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1500 questions
8
votes
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How to prevent future loops using a control qubit?
I am trying to construct a quantum multiplier using the method described here: https://arxiv.org/abs/quant-ph/0403048. However, it seems that the control qubit would only disable the following gates for one iteration. Afterward, the $|y\rangle$…
nikojpapa
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How does the conditional min-entropy $H_{\rm min}(A|B)_\rho$ relate to the conditional entropy $H(X|Y)_\rho$?
Suppose we have a classical quantum state $\sum_x |x\rangle \langle x|\otimes \rho_x$, one can define the smooth-min entropy $H_\min(A|B)_\rho$ as the best probability of guessing outcome $x$ given $\rho_x$. How does this quantity relate to…
john_smith
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8
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Depolarizing channel implementation on IBM Q
Given a single qubit in the computational basis, $|\psi\rangle =\alpha |0\rangle + \beta|1\rangle$, the density matrix is $\rho=|\psi\rangle\langle\psi|=\begin{pmatrix} \alpha^2 & \alpha \beta^*\\
\alpha^*\beta & \beta ^2\end{pmatrix}$.…
Mathist
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8
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2 answers
What's a Qubit on D-Wave 2000Q?
From D-Wave flyer:
The D-Wave 2000Q system has up to 2048 qubits and 5600 couplers. To reach this scale, it uses 128,000
Josephson junctions, which makes the D-Wave 2000Q QPU by far the most complex superconducting
integrated circuit ever…
0x90
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8
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1 answer
Rotationally invariant maximally entangled states in higher dimensions
Is there a straightforward generalization of the $\mathbb{C}^2$ Bell basis to $N$ dimensions? Is there a rotational invariant Bell state in higher dimensions? If yes, then what is the form of that state (how does it look like)? And, by rotational…
Vijeth Aradhya
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8
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2 answers
When would I consider using an outer product of quantum states, to describe aspects of a quantum algorithm?
I know the inner product has a relationship to the angle between two vectors and I know it can be used to quantify the distance between two vectors. Similarly, what's an use case for the outer product? You can exemplify with the simplest case. It…
R. Chopin
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In D-Wave's universal quantum computer, why does the YY term have to be driven along with the linear X term?
D-Wave has a new prototype annealer that uses a Hamiltonian which, if there was enough qubits and sufficient control, would be able to simulate any universal circuit-based quantum computer with at most polynomial overhead. It was presented at the…
user1271772 No more free time
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8
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1 answer
How to say "apply CNOT on qubit 1 controlled by qubit 2"?
Say you have $2$ qubits, namely $q_1, q_2$. What's the right language for saying apply CNOT on $q_1$ and $q_2$ where $q_1$ is the control qubit and $q_2$ is the target?
For instance, can I say "apply CNOT on $q_2$ controlled by $q_1$"? What's the…
R. Chopin
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8
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1 answer
Compact way of describing the set of all stabilizer groups for fixed number of physical qubits and encoded logical qubits
Fix $n$, the number of qubits and $k$, the number of encoded logical qubits. We can find a set of $(n-k)$ operators that all mutually commute and moreover form a group $S$. Let's assume that the group $S$ is a subgroup of the Pauli group. We can use…
Amara
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8
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17 answers
What are some common conventions in quantum computing?
Marked Community-Wiki
In many fields of human endeavor there are often conventional practices or other "house rules" that go unstated and initially appear arbitrary but might have historical or otherwise practical reasons. Think "the tie goes to the…
Mark Spinelli
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8
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2 answers
How does one obtain amplitude information in Q#?
Is it possible to obtain amplitude information (in lexicographic ordering) for a particular qubit register in Q#?
For example, in the following code:
operation GetProbabilityAmplitude() : Double[]
{
body
{
mutable result = new…
jman
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8
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1 answer
Difference between coherence transfer, polarization transfer and population transfer?
I asked a question on Physics Stack Exchange but no one answered the question and I didn't get enough views on it. I am asking it on QCSE because the question is related to experimental quantum computation realized through NMR.
For an ensemble of…
Jitendra
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8
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1 answer
Why is it helpful to use Majorana particle in topological quantum computing (they are just Fermions?!)
My basic understanding of topological quantum computing is that it makes use of anyons which are particles satisfying the following rule under wave-function swap:
$$ |\psi_1 \psi_2\rangle = e^{i \theta} |\psi_2 \psi_1\rangle$$
We recognize the…
Marco Fellous-Asiani
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8
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1 answer
Does a Wigner function uniquely determine a quantum state?
We know that the Wigner function of a Gaussian quantum state is (up to a constant) a Gaussian distribution. The first moment and the covariance of this distribution uniquely specify a quantum state. Therefore a Wigner function uniquely determines a…
taper
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8
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Can two states with the same entanglement be transformed into each other using local unitaries?
Take two pure bi-partite states $\psi$ and $\phi$ that have the same amount of entanglement in them as quantified by concurrence (does the measure make a difference?). Can any such states be transformed into each other using local unitaries?
user120404
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