Quantum Sensor for Photons

An ion between two spherical mirrors serves as a quantum sensor for light particles. (Source: K. Schüppert)

Physicist Tracy Northup is currently researching the develop­ment of quantum internet at the Univer­sity of Innsbruck. The American citizen builds interfaces with which quantum information can be transferred from matter to light and vice versa. Over such inter­faces, it is anticipated that quantum computers all over the world will be able to communi­cate with each other via fiber optic lines in the future. In their research, Northup and her team at the Department of Experimental Physics have now demons­trated a method with which visible light can be measured non-destructively. 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 was awarded the Nobel Prize in Physics in 2012.

In work led by postdoc Moonjoo Lee and PhD student Konstantin Friebe, the researchers place an ionized calcium atom between two hollow mirrors through which visible laser light is guided. “The ion has only a weak influence on the light,” explains Northup. “Quantum measurements of the ion allow us to make statis­tical predic­tions about the number of light particles in the chamber.”

The physicists were supported in their interpretation of the measurement results by the research group led by Helmut Ritsch, a Innsbruck quantum optician from the Depart­ment of Theoretical Physics. “One can speak in this context of a quantum sensor for light particles”, sums up Northup, who has held an Ingeborg Hochmair professor­ship at the Uni­versity of Innsbruck since 2017. One application of the new method would be to generate special tailored light fields by feeding the measurement results back into the system via a feedback loop, thus establishing the desired states.

Now, the researchers have limited thems­elves to classical states. In the future, this method could also be used to measure quantum states of light. The work was finan­cially supported by the Austrian Science Fund FWF and the European Union, among others. (Source: U. Innsbruck)

Reference: M. Lee et al.: Ion-Based Quantum Sensor for Optical Cavity Photon Numbers, Phys. Rev. Lett. 122, 153603 (2019); DOI: 10.1103/PhysRevLett.122.153603

Link: Quantum Optics & Spectroscopy, Institute for Experimental Physics, University of Innsbruck, Innsbruck, Austria

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