A NICE (Nulling Interferometry Cryogenic Experiment) update: Beyond 1e-5 and towards cryogenic operation

To find a world like our own, astronomers must look for a faint glow hiding in the blinding glare of a distant star. This is the fundamental challenge of searching for exoplanets: the star is millions of times brighter than the planet it hosts, making the planet nearly impossible to see directly. One of the most promising ways to solve this problem is a technique called nulling interferometry. Imagine two telescopes working together as a single instrument, collecting light from a star and combining it in a way that causes the starlight waves to cancel each other out, like two ripples in a pond meeting and flattening the water. If the cancellation is perfect, the star disappears, leaving only the faint light of any orbiting planets visible. However, achieving this perfect cancellation requires extreme precision, especially when looking for planets in the mid-infrared part of the spectrum, where the heat of the planets themselves shines brightest. To reach the necessary sensitivity, the entire instrument must be cooled to temperatures near absolute zero, a condition that introduces new physical challenges as materials shrink and shift.

At the Swiss Federal Institute of Technology in Zurich, a team of researchers is building a laboratory testbed called the Nulling Interferometry Cryogenic Experiment, or NICE, to prove that this difficult technology can work. Their goal is to prepare for the LIFE mission, a future space observatory designed to hunt for habitable worlds. In their latest report, the team describes how they have successfully demonstrated the ability to cancel out starlight to a level of one part in one hundred thousand, but only for a single color of light and a single polarization state. This is a crucial threshold for the mission, yet it falls short of the full broadband and polarization-agnostic requirements needed for the final space instrument. They achieved this using a setup that mimics the final space instrument, but they did so at room temperature first to ensure the basic concept works before attempting the much harder task of cooling the entire system to fifteen Kelvin, which is roughly minus 258 degrees Celsius. The researchers found that while they could reach the required darkness in a single color of light, stretching this performance across a broad range of colors and different polarizations of light revealed several stubborn obstacles that must be solved before the space mission can fly.

The experiment is built around a two-beam interferometer, a device that splits a laser beam into two paths and then recombines them. In the NICE setup, the light travels through a series of mirrors and lenses, guided by a sophisticated control system that constantly measures the position of the beams and makes tiny adjustments to keep them perfectly aligned. The researchers used a special type of fiber optic cable to filter the light, allowing only the cleanest, most uniform waves to pass through to the detector. This filtering is essential because it removes any messy distortions in the light wave that would prevent the starlight from canceling out completely. In their initial tests at room temperature, the team managed to create a "null," or a point of darkness, that was deep enough to meet the requirements for the LIFE mission, but only for a single, narrow color of light. They also showed that the system could maintain this darkness for a short period, proving the concept is viable.

However, the path to a working space telescope is paved with complications that only appear when the system is pushed to its limits. The researchers discovered that their current fiber optic cables were not perfect at blocking unwanted light patterns. Even though the main beam of light passed through cleanly, some stray light leaked through the outer layers of the cable, creating a faint background glow that prevented the starlight from being canceled out completely. This leakage was so subtle that it only became visible when they tried to reach the deepest levels of darkness. The team tested several different types of fiber materials and coatings to stop this leak, including exotic glass and special liquid metal coatings, but none of these attempts fully solved the problem. They found that while improving the alignment of the equipment could reduce the amount of stray light entering the fiber, this approach does not fix the underlying problem of mode leakage and actually reduces the allowed tolerances for the system.

Another major hurdle is the color of the light itself. The LIFE mission needs to observe a wide range of infrared colors, from four to eighteen micrometers, to detect the chemical signatures of life. The NICE team found that while they could cancel out one specific color of light, keeping the cancellation perfect across a broad spectrum is incredibly difficult. This is because the different colors of light travel at slightly different speeds through the glass and other materials in the instrument, causing them to fall out of step with one another. The researchers built a model to predict how much error this would cause and found that the manufacturing tolerances for the glass components would need to be incredibly tight, with errors smaller than the width of a human hair. They are currently developing a mechanical device that can shift a piece of glass back and forth by a tiny amount to correct this timing error, but they have not yet achieved a deep null across a broad band of colors.

The team also investigated how the polarization of light, which describes the direction in which the light waves vibrate, affects the cancellation. Since stars and planets emit light that vibrates in all directions, the instrument must be able to cancel out both types of vibration equally well. The researchers measured the polarization of the two beams and found that small differences in the angle of the mirrors caused the light to vibrate slightly differently in each path. This difference, though small, was enough to degrade the quality of the cancellation. They identified a specific mirror in the delay line system as the main culprit and corrected its alignment, which improved the results. However, they still need to determine if the remaining differences are small enough to meet the mission's strict requirements or if they will need to build a new mechanism to actively correct the polarization.

Looking ahead, the researchers are preparing to move their experiment from room temperature into a cryogenic environment. They have built a small, modular freezer called the "Ice Cube" to test how the materials and mechanical parts of their instrument behave as they cool down to fifteen Kelvin. This step is critical because the extreme cold causes materials to contract, which could throw the delicate alignment of the mirrors out of whack. The team plans to use this freezer to test different ways of mounting the optics and to verify that their control systems can still function in the cold. Once they have confirmed that the components can survive the cold, they will begin designing a much larger freezer that will house the entire instrument. The ultimate goal is to demonstrate that a deep, stable null can be achieved at these low temperatures, proving that the technology is ready for the space environment.

The roadmap for the NICE experiment is clear, though the work ahead is substantial. The team has already proven that the basic idea works and has identified the specific technical barriers that stand in the way of a full-scale space mission. They are now focused on solving the problems of stray light, broad color ranges, and polarization, all while preparing the system for the extreme cold of space. If they succeed, they will have provided the essential proof of concept needed to launch the LIFE mission, a project that could finally reveal the secrets of planets that might harbor life. The journey from a laboratory bench to a space telescope is long and fraught with challenges, but the NICE team has shown that the path forward is possible, one precise adjustment at a time.