Entropy

• How Does the Replica Trick Calculate Entanglement Entropy?
  The replica trick finds entanglement entropy from integer moments of a reduced density operator, then analytically continues them to n = 1.
• Do Individual Photons Gain Entropy as They Travel?
  A photon has no fixed entropy just from traveling: entropy belongs to its quantum state and can change locally through mixing or interactions.
• Why Is Entropy an Extensive Quantity?
  Entropy is extensive because independent subsystems have multiplying numbers of microstates, and the logarithm turns that product into a sum.