The development of new quantum technologies has showed significant promise for photons, the particles that make up a quantum of light. In particular, scientists have been investigating the potential for developing photonic qubits, which are quantum units of information that may be transferred over vast distances using photons.

Despite these encouraging findings, there are still a number of challenges that must be cleared before photonic qubits may be successfully used on a broad scale. For instance, photons are known to be incapable of interfering with one another and to be subject to propagation loss (i.e., a loss of energy, radiation, or messages as it travels from one point to another).

One of these difficulties, the absence of photon-photon interactions, has recently been addressed by researchers from the University of Copenhagen in Denmark, the Instituto de Fsica Fundamental IFF-CSIC in Spain, and Ruhr-Universität Bochum in Germany. Their approach, which was described in an article that was published in Nature Physics, may potentially help the creation of more complex quantum devices.

According to Peter Lodahl, one of the study's authors, "we have been working on the deterministic interfacing of single quantum emitters (quantum dots) to single photons for over 15 years and have created a highly powerful approach based on nanophotonic waveguides." However, inducing nonlinear operations on photons is another viable application. "We commonly applied these devices for deterministic single-photon sources and multi-photon entanglement sources," the author says.

The first proof-of-concept demonstration of nonlinear processes utilising individual photons was accomplished by Lodahl and his associates in 2015. However, as they dug deeper into this phenomenon, they found it challenging to fully comprehend the fundamental physics underpinning this intricate, single-photon, and nonlinear interaction.

In earlier research, Lodahl and colleagues discovered that the physics underlying the nonlinear interaction of light pulses was very rich and provided some novel potential for building photonic quantum gates and photon sorters. We have conducted the first experimental investigation of nonlinear quantum pulses coupled to a deterministically coupled quantum emitter that are subject to nonlinear interaction.

To enable nonlinear quantum interactions between single-photon wave packets in their new experiment, the researchers exploited the effective and coherent coupling of a single quantum emitter with a nanophotonic waveguide. They achieved this using a single quantum dot that was encapsulated in a photonic crystal waveguide. A quantum dot is a tiny particle (nm in size) that functions like a two-level atom.

Since the coupling in these systems is deterministic, even a single photon that enters the waveguide interacts with the quantum dot, according to Lodahl. "Since only one photon at a time may interact with the quantum dot, sending in pulses containing two or more photons causes quantum correlations. We may tune these correlations and the interaction between the photons by adjusting the quantum pulse's duration."

With the use of their quantum emitter and their experimental technique, Lodahl and his associates were effectively able to control a photon by means of a second photon. In other words, a nonlinear photon-photon interaction was effectively realised.

According to Lodahl, "We developed a mechanism to get photons to interact with each other efficiently through the coupling to quantum dots." We believe that this could open up new possibilities for creating deterministic photon sorter devices, which are crucial, for example, in quantum repeaters, or photon-photon quantum gates, which are the challenging gate in photonic quantum computing.

The innovative approach put out by this group of experts may have significant ramifications for both the study of quantum physics and the advancement of quantum technologies. Their approach, for instance, might create new opportunities for the creation of quantum optical gadgets while also enabling scientists to experiment with difficult sophisticated photonic quantum states.

Hanna Le Jeannic, another researcher working on the project, told Phys.org that "we have a spectrum of initiatives that extend the existing work." "We are interested in learning more about how passing through a single quantum dot affects the quantum states of light at a fundamental level. However, we are already imagining uses for this quantum interaction."

Currently, Lodahl, Le Jeannic, and their colleagues are attempting to model the vibrational dynamics of molecules using the nonlinear photon-photon interaction they realised in their recent study. This could be accomplished by projecting the photon propagation through sophisticated photonic circuitry onto the vibrational dynamics of complicated molecules.

Reference: Nature Physics, Nature Nanotechnology, Nature Communications

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