Quantum Computing Stack Exchange Community Digest

Top new questions this week:

Quantum teleportation and the reality of quantum states

This question is perhaps philosophical but it's been confusing me. Suppose Alice is teleporting some qubit state $|\phi\rangle$ to Bob via the quantum teleportation protocol. After Alice applies the ...

quantum-state teleportation communication foundations  
asked by Pedro 5 votes
answered by Mateus Araújo 1 vote

Could random quantum circuits be efficiently approximately simulated?

Google's landmark result last year was to compute a task with a quantum computer that a classical computer could not compute, and they chose random circuit sampling. Part of their justification was ...

complexity-theory quantum-advantage random-quantum-circuit  
asked by Sam Jaques 5 votes
answered by Condo 1 vote

Alternative definition of the coherent information of a quantum channel

Let $T: M_n \to M_n$ be a quantum channel. If I understand Definition 13.5.1 of the book "Quantum information theory" of Wilde, the coherent information $Q(T)=\max_{\phi_{AA'}} I(A \rangle B)...

quantum-state quantum-operation information-theory  
asked by Aaron 4 votes
answered by rnva 2 votes

Can I partially collapse the wavefunction in Qiskit?

I'm trying to do something similar to this question, where I want to partially measure the system before getting the output. In particular, say I have 4 qubits. I want to measure two of them, and then ...

qiskit measurement  
asked by Germ 4 votes

What is a maximal number factored by Shor's algorithm so far?

With reference to similar question here I would like to know what is the maximal number which has been factored with Shor's algorithm so far on actual quantum hardware. The reason I am asking similar ...

shors-algorithm  
asked by Martin Vesely 4 votes
answered by KAJ226 2 votes

How to perform this d-dimension unitary operation on IBM Q?

$U_{a,b}=\sum^{d-1}_{x=0}\omega^{bx}|x+a\rangle\langle x|$,$\omega=e^{\frac{2\pi i}{d}}$,$a,b\in\{0,1,2,...,d-1\}$ Can someone please give me the pic of the quantum circuit?

quantum-gate ibm-q-experience quantum-operation  
asked by Williamyang 3 votes
answered by tsgeorgios 2 votes

Why do the constant operations in a quantum computer need second qbits?

From what I'm reading, building the constant-1 and constant-0 operations in a quantum computer involves building something like this, where there's two qbits being used. Why do we need two? The ...

quantum-operation universal-gates  
asked by MysteriousWaffle 3 votes

Greatest hits from previous weeks:

How to quickly calculate the custom U3 gate parameters $\theta, \phi$ and $\lambda$ for any unitary?

In IBM Qiskit and Quantum Experience, the custom U3 gate is defined as $$ U(\theta, \phi, \lambda) = \begin{pmatrix} \cos\left(\frac{\theta}{2}\right) & -e^{...

programming qiskit resource-request ibm-q-experience  
asked by Sanchayan Dutta 7 votes
answered by tigerjack89 5 votes

Is there proof that the D-wave (one) is a quantum computer and is effective?

I'm admittedly a novice in this field, but I have read that, while the D-wave (one) is an interesting device, there is some skepticism regarding it being 1) useful and 2) actually a 'quantum computer'....

performance annealing d-wave classical-computing experiment  
asked by Discrete lizard 41 votes
answered by Andrew O 27 votes

How to understand the Haar measure from a quantum information perspective?

I found it a little difficult to understand it using Wikipedia and some mathematical documents. How to understand the Haar measure from a quantum information theory perspective? Are there any ...

quantum-information resource-request  
asked by raycosine 12 votes
answered by David Bar Moshe 15 votes

How to interpret a quantum circuit as a matrix?

If a circuit takes more than one qubit as its input and has quantum gates which take different numbers of qubits as their input, how would we interpret this circuit as a matrix? Here is a toy example:...

quantum-gate matrix-representation  
asked by Archil Zhvania 16 votes
answered by heather 18 votes

Why doesn't the Gottesman-Knill theorem render quantum computing almost useless?

The Gottesman-Knill theore states (from Nielsen and Chuang) Suppose a quantum computation is performed which involves only the following elements: state preparations in the computational basis, ...

simulation nonclassicality gottesman-knill  
asked by user2723984 7 votes
answered by DaftWullie 11 votes

How to input 2 qubits in 2 Hadamard gates?

Let's say we have a circuit with $2$ Hadamard gates: Let's take the $|00\rangle$ state as input. The vector representation of $|00\rangle$ state is $[1 \ 0 \ 0 \ 0]$, but this is the representation ...

quantum-gate quantum-state  
asked by Archil Zhvania 11 votes
answered by Sanchayan Dutta 15 votes

Can the Bloch sphere be generalized to two qubits?

The Bloch sphere is a nice visualization of single qubit states. Mathematically, it can be generalized to any number of qubits by means of a high-dimensional hypersphere. But such things are not easy ...

resource-request bloch-sphere state-space-geometry  
asked by James Wootton 19 votes
answered by Craig Gidney 16 votes

Can you answer these questions?

Abelian Hidden Subgroup Problem for arbitrary cyclic p-Groups

I had asked a question similar to this one here regarding how to handle the HSP for groups whose cyclic decomposition contains factors whose order is not a power of two. I also had some prior ...

shors-algorithm quantum-fourier-transform hidden-subgroup-problem fourier-sampling  
asked by dylan7 2 votes

What is known about quantum algorithms for graph isomorphism?

Shor's algorithm (for factoring integers) and Grover's algorithm (for searches) are the two most well-known quantum algorithms. I was wondering if there was a similar result in QC that dealt with the ...

adiabatic-model  
asked by paulinho 2 votes

How to implement the Mixer of Quantum Alternating Operator Ansatz for Max-Independent-Set

I am trying to implement the Mixer of the Max-Independent Set from The Quantum Alternating Operator Ansatz. From this paper: https://arxiv.org/pdf/1709.03489.pdf in Chapter 4.2 page 15 to 17. For ...

programming mathematics hamiltonian-simulation vqe qaoa  
asked by Hannah 1 vote
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