“Quantum Light” manipulation one step closer

For the first time, scientists have succeeded in identifying and manipulating interacting photons – light particles.

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 “light quanta”. Over a century ago, physicists grappling with the strange world of quantum mechanics discovered “wave-particle duality”. Photons, electrons and other subatomic particles behaved neither like particles nor like waves, but showed properties of both forms.

Einstein first suggested in 1916 (published in 1917) that one could cause atoms to emit photons by “exciting” the electrons in the atoms with energy. This type of photon scattering can now be observed every day with lasers (LASER = Light Amplification by Stimulated Emission of Radiation) with a large number of photons.

But this new research shows stimulated emission for single photons.

Scientists from the University of Sydney and the Swiss University of Basel have now 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 a photon and the scattering of a pair of photons at a single quantum dot.

A quantum dot is a type of artificial atom that is made with a nanometer-sized crystal structure. Quantum dots can convert light of one wavelength into a photon of another wavelength.

dr Sahand Mahmoodian. Photo credit: University of Sydney.

“This opens the door to manipulating what we can call ‘quantum light,'” says Dr. Sahand Mahmoodian from the University of Sydney. “This fundamental research paves the way for advances in quantum-based measurement techniques and photonic quantum computing.”

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

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

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Light has already proven itself through optical fibers as a promising replacement for electrical networks for almost distortion-free and ultra-fast transmission of information.

When we want light to interact, things get a bit messy.

For example, interferometers are common measurement tools today that work by combining two or more light sources to create an interference pattern. Interferometers are used in medical imaging and in some of the world’s most advanced experiments, like LIGO at Caltech, which first discovered gravitational waves in 2015.

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

dr Natasha Tomm. Source: University of Basel.

“The device we built induced such strong interactions between photons that we could observe the difference between one photon interacting with it versus 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. In this really strong photon-photon interaction, the two photons become entangled in a so-called two-photon bound state.”

Such “quantum light” devices promise a much higher resolution and sensitivity than interferometers, which have so far worked with classic laser light.

Researchers say this will be useful in areas like medical imaging, and further research will aim to manipulate quantum light to make 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 major technological step towards more advanced applications,” explains Tomm.

“We can apply the same principles to design more efficient devices that give us photon-bound states. This is very promising for applications in a variety of fields: from biology to advanced manufacturing to quantum information processing.”

The research is published in Nature Physics.