In brief

By using five crystals instead of just two, scientists have improved the sensitivity of interferometers for sensing or quantum engineering.

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Seeing (infra)red more clearly

9 Dec 2020

Adding more crystals to a quantum optical infrared sensor improves its sensitivity and opens the door to broader use.

Thin films on water display a mesmerizing array of colors as beams of light bounce off the water surface and ‘interfere’ with each other—a brilliant demonstration of a physical phenomenon called interference.

In the lab, devices for precise measurement based on interference patterns—called interferometers—are used in applications ranging from analyzing biological samples to measuring time. Major advancements in the burgeoning field of quantum optics have led to the development of nonlinear interferometers, which enable the characterization of a sample’s infrared properties using inexpensive components designed for visible light.

Nearly five years ago, Leonid Krivitsky, a Senior Scientist at A*STAR’s Institute of Materials Research and Engineering (IMRE), and colleagues were among the first to pioneer a low-cost, compact nonlinear interferometer that can detect infrared by measuring only visible light. However, the device used just two nonlinear crystals, placing a limit on its sensitivity.

“Conventional (linear) interferometers with N-elements are well known,” Krivitsky said. “The most common configuration is a Fabri-Pérot interferometer, which consists of two opposing mirrors with the light ‘bouncing’ between them. We found that if we substitute standard mirrors with nonlinear crystals, the working principle remains the same.”

And while theoretical studies on nonlinear interferometers have hinted that increasing the number of nonlinear elements can enhance their sensitivity, no such device existed until Krivitsky and his IMRE colleague, Anna Paterova, decided to build one—a stable and versatile nonlinear interferometer with up to five nonlinear crystals.

The researchers first performed a theoretical study to calculate ideal values for parameters including the wavelengths of the two quantum-linked beams and the orientation of each crystal. They confirmed that increasing the number of nonlinear crystals from two to five caused a narrowing of bright interference fringes or bands, improving the accuracy and precision of their device.

“The width of the interference fringes determines how accurately we can measure the phase shifts in the interferometer caused by the analyte,” explained Krivitsky. “Thus, by increasing the number of nonlinear elements, we improve the sensitivity of the interferometer.”

The researchers confirmed their theoretical findings by building and testing a nonlinear interferometer with two to five crystals. Importantly, they discovered that parameter setting, particularly the angle of each crystal, becomes increasingly important with the addition of more nonlinear crystals.

To show real-life applicability, the researchers confirmed experimentally that the five-crystal configuration had greater accuracy for detecting carbon dioxide gas over the two-crystal configuration.

“We are now working on realizing such interferometers using integrated optical chips,” concluded Krivitsky. “By developing interferometers on a chip, it will allow us to achieve an increase in sensitivity down to the molecular level on a compact and robust platform.”

The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).

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Paterova, A.V. and Krivitsky, L.A. Nonlinear interference in crystal superlattices. Light: Science & Applications 9, 82 (2020) | article

About the Researcher

Leonid Krivitsky is the head of the quantum technologies for engineering department at A*STAR’s Institute of Materials Research and Engineering (IMRE). Krivitsky obtained his PhD in Physics from Lomonosov Moscow State University and held postdoctoral positions at the National Metrology Institute, Max Planck Institute for the Science of Light, and the Technical University of Denmark. His scientific interests include quantum metrology and sensing and quantum information processing.

This article was made for A*STAR Research by Wildtype Media Group