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h(X)=-\int_{x\in \mathcal{X}} f_X (x) \log_2 \left(f_X (x)\right)
h(X)=-\int_{x\in \mathcal{X}} f_X (x) \log_2 \left(f_X (x)\right)
</math>
</math>
===Entropy in Quantum Information Theory===
In entangled systems, a useful quantity is the Von Neumann Entropy, defined (for a system with density matrix <math> \rho </math> by
<math>
H(\rho) = -\bold{Tr} \left( \rho \ln \rho \right)
</math>
where Tr() indicates taking the Trace of a matrix (the sum of the diagonal elements). This is a useful measure of entanglement, which is zero for a pure state, and maximal for a fully mixed state.
==See also==
==See also==