Ultra-cold quantum sensors have significantly improved the accuracy of X-ray measurements, offering new tools for evaluating nuclear materials. Scientists at the National Institute of Standards and Technology (NIST) have measured X-ray emissions from elements like plutonium, uranium, and neptunium with greater precision than ever before. This advancement helps filter out background X-ray noise, enabling more accurate assessments of nuclear material accumulation. According to Jonathan Dean, a physicist at NIST and the University of Colorado Boulder, these precise measurements support international efforts to monitor and safeguard nuclear materials by allowing more exact tracking of their presence in nuclear facilities. The researchers used transition edge sensors (TESs), which are tiny, highly sensitive thermometers. These sensors are made of a superconducting film kept at a temperature just a fraction of a degree above absolute zero. When an X-ray photon hits the sensor, it generates a tiny amount of heat, which increases the resistance of the superconducting film. This change in resistance corresponds directly to the energy of the photon, allowing for high-resolution energy measurements. The TESs have reduced the uncertainty in X-ray energy measurements by one-third to one-eighth compared to previous methods, greatly enhancing their precision. Accurately measuring and subtracting the background X-ray radiation allows TESs and similar gamma-ray detectors to better characterize nuclear materials. Determining the ratio of different isotopes in a sample is essential, as it can reveal whether the material is intended for use in a nuclear power plant or a nuclear weapon. For instance, uranium-235 makes up only 0.7% of natural uranium but must be enriched to a few percent for use in nuclear fuel and up to 90% for weapons-grade material. These precise measurements help ensure that nuclear materials are used appropriately and safely. The new measurement techniques allow scientists to assess the composition of nuclear materials more quickly, which could improve operations at nuclear power plants. Generating electricity from fission involves multiple steps, each of which requires reassessing the composition of the nuclear fuel. Measuring this more rapidly could reduce delays between steps, increasing efficiency and lowering costs. This advancement could lead to more effective and cost-efficient nuclear energy production. Despite their high sensitivity, TESs require cooling to temperatures just above absolute zero, and the necessary equipment is too large to be handheld. However, arrays of TES detectors can be used in any location with a reliable power source to run the refrigeration system. In places where TESs are not immediately available, scientists can collect samples and send them to specialized laboratories for analysis. NIST has already installed TES detectors at three Department of Energy laboratories and deployed them in research projects at national and international facilities, including the CERN particle accelerator in Switzerland. Looking ahead, the research team is working to further improve the accuracy of these sensors to aid in the search for new fundamental particles in collaboration with CERN and to install a new detector at NASA Goddard to study extreme environments in the universe. NIST is also working to simplify, miniaturize, and reduce the cost of the cooling equipment. Two U.S. companies have already adapted and are manufacturing a compact refrigeration system based on NIST designs, making the technology more accessible for future applications.