Researchers have developed a new laser that makes it possible to measure electron transition energies in small atoms and molecules with unprecedented precision. The instrument will help scientists test one of the bedrock theories of modern physics to new limits, and may help resolve an unexplained discrepancy in measurements of the size of the proton. The team will present their work during the Frontiers in Optics (FiO)/Laser Science (LS) conference in Rochester, New York, USA on 17 -21 October 2016.