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For questions about quantum channels or more generally quantum maps and the related formalism. For questions about unitary operations, please use quantum-gate instead.

2 votes
1 answer
138 views

What is the relevance of preservation of trace in completely postive trace preserving (CPTP)...

Why is the trace preserving part necessary? Is it not enough if it can take all matrices to matrices of trace 1?
Mahathi Vempati's user avatar
3 votes
0 answers
88 views

What do you specify when you physically apply a unitary?

In the Environment and Quantum Operations in Nielsen and Chuang, section 8.2.2, they say that when you apply a unitary on a state, you expect the output as the just the state transformed by the unitar …
Mahathi Vempati's user avatar
4 votes
1 answer
109 views

Structural Physical Approximation of Partial Transpose

To make the partial transpose a complete positive and therefore physical map, one has to mix it with enough of the maximally mixed state to offset the negative eigenvalues. The most negative eigenva …
Mahathi Vempati's user avatar
4 votes
1 answer
389 views

How are witness operators physically implemented?

Let's take an example of an entanglement witness of the form $W = | \phi \rangle \langle \phi | ^{T_2}$ where $ | \phi \rangle $ is some pure entangled state. If I wanted to test some state $\rho$, I …
Mahathi Vempati's user avatar
4 votes
1 answer
349 views

For 2x2 and 2x3 systems, is the partial transpose the only positive but not CP operation?

Question: For 2x2 and 2x3 systems, is the partial transpose the only positive but not completely positive operation that is possible? Why this came up: The criteria for detecting if a state $\rho$ is …
Mahathi Vempati's user avatar