QUINTO (Quantum Interacting Topological Optics) is a research project carried out by Błażej Jaworowski under the supervision of Prof. Darrick Chang in ICFO (Institute of Photonic Sciences) in Barcelona. The project is funded by Horizon Europe program under the Marie Skłodowska-Curie Action. The aim is to build a “bridge” between two fields of physics: quantum optics and condensed matter physics.
Quantum optics describes the interaction of atoms with light described in the quantum way (i.e. down to the level of individual photons). Typical systems studied within quantum optics are single atoms inside optical cavities (imagine placing the atom between two mirrors, so the light can bounce back and forth, passing the atom many times) or clouds of many atoms. Recently, a new paradigm emerged: arranging the atoms in ordered arrays. In such a case, one can use the interference between photons to suppress or enhance emission in a desired way – as the chain of atoms shown in the picture below, which emits light only at its ends. However, the picture shows a result for a single photon only. What if there are many of them? Usually, photons don’t “see” each other. But once they are absorbed by atoms, they start to interact, sometimes very strongly. The more interacting particles we have, the more difficult it is to describe them. This is the quantum many-body problem, notoriously difficult to solve, but potentially causing new, unexpected physical phenomena. Right now, quantum optics lacks paradigms to deal with the many-body physics.
In condensed-matter physics, on the other hand, a number of interesting many-body effects were already demonstrated, and have a good theoretical description. Condensed-matter physics studies systems such as electrons in crystals. The electrons naturally interact, which leads to phenomena such as magnetism (electrons aligning their tiny magnetic moments in one direction). Even more striking example are the “topological orders”, such as fractional quantum Hall effect, where the electrons confined in a 2-dimensional plane form a “liquid”, and tiny, local “dips” and “peaks” in the density are “quasiparticles” – they behave as particles. Actually, a very special kind of particles, While all the known elementary particles (electrons, protons, photons…) belong to one of two classes: bosons and fermions, the quasiparticles in topological orders are “anyons”, a theoretically-predicted type of particle which can exist only in 2D. This is not just a theoretical curiosity – it was proposed that by moving the anyons around each other and “braiding” their paths one can process and store quantum information. Such a “topological quantum computer” would be robust against disorder and errors. The lack of such robustness is one of the main obstacles to the development of powerful quantum computers, which could perform certain complex tasks much faster than the “classical” computers we use today.
In the QUINTO project, we plan to find and exploit analogies between the theory of topological orders and the physics of atomic arrays. This will provide us with a conceptual framework to understand certain many-body effects in atomic arrays. In turn, this will help to establish the atomic arrays as a new platform for the study of topological orders, with its own, characteristic experimental tools based on light.