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### What is the operator-sum representation of the two-qubit depolarizing channel?

As pointed out in the comments, you cannot use the one-qubit formula because something like $X\rho X$ does not make sense if $\rho$ is a 2-qubit state. In fact, for this reason the answer you based ...
• 2,823
Accepted

• 2,823

### Do POVM and generalized measurements really describe all possible measurements we can do on a quantum system (open dynamics)?

There's a few different things here, so let me try to break down a few different arguments: Why are POVMs the most general possible measurement? — I'd say POVMs are simply what you get when applying ...
• 26.3k

### Do POVM and generalized measurements really describe all possible measurements we can do on a quantum system (open dynamics)?

POVMs are the most general measurement as long as you insist that the probability $p_i(\rho)$ for any outcome $i$ is a linear function of $\rho$ (that is, any such linear $p_i(\rho)$ can be written as ...
• 7,653

### When performing a projective measurement on a subsystem X entangled with another system Y, can the evolution of Y be unitary?

Non existence for "forgetful" measurements Suppose after the entangling operation the state is some $\rho_{XY}$. Given a POVM $\{M_a\}_a$ we can describe the final state of system $Y$ and ...
• 6,216
1 vote
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### Mathematical properties used to derive Kraus operators

You have $$\rho_{\rm in}\otimes|e_0\rangle\!\langle e_0| = (\rho_{\rm in}\otimes I)(I\otimes |e_0\rangle\!\langle e_0|) = (I\otimes |e_0\rangle\!\langle e_0|)(\rho_{\rm in}\otimes I).$$ The main &...
• 26.3k

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