Researchers at the University of Innsbruck have demonstrated a method for measuring visible light particles nondestructively. The method could be used to investigate the quantum properties of light. The team placed an ionized calcium atom between two hollow mirrors, creating a cavity through which visible laser light was guided. “The ion has only a weak influence on the light,” said professor Tracy Northup. The researchers took quantum measurements of the ion. Based on these measurements, they were able to make statistical predictions about the number of light particles in the chamber. They used the measurements first to obtain the ion-cavity interaction strength. Then they reconstructed the cavity photon-number distribution for coherent states and for a state with mixed thermal-coherent statistics, finding overlaps above 99% with the calibrated states. An ion between two spherical mirrors could serve as a quantum sensor for light particles. Courtesy of Klemens Schüppert. The new method could be used to generate specially tailored light fields by feeding the measurement results back into the system via a feedback loop, thus establishing the desired states. In their current work, the researchers have limited themselves to classical states. In the future, this method could also be used to measure quantum states of light. The research was published in Physical Review Letters (https://doi.org/10.1103/PhysRevLett.122.153603).