A photon can trap another photon within a material, researchers have discovered.

ANU honours student Janet Zhong made the surprise discovery that a photon can be localised within a second photon while she was modelling a very simple collection of atoms.

The phenomenon is an example of hybrid particles made from light and matter, known as polaritons, and could lead to future way to store and transmit information, Zhong said.

“Quantum optics effect like this are important for quantum computing; if we can find better ways to control light-matter interactions it will bring lasting benefits for the field,” said Zhong, a student in the RSPhys Nonlinear Physics Centre led by Professor Yuri Kivshar.

Zhong’s discovery differs from other instances of localisation, which are quite rare, and caused by disorder or effects at the edge of a large piece of material. In contrast, the newly-found localised photons are widespread and occur in the simplest systems.

“These localisations are way deeper than we initially expected, way more fundamental than we realised,” Zhong said.

“It’s cool that you can get such a rich variety from a simple quantum optics system.”

The discovery came early in Zhong’s honours research, while she was searching for a different phenomenon - topological states, strange effects near the edge of materials.

“They’re really hard to find, so I plotted every state and searched for them manually – a pretty crude method,” said Zhong.

“I noticed that about half the states were these really strange states where the photon trapped in the middle, which is very unexpected and very weird – no one had discovered this before. Localised states are not supposed to happen in the middle of the array!”

Although Zhong was sure she’d made a mistake in her code, her supervisor Alexander Poddubny, working with Alexander Poshakinskiy, found an analytical explanation for the initial numerical results, and realised she had stumbled across an important phenomenon.

“I really do think this is a very interesting, general and fundamental result - one of the most beautiful I ever had,” wrote Poddubny in an email to Zhong.

Once Yongguan Ke, a visiting researcher from Sun Yat-Sen University in China, had thoroughly checked the numerics and helped provide important interpretations, the team published their results in Physical Review Letters . Their paper describes how each particle is modelled as a simple two-level quantum system, with spacing much smaller than the wavelength of the light.

Using two-excitation Hilbert space they modelled how the two photons interact with the array of atoms and form polaritons. This model revealed that the two polaritons have very different masses and scales, which is why one can get trapped inside the other.

“One can think of one polariton in a standing wave as creating a cosine potential. This cosine potential is kind of like an optical lattice that traps stuff in its nodes.” Zhong said.

The model showed that the photon localisation occurs in one-dimensional arrays of as few as 15 particles. That the effect is commonplace in such simple systems makes the fact they’ve never been noticed before even more surprising, Zhong said.

“They actually happen everywhere, it’s quite hard to get rid of this effect!”