Indian scientists design new technique to better study absolute zero atoms

Indian scientists have designed a new image-correction technique capable of getting better images during the study of atoms at absolute zero temperature. The technique can get rid of 50% unwanted interference fringes in the images, which are important for understanding the intriguing quantum mechanics governed properties of atoms at cold temperatures better, the Science and Technology Ministry said in a recent statement.

A research group at the Raman Research Institute (RRI), an autonomous institute of the Ministry, has developed an image-correction solution. At low temperatures near absolute zero, the original properties of atoms based on classical mechanics are replaced and then governed by the laws of quantum mechanics. They hold the potential to offer a possibility to study and better understand the atomic properties at such low temperatures, according to the Ministry.

The commonly used technique for the study of ultracold atoms is by deploying magneto-optical traps with high-power laser cooling techniques. Cold atoms of elements like sodium, potassium, and rubidium are commonly studied. Detection techniques, namely fluorescence, absorption or phase-contrast imaging techniques are used. Of these, imaging through fluorescence or absorption techniques is widely used.

However, the images obtained using these techniques often suffer due to unwanted interference fringes which are unwanted dark-bright patterns imprinted on the actual images, thus lowering the quality of results. The presence of unwanted interference fringes has the potential to derail the accurate calculation of important parameters -- the atom number, temperature and dynamics in smaller time scales.

The newly developed algorithm by the team of the Raman Research Institute is based on the existing eigenface recognition coupled with a smart masking technique aimed to obtain images with minimal interference fringes. This eigenface recognition is quite similar to finding a correct image of a person or an object from a group of images based on the features of the object. Our cell phones use this as a base technology, however, modern-day smartphones have modified this with additional machine-learning-based technology to improve the feature but the idea remains the same.

"While dealing with the cold atoms, it is necessary to calculate the Optical Density (OD) from which one can determine the temperature, size, density and other useful parameters," said Gourab Pal, a PhD student at the Raman Research Institute. In this algorithm, researchers need to calculate an important parameter known as the Optical Density, which is the logarithmic subtraction of two frames -- one containing the cold cloud (denoted as S) and the other is the probe light (denoted by L). Under ideal circumstances, both the L and S frames have identical interference fringes, which when logarithmically subtracted result in the removal of fringes.

"But in reality, while working in the lab, these frames do not showcase identical interference fringes, making the situation challenging and requiring a de-fringing method to obtain a clean Optical Density," explained Pal, who is the first author of the paper titled 'Efficient denoising of cold atom images using optimised eigenface recognition algorithm'.

In the paper published recently in the journal Applied Optics, the RRI team claimed the proposed technique could reduce the interference fringes in the absorption imaging of cold atoms by nearly 50%. In addition, there was a marked improvement, of the order of 50%, obtained in the temperature uncertainties in cold Rubidium atoms, when this algorithm was applied. Scientists contend the absorption imaging technique is popular in the cold atom community and has a wide range of applications.

"This is particularly useful where the number of atoms is fewer. Absorption imaging can be used to find the density profile of cold and ultracold atoms. In this technique, we find the temperature of a cold atom cloud via time-of-flight measurements. The basis of the quantum gas microscopy is the absorption imaging. In addition, this method is used to perform in-situ measurements of trapped atoms," said co-author of the paper, Saptarishi Chaudhuri, head QuMix laboratory at the Institute.