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Researchers at the University of Sydney and the University of Basel in Switzerland have achieved a groundbreaking feat by manipulating and identifying small numbers of interacting photons, or packets of light energy, with high correlation. This achievement is a significant milestone in the development of quantum technologies and has been published in Nature Physics.

Stimulated Emission Observed for Single Photons

Stimulated light emission, which was postulated by Einstein in 1916, has been widely observed for large numbers of photons and was the basis for the invention of the laser. However, with this research, scientists have now observed stimulated emission for single photons. Specifically, the researchers were able to measure the direct time delay between one photon and a pair of bound photons scattering off a single quantum dot, which is a type of artificially created atom.

“This opens the door to the manipulation of what we can call ‘quantum light’,” said Dr. Sahand Mahmoodian, joint lead author of the research and from the University of Sydney School of Physics. “This fundamental science opens the pathway for advances in quantum-enhanced measurement techniques and photonic quantum computing.”

Light Interacting with Matter

Scientists discovered more than a century ago that light was not just a beam of particles or a wave pattern of energy, but exhibited both characteristics, known as wave-particle duality. The way light interacts with matter continues to captivate scientists and the human imagination, both for its theoretical beauty and its powerful practical application.

Research into light is a vital science with important practical uses. Without these theoretical underpinnings, practically all modern technology would be impossible. No mobile phones, no global communication network, no computers, no GPS, no modern medical imaging.

Advantages of Using Light in Communication

One advantage of using light in communication through optic fibers is that packets of light energy, or photons, do not easily interact with each other. This creates near distortion-free transfer of information at light speed. However, sometimes we want light to interact, and this is where things get tricky.

For instance, light is used to measure small changes in distance using instruments called interferometers. These measuring tools are now commonplace, whether it be in advanced medical imaging, for important but perhaps more prosaic tasks like performing quality control on milk, or in the form of sophisticated instruments such as LIGO, which first measured gravitational waves in 2015.

Quantum Light for More Sensitive Measurements

The laws of quantum mechanics set limits as to the sensitivity of such devices. This limit is set between how sensitive a measurement can be and the average number of photons in the measuring device. For classical laser light, this is different from quantum light.

Quantum light has an advantage in that it can, in principle, make more sensitive measurements with better resolution using fewer photons. This can be important for applications in biological microscopy when large light intensities can damage samples and where the features to be observed are particularly small.

Future Research

“By demonstrating that we can identify and manipulate photon-bound states, we have taken a vital first step towards harnessing quantum light for practical use,” said Dr. Mahmoodian. “The next steps in my research are to see how this approach can be used to generate states of light that are useful for fault-tolerant quantum computing, which is being pursued by multimillion dollar companies, such as PsiQuantum and Xanadu.”

“This experiment is beautiful, not only because it validates a fundamental effect — stimulated emission — at its ultimate limit, but it also represents a huge technological step towards advanced applications,” said Dr. Natasha Tomm, joint lead author of the research and from the University of Basel. “We can apply the same principles to develop more-efficient devices that give us photon bound states. This is very promising for applications in a wide range of areas: from biology to advanced manufacturing and quantum information processing.”

Collaboration and Research

The research was a collaboration between the University of Basel, Leibniz University Hannover, the University of Sydney, and Ruhr University Bochum. The lead authors are Dr. Natasha Tomm from the University of Basel and Dr. Sahand Mahmoodian at the University of Sydney, where he is an Australian Research Council Future Fellow and Senior Lecturer. The artificial atoms (quantum dots) were fabricated at Bochum and used in an experiment performed in the Nano-Photonics Group at the University of Basel. Theoretical work on the discovery was carried out by Dr. Mahmoodian at the University of Sydney and Leibniz University Hannover.


Source material by: University of Sydney. Content has been edited for readability.