# Tag Info

## Hot answers tagged information-theory

20 votes
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### How can classical bits be copied if qubits cannot be copied?

TL;DR: The ban on copying is not nearly as universal as it might seem. No-cloning theorem actually allows copying as long as it is limited to orthogonal states. Classical information is the type of ...
• 18.2k
12 votes
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### Proof of an Holevo information inequality for a classical-classical-quantum channel

It appears that the statement is not true in general. Suppose $X = Y = \{0,1\}$, $\mathcal{H}$ is the Hilbert space corresponding to a single qubit, and $W$ is defined as \begin{align} W(0,0) & = |...
• 4,853
10 votes
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### Partial trace over a product of matrices - prove that ${\rm Tr}(\rho^{AB}(\sigma^A\otimes I))={\rm Tr}(\rho^A\sigma^A)$

The equation at the top of the question is not correct: there is a missing factor of $1/d$ on the right-hand side. Let's eliminate this factor from the left-hand side to make it simpler, so that the ...
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10 votes
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• 4,853
8 votes
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### Can $2^n$ bits be sent with $n$ instances of quantum teleportation?

Quantum teleportation can send a single qubit from Alice to Bob, with two classical bits Correct, on the condition that Alice and Bob also have an entangled qubit pair shared between them. This ...
• 1,050
7 votes

• 6,353
7 votes
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• 4,853
5 votes
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### Quantum circuit for computing fidelity

Prohibited device Such a circuit $C$ would enable faster-than-light communication and therefore does not exist. Suppose Alice and Bob share a Bell pair \$|\psi\rangle = \frac{1}{\sqrt{2}}(|01\rangle - |...
• 18.2k

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