02.07.14 - EPFL scientists show for the first time a method for tuning polariton collisions, with great implications for quantum physics research and superfluidity.

Polaritons fall under the category of ‘quasiparticles’, which is a term that refers to phenomena that emerge from the behavior of particles like electrons inside solids. Polaritons are made up by combining a photon with an electron, and are of enormous interest today, as they can be used to make a new type of laser that uses considerably lower energy than conventional ones. But polaritons also hold the key to a deeper understanding of quantum physics by studying exotic forms of matter like superfluids and Bose-Einstein condensates. These interesting phenomena can only be observed due to the unique nature of polaritons. In particular, they experience strong nonlinear behavior inherited from their mutual interactions. Publishing in Nature Physics, EPFL scientists have developed an unprecedented method for tuning the nonlinear behavior of polaritons, paving the way to a new chapter in quantum research. This method is also introduced for the first time in a solid-state system using microcavity polaritons.

Polaritons are formed when an excited electron combines with a photon. There are several types of polaritons depending on the light wavelength and type of matter particle. These can include ‘conventional’ particles like electrons or other exotic particles like phonons and excitons, which occur from the behavior of electrons in materials. Polaritons themselves appear both as waves with measurable frequencies, amplitudes and wavelengths, and can also be described as ‘particles’ with properties such as mass, spin, and velocity.

The group of Benoît Deveaud has shown how polaritons can be controlled inside a semiconductor microresonator, which is a device that can ‘hold’ polaritons for a prolonged period of time. The researchers trapped two polaritons with anti-parallel spins inside the microresonator and tuned their energies to be equal to the energy of the excitonic molecule, the so-called biexciton. When the energies of the polaritons and the molecular states “match”, the strength of collisions between polaritons strongly increases revealing large nonlinearities. Therefore polaritons might display either repulsive or attractive interactions depending on their relative energies respect to the biexciton state.

This method is called “Feshbach resonance” is a remarkable physical phenomenon allowing engineering of the fundamental interactions and the scattering properties of free particles. It is also a key tool for understanding Bose–Einstein condensates (BECs), which are unconventional states of matter that occur in extremely low temperatures. In a BEC, particles agglomerate into their lowest possible state of energy, giving rise to phenomena like superconductivity (zero electrical resistance) and superfluidity (zero liquid viscosity). The introduction of Feshbach resonance can instigate a wide range of major observations in cold-atom systems.

The observation carried out by Deveaud’s group imparts a comprehensive understanding of Feshbach resonance in spinor polariton gases, which widens the palette of Feshbach resonance techniques already explored in cold atom physics. Moreover, it provides an unprecedented tool for controlling the behavior of polaritons, which can pave the way for novel device principles such as single-photon generation based on “Feshbach blockade” or polariton switches.

Takemura N, Trebaol S, Wouters M, Portella-Oberli MT, Deveaud B. Polaritonic Feshbach resonance. Nature Physics (2014) DOI: 10.1038/nphys2999