Scientists 'control' quantum light for the first time, achieving landmark

"This opens the door to the manipulation of what we can call 'quantum light'," Dr. Sahand Mahmoodian from the University of Sydney School of Physics and joint lead author of the research said in a press release. 

"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," explained Joint lead author Dr. Natasha Tomm.

"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."

This type of quantum light has the benefit that it can theoretically use fewer photons to do more sensitive measurements with better resolution. This can be crucial for biological microscopy applications where tiny features must be detected, and high light intensity can harm samples.

"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," Dr. Mahmoodian said.

"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," he added. 

Dr. Tomm went on to say that the experiment is beautiful not only because it validates a fundamental phenomenon- stimulated emission- at its ultimate limit but also marks a significant technological advancement for future uses.

"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," she concluded.

The full study was published in Nature on March 20 and can be found here. 

Study abstract:

The interaction between photons and a single two-level atom constitutes a fundamental paradigm in quantum physics. The nonlinearity provided by the atom leads to a strong dependence of the light–matter interface on the number of photons interacting with the two-level system within its emission lifetime. This nonlinearity unveils strongly correlated quasiparticles known as photon bound states, giving rise to key physical processes such as stimulated emission and soliton propagation. Although signatures consistent with the existence of photon bound states have been measured in strongly interacting Rydberg gases, their hallmark excitation-number-dependent dispersion and propagation velocity have not yet been observed. Here we report the direct observation of a photon-number-dependent time delay in the scattering off a single artificial atom—a semiconductor quantum dot coupled to an optical cavity. By scattering a weak coherent pulse off the cavity–quantum electrodynamics system and measuring the time-dependent output power and correlation functions, we show that single photons and two- and three-photon bound states incur different time delays, becoming shorter for higher photon numbers. This reduced time delay is a fingerprint of stimulated emission, where the arrival of two photons within the lifetime of an emitter causes one photon to stimulate the emission of another.