Physics

Bound state

Topological quantum number

Polariton

Photon

Superfluidity

Mesoscopic physics

Condensed matter physics

Bose–Einstein condensate

Quantum mechanics

Nature

Bound states in the continuum (BICs)1–3 are peculiar topological states that, when realized in a planar photonic crystal lattice, are symmetry-protected from radiating in the far field despite lying within the light cone4. These BICs possess an invariant topological charge given by the winding number of the polarization vectors5, similar to vortices in quantum fluids such as superfluid helium and atomic Bose–Einstein condensates. In spite of several reports of optical BICs in patterned dielectric slabs with evidence of lasing, their potential as topologically protected states with theoretically infinite lifetime has not yet been fully exploited. Here we show non-equilibrium Bose–Einstein condensation of polaritons—hybrid light–matter excitations—occurring in a BIC thanks to its peculiar non-radiative nature, which favours polariton accumulation. The combination of the ultralong BIC lifetime and the tight confinement of the waveguide geometry enables the achievement of an extremely low threshold density for condensation, which is reached&nbsp;not in the dispersion minimum but at a saddle point in reciprocal space. By bridging bosonic condensation and symmetry-protected radiation eigenmodes, we reveal ways of imparting topological properties onto macroscopic quantum states with unexplored dispersion features. Such an observation may open a route towards energy-efficient polariton condensation in cost-effective integrated devices, ultimately suited for the development of hybrid light–matter optical circuits. Using a patterned waveguide, a Bose–Einstein condensate of polaritons is realized in a bound state in the continuum, with a low condensation threshold density that occurs at a dispersion saddle point.

Polariton Bose–Einstein condensate from a bound state in the continuum