Fundamental first step for the practical use of quantum light

For the first time, the ability to manipulate and identify small amounts of interacting photons (packets of light energy) with a high correlation has been demonstrated.

This unprecedented achievement represents a major milestone in the development of quantum technologies, according to scientists from the Universities of Sydney and Basel who publish the breakthrough in Nature Physics.

Stimulated light emission, postulated by Einstein in 1916, is widely observed for large numbers of photons and laid the foundation for the invention of the laser. With this research, the stimulated emission of single photons has now been observed.

Specifically, the scientists were able to measure the direct time lag between a photon and a pair of bound photons scattering into a single quantum dot, a type of artificially created atom.

“This opens the door to the manipulation of what we can call ‘quantum light,'” Dr Sahand Mahmoodian of the University of Sydney’s School of Physics and co-lead author of the research said in a statement.

Dr Mahmoodian said: “This fundamental science paves the way for advances in quantum-enhanced measurement techniques and photonic quantum computing.”

By observing how light interacted with matter more than a century ago, scientists discovered that light was neither a beam of particles nor a wave pattern of energy, but instead 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.

Whether it’s how light traversed the vast spaces of the interstellar medium or the development of the laser, light research is a vital science with important practical uses. Without these theoretical foundations, virtually all modern technology would be impossible. No mobile phones, no global communication network, no computers, no GPS, no modern medical imaging.

One advantage of using light in communication, via fiber optics, is that the packets of light energy, the photons, do not easily interact with each other. This creates a nearly distortion-free transfer of information at the speed of light.

However, sometimes we want light to interact. And here, things get complicated.

For example, light is used to measure small changes in distance using instruments called interferometers. These measurement tools are now commonplace, either in advanced medical imaging, for important but perhaps more mundane tasks like performing milk quality control, or in the form of sophisticated instruments like LIGO, which first measured gravitational waves. in 2015.

The laws of quantum mechanics set limits on the sensitivity of such devices.

This limit is set between how sensitive a measurement can be and the average number of photons in the measurement device. For classical laser light, this is different than quantum light.

Co-lead author Dr. Natasha Tomm from the University of Basel said in a statement: “The device we built induced such strong interactions between photons that we were able to observe the difference between one photon interacting with it compared to two.” .

“We observed that one photon was delayed longer 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.”

Quantum light like this has the advantage that, in principle, you can make more sensitive measurements with better resolution using fewer photons. This may be important for applications in biological microscopy where high 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 critical first step in harnessing quantum light to practical use,” said Dr. Mahmoodian.

“The next steps in my research are to see how this approach can be used to generate light states that are useful for fault-tolerant quantum computing, which is what multibillion-dollar companies like PsiQuantum and Xanadu are pursuing.”