# Polaritons

When Vladimir Agranovich was made Dr. Honoris Causa of the Université Blaise Pascal, some time in 2003, Alexey Kavokin gave one of his most beautiful speeches. He opened it by stating that the interaction of light and matter is one of oldest topic to have perplexed the curiosity of man. He observed that already in the bible, and from its very opening, the account of creation starts with God creating matter on the one hand and light on the other. All that follows results from their continuous interaction.

Polaritons are molecules of light and matter. Much like an atom binds an electron to a nucleus, a polariton binds a photon (a particle of light) to an exciton (a particle of matter). Such a bonding is made possible by confining both excitations in a cavity, where they are both trapped and therefore compelled to interact. Without the cavity, which consists in two mirrors facing each others, the photon would simply fly away. But the mirrors make the photons bounce back and forth, hitting the crystal in the process and once in a while creating an excitation—that's our exciton—as the photon gets absorbed. The exciton is unstable, like a positron (it is in fact a bound electron-hole pair), an after a while, recombines to create another photon. The process repeats itself, for as long as the photon stays in the cavity. According to Quantum Mechanics, this generates a quantum superposition of the two possible states, just like the Schrödinger cat, but instead of being alive and dead simultaneously, the particle is light and matter simultaneously:

$$\ket{\mathrm{photon}}+\ket{\mathrm{exciton}}$$

These half-light/half-matter particles have fantastic properties, which they inherit from their underlying components:

• Polaritons are extremely coherent, like light.
• Polaritons interact strongly, like matter.

They have a mass, like matter, but a very light-one (the photon has none in vacuum and a small effective one when confined in a cavity).