Exciton-Biexciton Ladder
|XX⟩ → |X_H⟩ + ℏω_XX (biex decay)
|XX⟩ → |X_V⟩ + ℏω_XX'
|X_H⟩ → |G⟩ + ℏω_X (exciton decay)
|X_V⟩ → |G⟩ + ℏω_X'
Antibunching: g²(τ) = 1-exp(-Γτ)
g²(0) = 0 (single photon source!)
Purcell factor: F_p = (4g²/κγ)
E_bind = E_XX - 2E_X (few meV)
Quantum dots are "artificial atoms" — nanoscale semiconductor heterostructures with discrete energy levels. The biexciton (two electron-hole pairs) decays by emitting two photons sequentially, producing polarization-entangled pairs. Because only one photon can be emitted at a time (antibunching), quantum dots are ideal single-photon sources for quantum key distribution.