Ensemble

An '''ensemble''' of quantum states is a set of quantum states with corresponding probabilities. An ensemble $\backslash mathcal\{E\}$ is written :$\backslash mathcal\{E\}\; =\; \backslash \{\; (p\_i,\; \backslash rho\_i)\; \backslash \}$, where $p\_i$ is the probability for the state $\backslash rho\_i$. To each ensemble there is a corresponding ''average state'' :$\backslash bar\{\backslash rho\}\; =\; \backslash sum\_i\; p\_i\; \backslash rho\_i$, 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 state]]s. == Usage == * For sending classical information over quantum [[channel]]s, 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]]