Crystal Lattice Created from Hybrid Photon-Electron Quasiparticles

Using hybrid photon-electron quasiparticles – polaritons, an international research team has created an analog of a solid-body crystal lattice. In the resulting polariton lattice, some particles’ energy does not rely on their speed. Simultaneously, the lattice’s geometry, particle concentration, and polarization properties can still be altered.

This paves the way for new perspectives for the investigation of quantum effects and the application of optical computing. The findings of the study were published in Physical Review Letters.

A solid body is formed around a crystal lattice created by atomic nuclei. Lattice geometry may impact the relation between a particle’s energy and velocity. There are several kinds of lattices, divided based on their geometrical properties. Some of them, such as the Lieb lattice, have so-called flat bands: a state of particles where they display no energy-velocity relation at all. From a formal perspective, particles in flat bands have unlimited effective mass.

Flat bands are of significant interest for core science. They are used to explore ferromagnets, superconductors, and other quantum phases in electrons. However, quantum phases can also be witnessed in light elementary particles – photons. This requires forming an artificial photonic analog of a solid body: a so-called photonic crystal with modifiable geometry. Such conditions enable researchers to see and manage numerous quantum properties of particles a lot easier.

Physicists from ITMO University and University of Sheffield have created a photonic analog of a Lieb lattice and established that quantum effects in a photonic structure are truly stronger.

Strictly speaking, we were dealing with polaritons rather than photons. This hybrid condition occurs when excited electrons mix with photons. Such hybrid particles interact with each other, much like electrons do in a solid body. We used polaritons to create a crystal lattice and studied their new properties. Now we know how polaritons condense in flat bands, how their interaction breaks the radiation symmetry and how their spin or polarization properties change.

Dmitry Kryzhanovsky, Professor, University of Sheffield

Since polaritons maintain their spin rotation uninterruptedly, researchers are, at present, able to witness polarization for a long time. Also, easy control over polariton concentration in the lattice offers additional options for precise management of the system.

From a fundamental viewpoint, polariton crystals are interesting in that they provide a great variety of quantum phases and effects that we cannot study in standard crystals. Polarization can serve as an information storage element. All calculations are based on a binary system. There must be 0 and 1, so to implement optical computing we need two corresponding states. Polarization, right and left, with a number of intermediate combinations, is an ideal candidate for quantum-level information processing.

Ivan Shelykh, Head of the International Laboratory of Photoprocesses in Mesoscopic Systems, ITMO University

A significant contribution to the development and study of the polariton crystal lattices was made by staff of the University of Sheffield. Professor from the University of Sheffield heads a project on hybrid states of light along with Ivan Shelykh.

All the experiments were carried out in Sheffield, while theoretical modeling and analysis of the results were done at ITMO University. I consider this work a good example of what science should look like. Results of an experiment are incomprehensible when published without any interpretation. Similarly, raw theory using unrealistic parameters is difficult to apply in practice. But here we combined theory with experiment – and we plan to keep doing it this way. Our next goal is to obtain and investigate the topological boundary conditions of such a lattice.

Ivan Shelykh, Head of the International Laboratory of Photoprocesses in Mesoscopic Systems, ITMO University