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Physics LibreTexts

7: The Density Matrix

Thumbnail: The incandescent light bulb (1) emits completely random polarized photons (2) with mixed state density matrix \(\rho\). After passing through vertical plane polarizer (3), the remaining photons are all vertically polarized (4) and have pure state density matrix \(\rho\). Image used with permission (CC BY-SA 3.0; Kaidor).