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1500 questions
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One-qubit gate results in QISKit
I found it odd that the result of the action of identity gate (namely a $2\times2$ identity matrix) on a pure state $|0\rangle$ (namely the vector corresponding to the $2\times1$ matrix $\begin{bmatrix} 1\\0 \end{bmatrix}$) becomes a $2\times2$…
Mathist
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How is a single qubit fundamentally different from a classical coin spinning in the air?
I had asked this question earlier in the comment section of the post: What is a qubit? but none of the answers there seem to address it at a satisfactory level.
The question basically is:
How is a single qubit in a Bell state
…
Sanchayan Dutta
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7
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Circuit from finite group of gates and classical simulations
Let $ G $ be a finite group of quantum gates. Is it true that any circuit made using only gates from the finite group $ G $ can be efficiently simulated on a classical computer?
Here by circuit made from $ G $ I mean a circuit in which all gates…
Ian Gershon Teixeira
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2 answers
What is this circuit doing?
Can anyone help me understanding what is this circuit doing ?
The circuit can be reproduced with qiskitas follows:
qc =…
mpro
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What is the complexity of hidden subgroup problems?
It is often stated that some of the "hidden subgroup problems" can be efficiently solved by quantum computers if the group is abelian, while no efficient algorithm is known for the non-abelian case. The problems of the first case include…
Doriano Brogioli
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3 answers
When can we expect the first (universal) quantum computer being able to do something useful outside the academic world?
The governments, big companies (list of quantum processors) and smaller ones are in the competition of building bigger and bigger quantum computers.
Not unexpectedly the number of qubits of those quantum computers seem to double every year but those…
JanVdA
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Why do we need to keep of history of $d$ cycle for surface code in order to resist against data *and* measurement errors
I am trying to understand why we need to keep in memory $d$ clock cycles in order to correct efficiently for the surface code where the syndrome qubits are also faulty ($d$ is the code distance).
[edit]: It seems that there is no real "proof" of…
Marco Fellous-Asiani
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How did Lov Grover think up his search algorithm? How might one have discovered it themselves?
There are two related but distinct parts to my question and I'm happy to hear perspectives on either:
Any historical details, testimonies, papers that shed insight into what Lov Grover was working on which led him to think up his algorithm.
What…
shashvat
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Qubit fidelity of DWAVE device
Since DWAVE quantum device is constructed using superconducting flux qubits, each qubit cannot be produced identically so that the fidelity of the qubit must be different. DWAVE only provides the information of their devices in terms of number of…
peachnuts
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Quantum XOR Linked List Construction
After getting help here with XNOR & RCA gates I decided to dive into XOR Swaps & XOR linked lists. I was able to find this explanation for quantum XOR Swapping which seems sufficient for the time being. I am not able to find any information on…
user820789
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7
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Closest quantum state with a fixed marginal: Analytical solution?
Let $\rho_{AB}$ be a bipartite state and let $\sigma_{B}$ be another state. What state $\tilde{\rho}_{AB}$ is closest to $\rho_{AB}$ and satisfies $\tilde{\rho}_B = \sigma_B$? We can define closeness in many ways but I pick fidelity here…
user1936752
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7
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2 answers
How to check if 2 qubits are entangled?
I know that 2 qubits are entangled if it is impossible to represent their joint state as a tensor product. But when we are given a joint state, how can we tell if it is possible to represent it as a tensor product?
For example, I am asked to tell if…
Archil Zhvania
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7
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Zero-distance self-dual GF(4) quantum codes and constructing k > 0 codes from them
During a description of zero-dimensional self-dual $\text{GF}(4)$ quantum codes in "On self-dual quantum codes, graphs, and Boolean functions" by L.E. Danielsen, it states:
A zero-dimensional stabilizer code with high distance represents a single…
SLesslyTall
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7
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2 answers
Confusion regarding projection operator
Suppose we have a qutrit with the state vector $|\psi\rangle = a_0|0\rangle + a_1|1\rangle + a_2|2\rangle$, and we want to project its state onto the subspace having the basis $\{|0\rangle,|2\rangle\}$, I know the projection operator would be…
alphauser
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What do "$i$-th basic network", "quantum multiplexers" and "quantum parallelism" mean in this context? How are they beneficial?
I have been reading the paper A quantum-implementable neural network model (Chen et al., 2017) for a few days now, but failed to understand how exactly their algorithm offers a speedup over the classical neural network models.
In particular I'm…
Sanchayan Dutta
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