6
votes
Accepted
Finding the eigenvalues of a qutrit state
Maybe you forgot about the coefficient $\frac{1}{\sqrt{2}}$. The correct reduced density matrix is $$\frac12 (\left|1\right>\left<1\right| + \left|2\right>\left<2\right|)$$
- 208
4
votes
Accepted
Is possible to write a separable state as a finite or countable infinite sum of product states?
Yes, you can always write an integral of separable states as a (finite) convex combination of product states, owing to the fact that the set of separable states is compact.
A quick way to see this is ...
- 5,158
2
votes
Accepted
Problem with eigenvalue evaluation algorithm application on matrix $U$
TL;DR: Qubit order in the top register is reversed.
QFT qubit order in Quirk
Quirk's QFT gate treats the top qubit as the least significant and the bottom qubit as the most significant. Thus, if you ...
- 20.8k
2
votes
What is known about the size of the spectral gap of unital quantum channels?
TL;DR: Spectral gap depends on the specific channel. Moreover, for any $g\in[0,1]$ there is a channel with spectral gap $g$.
Non-peripheral eigenoperators are traceless
We can make simple observations ...
- 20.8k
1
vote
Finding the "dual" basis of an overcomplete basis for Quantum State Tomography
A general procedure to find the dual of any given frame $\{v_k\}_k$ is to compute the frame operator $S$, and then the (canonical) dual frame elements as $\tilde v_k\equiv S^{-1} v_k$. The dual frame ...
glS♦
- 23.4k
1
vote
Accepted
Why $\sqrt{\rho} = P \sqrt{\rho}$ in the proof of quantum error correction conditions in Nielsen & Chuang?
Recall from the sentence immediately preceding $(10.20)$ that $\rho$ is a state in the code subspace $C$ and from the statement of theorem $10.1$ that $P$ is a projector onto $C$. We will show that $\...
- 20.8k
1
vote
Why $\sqrt{\rho} = P \sqrt{\rho}$ in the proof of quantum error correction conditions in Nielsen & Chuang?
$\rho$ is a state in the code. $P$ is the projector onto the codespace. That means (by definition) that for any state $|\psi\rangle$ in the code, $P|\psi\rangle=|\psi\rangle$.
$\rho$ is one such state ...
- 55.7k
1
vote
Accepted
Problem with the mathematical definition of the eigenvalue algorithm on a specific exercise
Calculate the wavefunction step by step by applying each gate in sequence
$$
\begin{align}
|000\rangle&\xrightarrow{H\otimes H}\frac12\sum_{a,b=0}^1|a\rangle|b\rangle|0\rangle\tag1\\
&\...
- 20.8k
1
vote
Accepted
How to get the Kraus operator $M_0=\sqrt{1-p}\, I$ for the depolarizing channel, from its isometric representation?
The unitary representation specifies how the isometry $U$ acts on pure states. The channel $\Phi$ is related to $U$ by $\Phi(\rho)=\operatorname{tr}_E[U\rho U^\dagger]$. Notice that on pure states ...
glS♦
- 23.4k
1
vote
Is there a general method for calculating expectation values for time-dependent wavefunctions?
I won't give the final answer (close to it), but instead try and point you in the general direction.
Initial State: The given initial state is $ | \psi(t=0) \rangle = | 0 \rangle $.
Pauli X Matrix: ...
- 755
1
vote
What do the values in a unitary matrix represent/what do they mean? How do you figure out what gate a unitary matrix represents?
I don't know what your background is, but I find it helpful to come at this from the starting point of classical probability:
Imagine you have a two state system (such as a coin). It can either be ...
- 55.7k
1
vote
What do the values in a unitary matrix represent/what do they mean? How do you figure out what gate a unitary matrix represents?
Every unit vector in your Hilbert Space corresponds to a valid quantum state. Multiplying that matrix with your vector is just the linear transformation.
It is like, given that all possible states, ...
- 1,311
1
vote
Accepted
How to know what eigenvalue corresponds to measurements of individual qubits in a multiqubit system?
If you consider the state
$$\Omega_- = \frac{1}{\sqrt{2}}(|00\rangle + |11\rangle).\tag{1}$$
you will find that there is NO violation. Let me show this. Note that the approach I will use for solving ...
- 126
1
vote
How to know what eigenvalue corresponds to measurements of individual qubits in a multiqubit system?
From your current description, it seems that you have found each eigenvector $\{|v_1\rangle,...,|v_4\rangle\}$ and its associated eigenvalue $\lambda_1,...,\lambda_4$. These eigenvalues represent the ...
- 93
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