Quantum Polaritonics

An international research partnership of state-of-the-art experimental laboratories and theoretical groups between the University of Cambridge, MIT-Skoltech, and the University of Southampton.

At the laboratories for Quantum Polaritonics we investigate the exotic physics of Bose-Einstein condensates in semiconductor devices and utilise their properties in engineering a quantum simulator.

Snapshot on polaritonics: Bose-Einstein condensation (BEC) is an exotic state of matter, wherein particles coalesce to a macroscopically occupied coherent state. BECs have been observed for a broad range of systems such as 4He, alkali-metal atoms, magnons, and polaritons. Beyond the beauty of the underlying physics describing the fundamental properties and dynamics of BECs, there is a range of applications that utilise the coherence of their massive wavefunctions, especially in the rapidly developing field of quantum technologies. In semiconductor microcavities, polaritons result from the admixture of cavity photons and excitons in the strong coupling regime. Above a critical density, polaritons have been shown to undergo BEC. Unlike other BECs, polariton condensates (hosted in semiconductor slabs embedded in optical cavities) can be optically pumped, while they decay by emitting coherent light, very much like an optically pumped laser. By appropriate choice of the crystalline semiconductor host, polariton condensates can form even at room temperature. Our research in the field of polaritonics spans a broad range of fundamental physics and applications including polariton simulators, polariton circuits and room temperature polaritonics.

Background physics:

Semiconductor microcavities are nanostructures that consist of a planar Fabry-Perot cavity with one or more embedded quantum structures (wells, wires, dots etc), sandwiched between two Bragg mirrors. Coupling between the exciton resonance and the cavity mode may lead to either crossing or anticrossing of the real parts of the eigenfrequencies of the structure modes (called exciton – polariton modes). Excitons-polaritons are half light, half matter quasi particles, combining the properties of excitons and photons, they can be interpreted as virtual exciton-photon pairs, whose propagation in the crystal is just a result of multiple virtual absorption and emission processes of photons by the excitons.

The cavity and the exciton modes in the momentum space anticross in the strong-coupling regime and new eigenstates are formed, which we call them polaritons. Polaritons are mixed cavity photon and exciton bosonic quasiparticles. Due to their photonic component, they have 8 orders of magnitude lighter mass than atomic gases and can condense at temperatures as high as the room temperature. Polaritons have two main modes, which are called the upper polariton and the lower polariton mode. The lower polariton mode acts like a trap. However, note that these modes are only in the momentum space (k) and not in the real space!

When weakly interacting Bosonic particles trapped in an external potential are below the temperature of quantum degeneracy they go through a thermodynamic phase transition during which phase correlations spontaneously build up and the system enters a macroscopic coherent state. The critical density is the density at which the interparticle distances are comparable to their de Broglie wavelength. Under such condition, a large fraction of the particles condense in the lowest quantum state of the external potential and quantum effects become apparent on macroscopic scales. This state of matter is called a Bose-Einstein condensate. For atoms and molecules the cooling is initially done by laser cooling and the particles are then further cooled by evaporative cooling techniques.

We optically pump the cavity using (a usually off-resonant laser) and create an electron-hole plasma in the quantum wells. The electron holes immediately form bound excitons. The excitons cool by exciton-exciton and exciton-phonon relaxations (right panel). Further cooling to the ground state occurs by parametric scatterings (such as in optical parametric oscillators (OPOs)). In the OPO scattering two polaritons collide, with one going to higher energies and the other one falling into the bottom of the trap. Since the polaritons are half photons, they have a very short lifetime (10s of picosecond). But if the cooling process occurs faster than their decay rate, polaritons at the ground state could reach critical densities and a condensate could form at the bottom of the dispersion.

Microcavities are a fascinating system for studying condensed matter physics ranging from fundamental physics such as superfluidity to future possible applications like spinoptronic devices and polariton simulators.