“Quantum light” manipulation a step closer, with potential in medical imaging and quantum computing

For the first time, scientists have been able to identify and manipulate photons – particles of light – which are interacting with each other.

The breakthrough has implications for quantum technologies including advances in medical imaging and quantum computing.

Photons can also be thought of as packets of light energy, or “quanta” of light. Over a century ago, physicists coming to grips with the weird world of quantum mechanics discovered “ wave-particle duality.” Photons, electrons and other subatomic particles behaved not as either particles or waves, but exhibited characteristics of both forms.

Einstein first proposed in 1916 (published in 1917) that you could get atoms to emit photons by “exciting” the electrons in the atoms with energy. This type of photon scattering is seen every day now in lasers (LASER = Light Amplification by Stimulated Emission of Radiation) with large numbers of photons.

But this new research shows stimulated emission for single photons.

Now, scientists at the University of Sydney and Switzerland’s University of Basel teamed up to observe stimulated emission for single photons for the first time.

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The physicists were able to directly measure the time delay between one photon and a pair of photons scattering of a single quantum dot.

A quantum dot is a type of artificial atom produced using a nanometre-sized crystal structure. Quantum dots can convert light of one wavelength into another wavelength photon.

“This opens the door to the manipulation of what we can call ‘quantum light’,” says the University of Sydney’s Dr Sahand Mahmoodian. “This fundamental science opens the pathway for advances in quantum-enhanced measurement techniques and photonic quantum computing.”

Understanding the nature of light not only captures the imagination, but underpins much of modern technology including mobile phones, global communications networks, computers, GPS and modern medical imaging.

Further advances in our knowledge of how light works promises to underpin new technological innovations.

Light has already shown promise, through optical fibres, as a replacement for electrical networks for near distortion-free and ultra-fast transfer of information.

It’s when we want light to interact that things get a little messy.

For example, interferometers are now common measuring tools that work by merging two or more light sources to create an interference pattern. Interferometers are used in medical imaging and in some of the most advanced experiments in the world such as LIGO at Caltech which was the first to detect gravitational waves in 2015.

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Interferometers are limited in their sensitivity by quantum effects which make it difficult to tell the many photons in the device apart.

“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,” says Dr Natasha Tomm from the University of Basel. “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.”

Such “quantum light” devices, as opposed to interferometers which until now have used classical laser light, promise to have far higher resolution and sensitivity.

The researchers say this will be useful in fields such as medical imaging, and further research will be aimed at manipulating quantum light to produce fault-tolerant quantum computers.

“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,” Tomm explains. 

“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.”

The research is published in Nature Physics.