# Tag Info

Accepted

### 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 ...
Accepted

### 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) & = |...
Accepted

### 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 ...
Accepted

Accepted

### 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 ...

Accepted

Accepted

### No-cloning theorem and distinguishing between two non-orthogonal quantum states revisited

TL;DR: The assumption of non-orthogonality is implicitly used by the linked answer. It is needed due to a "loophole" in the no-cloning theorem that allows cloning of known orthogonal states. ...
Accepted

### What is the root of the non-trace-preserving bit-flip map

Assuming w.l.o.g. that $p\in\mathbb{R}$, the linear map in the question may be rewritten as $$\mathcal{E}(\rho) = p^2\rho+p^2X\rho X = 2p^2\left(\frac12\rho + \frac12 X\rho X\right)$$ where $X$ is ...
You don't need any additional conditions beyond those already stated in the question. That is, for any isometry $V: A \rightarrow A\otimes B$ and any unit vector $|\psi\rangle_B$, there will always be ...