Quantum sensor for photons

A photodetector converts the light into an electrical signal, which results in its loss. Researchers led by Tracy Northup of the University of Innsbruck have developed a quantum sensor capable of measuring light particles non-destructively. It can be used to deepen the quantum properties of light.

The physicist Tracy Northup is currently studying quantum internet development at the University of Innsbruck. The American citizen builds interfaces with which quantum information can be transferred from matter to light and vice versa. Through these interfaces, it is expected that quantum computers around the world will be able to communicate with each other through fiber optic lines. In their research, Northup and his team at the Department of Experimental Physics have now shown a method of measuring non-destructively visible light. The development follows the work of Serge Haroche, who characterized the quantum properties of microwave fields with the help of neutral atoms in the 1990s and received the Nobel Prize in Physics in 2012.

In postdoctoral work by Moonjoo Lee and Ph.D. student Konstantin Friebe, the researchers placed an ionized calcium atom between two hollow mirrors through which visible laser light is guided. "The ion has only a slight influence on light," says Tracy Northup. "Quantum ion measurements allow us to make statistical predictions about the number of light particles in the chamber." Physicists were supported in their interpretation of the measurement results by the research group led by Helmut Ritsch, a quantum optician from Innsbruck's Department of Theoretical Physics. "One can speak in this context of quantum sensor for light particles", summarizes Northup, Ingeborg Hochmair Chair at the University of Innsbruck since 2017. One of the applications of the new method would be to generate custom specific light fields return the measurement results to the system via a feedback loop, thereby establishing the desired states.

In the current work in Letters of physical examinationresearchers have limited themselves to classical states. In the future, this method could also be used to measure the quantum states of light. The work has been financially supported by the Austrian Science Fund FWF and the European Union, among others.