Electromagnetically Induced Transparency

Physics: EIT: quantum interference between two excitation pathways in a Λ-system (|1⟩, |2⟩ ground states; |3⟩ excited) cancels probe absorption at two-photon resonance. The dark state |D⟩ = cosθ|1⟩ − sinθ|2⟩ is uncoupled from light. Susceptibility: χ = Nμ²/(ε₀ℏ) · δ/(δ² − Ω_c²/4 + iγδ/2). Group velocity v_g = c/(1 + ω·dχ'/dω) → 17 m/s demonstrated (Hau et al. 1999) in cold Na BEC. Stored light: adiabatically turn off coupling field → photon stored as spin wave; turn on → photon retrieved. Application: quantum memory for single photons in quantum networks.