Polariton Quantum Fluids Bramati Group · LKB
Research

Quantum fluids of light and polaritons

Photons confined in a semiconductor microcavity and dressed by excitons form exciton-polaritons: hybrid light-matter quasiparticles with an effective mass and strong interactions. Under continuous laser driving they condense and flow as a genuine quantum fluid, one that is intrinsically open and out of equilibrium, giving access to physics with no equilibrium counterpart.

Our program grew from a series of landmark results on polariton hydrodynamics and now spans open-system criticality, analogue gravity, and quantum information, unified by a common experimental toolbox for creating, probing and controlling these fluids of light.

The physics, in motion

Four ideas at the core of the group's work, from how light and matter fuse into polaritons, to how a flowing fluid of light mimics the horizon of a black hole.

Strong coupling
Light + matter → polaritons
A cavity photon and a quantum-well exciton hybridize. Their bare modes anticross into upper and lower polariton branches, split at resonance by the Rabi energy ℏΩ.
Bogoliubov spectrum
Sound from interactions
Interactions bend the free-particle parabola into a linear, phonon-like dispersion at low k. The fluid carries sound at speed c_s, the hallmark of superfluidity.
Goldstone mode
The gapless phase mode
Spontaneously breaking the U(1) phase symmetry of the condensate produces a soft, gapless excitation: a slow, long-wavelength twist of the order parameter.
Analogue gravity
A horizon in a fluid of light
Where the flow turns supersonic, waves can no longer swim upstream, an acoustic horizon that emits correlated Hawking pairs, just as a black hole radiates.

A foundation in polariton hydrodynamics

2009
First observation of superfluidity of polaritons
Nature Physics 5, 805
2011
Hydrodynamics and quantized flow of polariton fluids
Science 332, 1167 · Nature Photonics 5, 610
2012
Half-solitons in a polariton quantum fluid
Nature Physics 8, 724
2022
High-resolution coherent probe spectroscopy of the Bogoliubov dispersion
Physical Review Letters 129, 103601
ε(k) · Enabling technique

Reading the Bogoliubov dispersion of a fluid of light

A high-resolution coherent-probe spectroscopy method developed in the group maps the full Bogoliubov excitation spectrum of a polariton quantum fluid directly. Measuring the dispersion (its gaps, its speed of sound, its stability) turns qualitative signatures into quantitative observables, and opens the door to each of the projects below.