An important achievement taking our understanding of the quantum properties of light to a new level was made by scientists from the UP Faculty of Science. They have developed a unique method of recognising special quantum multiphoton states, even from emitters with limited efficiency. Their theoretical method has been confirmed by an experiment, for which they developed their own, precisely controlled light emitter and detector. The findings were recently published in the journal Nature NPJ: Quantum Information, and PhD students Lukáš Lachman and Ivo Straka played an important part in the research. The method has broad application potential.
While classical light emitted in nature only has wave properties, quantum optics sees light as a stream of particles, called photons. These particle properties are explored in laboratories. “From the quantum point of view, photons have one interesting property, called quantum non-Gaussianity, which means one photon cannot be prepared by quadratic non-linear processes. This property had been measured in single-photon states earlier in our lab, then in many others around the world. However we were interested whether it could be also detected in emitters generating multiple photons. In the article we have brought evidence that it is possible indeed. We have developed theoretical tools that enabled us to reliably verify this property, and in addition to this we have managed to conduct an experiment which has witnessed this property directly, on up to nine photons,” said Lukáš Lachman, who specialised in the theoretical part of the research.
Non-classical states of light are important for some applications in quantum metrology and in quantum information processing. However, they are difficult to obtain. Experimental work with multiple photons is quite a difficult task for optical physicists. “These methods are in the early stages of development. For instance, to acquire exactly five photons in a controlled manner is a great challenge, and so is their simultaneous measurement. That’s why we have developed a method that makes it possible. We suppose that it might be of interest even for foreign laboratories, where such quantum states are generated by means of other methods, but until now they have not had sufficient tools to monitor their quality,” explained the experimenter, Ivo Straka.
During the experiment, the optical physicists generated a controlled stream of photons from the light source, and witnessed the properties of individual light particles via their detectors. “In neither of these two crucial stages of the experiment did we have at hand a ready-made device; in order to make our measurements, we had to design our own device and build it from available components. Both the emitter and the detector. Their development is demanding in itself,” emphasised the leader of the project, Radim Filip.
According to Filip, the detection of quantum non-Gaussianity is important for advanced diagnostics of new nonlinear optical processes in developing quantum technologies. “These diagnostics allow us to go beyond the results obtained by optical spectroscopy of materials. This is where we see the future of our method,” added Filip.
The huge application potential of the method has been confirmed by the laboratory team leader, Miroslav Ježek. “The method will enable the foundation of quantum microscopy, which will be able to study not only the spatial structure, but especially the type of the process that is occurring in the live sample, all the way down to the level of individual molecules. We could get to functionality and resolution far beyond the limits of existing microscopes. So we will further investigate this direction,” said Ježek.
The published article is another culmination of research that has been going on at the department for about seven years. The necessary prerequisite for success is the top-notch equipment in the laboratory. “The development of the method demanded a great deal of invention, and the experiment demanded precision and controllability. Then the processing of the huge amount of data was very complicated. It was a lengthy process; we built on previous findings. At this moment, we’re in a stage when the methodology we work with is on the verge of technical feasibility. Often what is needed is just to overcome certain limitations in our minds and think outside the box,” concluded Filip.