A new method to enable efficient interactions between photons

Photons, particles representing a quantum of light, have shown great potential for the development of new quantum technologies. More specifically, physicists have been investigating the possibility of creating photonic qubits (quantum units of information) that can be transmitted over long distances using photons.

Despite some promising results, there are still some obstacles to be overcome before photonic qubits can be successfully implemented on a large scale. For example, photons are known to be prone to propagation losses (i.e. loss of energy, radiation or signal as they travel from one point to another) and do not interact with each other.

Researchers from the University of Copenhagen in Denmark, the Instituto de Física Fundamental IFF-CSIC in Spain and the Ruhr-Universität Bochum in Germany have recently developed a strategy that could help overcome one of these challenges, namely the lack of photon-photon interactions . Their method, presented in an article published in natural physicscould eventually support the development of more sophisticated quantum devices.

“We have been working on the deterministic coupling of single quantum emitters (quantum dots) to single photons for over 15 years and have developed a very powerful method based on nanophotonic waveguides,” says Peter Lodahl, one of the researchers who carried out the study. said Phys.org. “We have generally applied these devices to deterministic single-photon sources and multi-photon entanglement sources, but another potential application would be to induce nonlinear operations on photons.”

As early as 2015, Lodahl and his colleagues realized the first proof-of-concept demonstration of nonlinear operations with single photons. However, as they investigated this effect further, they encountered difficulties in thoroughly understanding the fundamental physics underlying this complex, nonlinear, single-photon interaction.

“In our earlier work, we found that the physics governing the nonlinear interaction of light pulses was remarkably rich, and created some new possibilities for building photonic quantum gates and photon sorters,” Lodahl said. “We have performed the first experimental investigation of nonlinear quantum pulses, which undergo nonlinear interaction due to coupling with a deterministically coupled quantum emitter.”

In their new experiment, the researchers used the efficient and coherent coupling of a single quantum emitter with a nanophotonic waveguide to enable nonlinear quantum interactions between single-photon wave packets. To do this, they used a single quantum dot, a nm-sized particle that behaves like a two-level atom embedded in a photonic crystal waveguide.

“In such systems, the coupling is deterministic, so that even a photon coupled into the waveguide interacts with the quantum dot,” explains Lodahl. “Sending pulses with two or more photons induces quantum correlations, since only one photon can interact with the quantum dot at a time. By controlling the duration of the quantum pulse, we can tailor these correlations and the interaction between the photons.”

Using their experimental method, Lodahl and his colleagues essentially succeeded in steering a photon through a second photon mediated by their quantum emitter. In other words, they successfully realized nonlinear photon-photon interaction.

“We developed a method to get photons to efficiently interact with each other, mediated by coupling to quantum dots,” Lodahl said. “We think this could open new directions for fabricating photon-photon quantum gates (the difficult gate in photonic quantum computing) or deterministic photon sorters, which are essential for quantum repeaters, for example.”

The new strategy put in place by this research team could have important implications for both quantum physics research and quantum technology development. For example, their method could open up new possibilities for the development of quantum optical devices and also allow physicists to experiment with tailor-made complex photonic quantum states.

“We have a number of activities that expand on the current work,” Hanna Le Jeannic, another researcher involved in the study, told Phys.org. “At a fundamental level, we are trying to understand more deeply how quantum states of light are affected as it travels through a single quantum dot. But we already foresee applications of this quantum interaction.”

At the moment, Lodahl, Le Jeannic and their colleagues are trying to use the nonlinear photon-photon interaction realized in their recent study to simulate the vibrational dynamics of molecules. This could be achieved by mapping the vibrational dynamics of complex molecules to the propagation of photons in advanced photonic circuits.

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