Questions tagged [kraus-representation]
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17
questions
4
votes
1answer
67 views
Unitary over bipartite states that can turn a non-product state into a product state
Consider a bipartite quantum state $\rho_{AB}$ over a product of finite-dimensional Hilbert spaces $\mathcal{H}_A \otimes \mathcal{H}_B$. Does there exists a unitary $U$ over $\mathcal{H}_A \otimes \...
1
vote
1answer
36 views
Prove that different Kraus decompositions are related through a unitary, using the Choi isomorphism
I consider a process $\mathcal{E}$ that is at least CP and hermitian preserving.
I know that the Choi matrix then has the form:
$$ M = \sum_k |M_k \rangle \rangle \langle \langle M_k | $$
Where $|M_k \...
0
votes
1answer
57 views
Choi matrix in QETLAB
I am using QETLAB, a package for working with quantum information theory in Matlab and I have some doubts. I am trying to calculate diamond norms using such for some quantum channels. However, when ...
3
votes
1answer
103 views
How to find the unitary operation of a depolarizing channel?
Suppose we have a depolarizing channel operation
$$E(\rho)=\frac{p}{2}\textbf{1}+(1-p)\rho$$
acting on a Spin$\frac{1}{2}$ density matrix of the form $\rho=\frac{1}{2}(\textbf{1}+\textbf{s}\cdot\...
2
votes
1answer
53 views
Finding the irreducible representation of Kraus operators of a dephasing channel
I would like to understand an example of finding a noiseless subsystem of a quantum channel from the irreducible representation of its Kraus operators.
Assume we have $2$ dephasing channels acting on ...
2
votes
1answer
54 views
Can any channel be written as $\Phi(X)=\operatorname{Tr}_{\mathcal Z}[U(X\otimes \sigma)U^\dagger]$ for any state $\sigma$?
We know that every CPTP map $\Phi:\mathcal X\to\mathcal Y$ can be represented via an isometry $U:\mathcal X\otimes\mathcal Z\to\mathcal Y\otimes\mathcal Z$, as
$$\Phi(X) = \operatorname{Tr}_{\mathcal ...
1
vote
1answer
51 views
What is the Kraus representation of quantum-to-classical channels?
As discussed in Watrous' book, quantum-to-classical channels are CPTP maps whose output is always fully depolarised. These can always be written as
$$\Phi_\mu(X) = \sum_a \langle X,\mu(a)\rangle E_{a,...
2
votes
4answers
126 views
Can a Kraus representation act as the identity on any operator?
In the textbook “Quantum Computation and Quantum Information” by Nielsen and Chuang, it is stated that there exists a set of unitaries $U_i$ and a probability distribution $p_i$ for any matrix A,
$$\...
2
votes
2answers
112 views
Find the Kraus operators of a combined amplitude and phase damping channel
I am going through the paper Surface code with decoherence: An analysis of three superconducting architectures and I have a doubt about how the authors get what they refer to as the combined channel ...
2
votes
2answers
154 views
Prove that the depolarizing channel is completely positive
In two dimensions, for a density operator $\rho$ and probability $\lambda$, a depolarizing channel can be written as:
$$\mathcal{E}(\rho) = (1-\lambda) \frac{\mathbb{I}}{2} + \lambda\rho$$
In ...
1
vote
2answers
49 views
Proving that Partial Trace is a Quantum Operation
I am referring to Nielsen and Chuang Quantum Computation and Quantum Information 10th Anniversary Edition Textbook, Chapter 8.3.
A linear operator $E_i:H_{QR}\longrightarrow H_Q $ is defined by:
$$...
2
votes
1answer
55 views
Implementing sum on Boolean with Grover algorithm
We are trying to implement a "sum over 4 booleans = k" in the spirit of Grover search. First, we have 4 qubits, one for each boolean q00, q01, q02, q03,; then 4 ...
1
vote
0answers
105 views
Affine Map of the Bloch sphere
I am referring to Equation (8.89) to (8.92) in Chapter 8 of "Quantum Computing and Information 10th Anniversary Edition" by Nielsen and Chuang. This section deals with the geometric picture of single ...
2
votes
1answer
133 views
How does qiskit finally implement a noise model?
I have been reading qiskit documentation for hours and I still don't get how does it implements noise in the circuit. I have understood that it works with a objects of the class QuantumError which ...
3
votes
1answer
172 views
What does it mean “less than identity” in the operator sum representation?
In Quantum Computation and Quantum Information by Isaac Chuang and Michael Nielsen, section 8.2.3, $\mathcal{E}=\sum_{k}E_k\rho E_k^{\dagger}$ gives the operator-sum representation. In general, it ...
6
votes
1answer
340 views
Can the Kraus decomposition always be chosen to be a statistical mixture of unitary evolutions?
If $\mathcal{E}$ is a CPTP map between hermitian operators on two Hilbert spaces, then we can find a set of operators $\{K_j\}_j$ such that
$$\mathcal{E}(\rho)=\sum_j K_j\rho K_j^\dagger $$
in the ...
4
votes
2answers
1k views
How to find the operator sum representation of the depolarizing channel?
In Nielsen and Chuang (page:379), it is shown that the operator sum representation of a depolarizing channel $\mathcal{E}(\rho) = \frac{pI}{2} + (1-p)\rho$ is easily seen by substituting the identity ...
