Scientists one step closer to manipulating 'quantum light'

Scientists at the University of Sydney and the University of Basel in Switzerland have demonstrated the ability to manipulate and identify small numbers of interacting photons — packets of light energy — with high correlation. This finding represents an important achievement in the development of quantum technologies. The research was published in Nature Physics.

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

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

By observing how light interacted with matter more than a century ago, scientists discovered light was not a beam of particles, nor a wave pattern of energy — but exhibited both characteristics, known as wave–particle duality. The way light interacts with matter continues to interest scientists, both for its theoretical beauty and its practical applications. One advantage of using light in communication — through optic fibres — is that packets of light energy, photons, do not easily interact with each other. This creates near distortion-free transfer of information at light speed. However, sometimes interaction is needed, which can present challenges.

For instance, light is used to measure small changes in distance using instruments called interferometers; these measuring tools are used in advanced medical imaging, for performing quality control on milk, or in the form of sophisticated instruments like LIGO, which measures gravitational waves. The laws of quantum mechanics set limits as to the sensitivity of such devices. The 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.

Joint lead author Dr Natasha Tomm from the University of Basel said the device built by the researchers induced such strong interactions between photons that the researchers could observe the difference between one photon interacting with it compared to two. “We observed that one photon was delayed by a longer time compared to two photons. With this really strong photon–photon interaction, the two photons become entangled in the form of what is called a two-photon bound state,” Mann said.

Quantum light like this 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.

“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. 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,” Mahmoodian said.

Tomm said the experiment validates a fundamental effect — stimulated emission — at its ultimate limit, and also represents a huge technological step towards advanced applications. “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,” Tomm said.

The research was a collaboration between the University of Basel, Leibniz University Hannover, the University of Sydney and Ruhr University Bochum. The artificial atoms (quantum dots) were fabricated at Bochum and used in experiment performed in the Nano-Photonics Group at the University of Basel. Theoretical work on the discovery was carried out by Mahmoodian at the University of Sydney and Leibniz University Hannover.

Image caption: Artist’s impression of how photons bound together after interaction with artificial atom. Image credit: University of Basel.