32 lines
1.1 KiB
Markdown
32 lines
1.1 KiB
Markdown
# Math 401, Paper 1, Side note 2: Page's lemma
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The page's lemma is a fundamental result in quantum information theory that provides a lower bound on the probability of error in a quantum channel.
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## Statement
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Choosing a random pure quantum state $\rho$ from the bi-partite pure state space $\mathcal{H}_A\otimes\mathcal{H}_B$ with the uniform distribution, the expected entropy of the reduced state $\rho_A$ is:
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$$
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\mathbb{E}[H(\rho_A)]\geq \ln d_A -\frac{1}{2\ln 2} \frac{d_A}{d_B}
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$$
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## Page's conjecture
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A quantum system $AB$ with the Hilbert space dimension $mn$ in a pure state $\rho_{AB}$ has entropy $0$ but the entropy of the reduced state $\rho_A$, assume $m\leq n$, then entropy of the two subsystem $A$ and $B$ is greater than $0$.
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unless $A$ and $B$ are separable.
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In the original paper, the entropy of the average state taken under the unitary invariant Haar measure is:
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$$
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S_{m,n}=\sum_{k=n+1}^{mn}\frac{1}{k}-\frac{m-1}{2n}
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$$
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## References
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- [Page's conjecture](https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.71.1291)
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- [Page's conjecture simple proof](https://journals.aps.org/pre/pdf/10.1103/PhysRevE.52.5653) |