle tracking. He and his colleagues are now adapting their design concept to take images of atom clouds in a magneto-optical trap at Stanford, while in the longer term they would like to develop an in-vacuum version of the system to install at MAGIS-100.

The present work is detailed in the Journal of Instrumentation.

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