An ensemble of quantum states is a set of quantum states with corresponding probabilities. An ensemble E is written

E = {(pi, ρi)}, where pi is the probability for the state ρi.

To each ensemble there is a corresponding average state

ρ̄ = ∑ipiρi, but any mixed state can be realized by many different ensembles, even if the states in the states in the ensemble are restricted to pure states.


  • For sending classical information over quantum channels, one has to choose an ensemble of states to encode the information. The maximum information which is possible to transmit with a given ensemble is called the accessible information.
  • A (non-destructive) measurement on a quantum state will define an ensemble of post-measurement states. If the measurement outcome is discarded, the state will be in the average state.
  • Different ensembles of pure states is used to define the entanglement of formation.

Category: Quantum States

Last modified: 
Monday, October 26, 2015 - 17:56