Repellent photons

The particles of light do not normally "feel" because there is no interaction between them. ETH researchers have managed to manipulate photons inside a semiconductor material so as to repel them nonetheless.

Two intersecting light beams do not deviate. Indeed, according to the laws of quantum physics, there is no interaction between light particles or photons. Therefore, during a collision, two photons simply cross each other instead of bouncing over each other – unless one of them does not help them. In fact, researchers have been trying for some time to find techniques that allow photons to "feel" each other. The hope is that this will result in many new research opportunities as well as practical applications. Ataç Imamoğlu, a professor at ETH Zurich's Quantum Electronics Institute, and his collaborators have taken an important step in producing strong interaction photons. The results of their research were recently published in the scientific journal Nature Materials.

Transformation into polaritons

"The photons in strong interaction are a little holy grail in our field of research, photonics," explains Aymeric Delteil, postdoctoral fellow in Imamoğlu's laboratory. For light particles to repel each other, he and his colleagues must go a step further. Using an optical fiber, they send short laser pulses into an optical resonator, within which the light is highly focused and finally reaches a semiconductor material. This material (produced by colleagues from Imamoğlu in Würzburg and St. Andrew's in Scotland) is cooled in a cryostat – a kind of extremely powerful refrigerator – up to minus 269 degrees Celsius. At these low temperatures, photons can combine with electronic excitations of the material. This combination results in what are called polaritons. At the opposite end of the material, polaritons return to photons, which can then exit the resonator.

As there are electromagnetic forces acting between the electronic excitations, an interaction also appears between the polaritons. "We were able to detect this phenomenon some time ago," Imamoğlu said. "However, at the time, the effect was so weak that only interactions between a large number of polaritons played a role, but not the pairwise repulsion between individual polaritons."

Correlations signal interactions

In their new experiment, researchers have now been able to demonstrate that unique polaritons – and thus, indirectly, photons contained in them – can actually interact with each other. This can be deduced from the way in which the photons leaving the resonator correlate with each other. To reveal these so-called quantum correlations, we measure the probability that a second photon leaves the resonator shortly after another. If photons get bothered by their polaritons inside the semiconductor, this probability will be lower than one would expect from photons without interaction.

In the extreme case, there should even be a "photon blockade", an effect that imamoğlu had already postulated 20 years ago. A photon in the semiconductor that has created a polariton then completely prevents a second photon from entering the material and becoming a polariton itself. "We are far from realizing this," admits Imamoğlu, "but in the meantime, we have further improved our result that has just been published, which means we are on the right track." The long-term goal of Imoamoğlu is to make sure that photons interact so strongly that they start to behave like fermions – like quantum particles, in other words, that can never be found in the same place.

Interest for strongly interacting polaritons

In the first place, Imamoğlu is not interested in applications. "It's really a basic research," he says. "But we hope to one day be able to create polaritons that interact so strongly that we can use them to study new effects in quantum physics that are difficult to observe otherwise." The physicist is particularly interested in situations in which polaritons are also in contact with their environment and exchange energy with it. This exchange of energy, associated with the interactions between the polaritons, should, according to the calculations of theoretical physicists. lead to phenomena for which only rudimentary explanations exist so far. Experiments such as those carried out by Imamoğlu could therefore help to better understand the theoretical models.

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More information:
Aymeric Delteil et al. Towards the blocking of polaritons of confined excitons – polaritons, Nature Materials (2019). DOI: 10.1038 / s41563-019-0282-y