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1500 questions
26
votes
2 answers
Who first proposed the idea of quantum computing using qubits?
As I understand it, the field of quantum mechanics was started in the early 20th century when Max Planck solved the black-body radiation problem. But I don't know when the idea of computers using quantum effects spread out.
What is the earliest…
luap42
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26
votes
2 answers
Why can't quantum annealing be described by a gate model?
This is a question I was inspired to ask based on this question, which notes that quantum annealing is an entirely different model for computation than the usual circuit model. I've heard this before, and it's my understanding that the gate-model…
Emily Tyhurst
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25
votes
4 answers
How to calculate circuit depth properly?
Is the circuit depth the longest sequence of gates applied on one of the qubits?
Or is it something more complicated?
C-Roux
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25
votes
5 answers
How would I implement the quantum oracle in Deutsch's algorithm?
I am trying to simulate Deutsch's algorithm (elementary case of Deutsch-Jozsa algorithm), and I am not entirely sure how I would go about implementing the quantum oracle necessary for the algorithm to function, without defeating the purpose of the…
Jack Ceroni
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25
votes
1 answer
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:
Archil Zhvania
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25
votes
1 answer
What are examples of Hamiltonian simulation problems that are BQP-complete?
Many papers assert that Hamiltonian simulation is BQP-complete
(e.g.,
Hamiltonian simulation with nearly optimal dependence on all parameters and Hamiltonian Simulation by Qubitization).
It is easy to see that Hamiltonian simulation is BQP-hard…
groupsgroupsgroups
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25
votes
2 answers
What is the status of confirming the existence of anyons?
In a comment on my answer to the question: What exactly are anyons and how are they relevant to topological quantum computing? I was asked to give specific examples of occurrence of anyons in nature. I've spent 3 days searching, but every article…
user1271772 No more free time
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25
votes
3 answers
What is the argument that practical quantum computers cannot be built?
An answer to another question mentions that
There are arguments that suggests that such machines ["quantum Turing machines"] cannot even be built...
I'm not sure I fully understand the problem, so perhaps I'm not asking the right question, but…
BurnsBA
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25
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1 answer
Explicit Lieb-Robinson Velocity Bounds
Lieb-Robinson bounds describe how effects are propagated through a system due to a local Hamiltonian. They are often described in the form
$$
\left|[A,B(t)]\right|\leq Ce^{vt-l},
$$
where $A$ and $B$ are operators that are separated by a distance…
DaftWullie
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25
votes
3 answers
Why do optical quantum computers not have to be kept near absolute zero while superconducting quantum computers do?
This is a follow-up question to @heather's answer to the question : Why must quantum computers be kept near absolute zero?
What I know:
Superconducting quantum computing: It is an implementation of a quantum computer in a superconducting electronic…
Sanchayan Dutta
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24
votes
3 answers
Which quantum error correction code has the highest threshold (as proven at the time of writing this)?
Which quantum error correction code currently holds the record in terms of the highest threshold for fault-tolerance? I know that the surface code is pretty good ($\approx10^{-2}$?), but finding exact numbers is difficult. I also read about some…
M. Stern
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24
votes
3 answers
What level of "confidence" of the result from a quantum computer is possible?
At a very basic level, reading or measuring a qubit forces it to be in one state or the other, so the operation of a quantum computer to gain a result collapses the state into one of many possibilities.
But as the state of each qubit is…
Rory Alsop
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24
votes
2 answers
Why does the "Phase Kickback" mechanism work in the Quantum phase estimation algorithm?
I've probably read the chapter The quantum Fourier transform and its applications from Nielsen and Chuang (10 th anniversary edition) a couple of times before and this took this thing for granted, but today, when I looked at it again, it doesn't…
Sanchayan Dutta
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24
votes
3 answers
Is quantum cryptography safer than classical cryptography?
Quantum computing allows us to encrypt information in a different way compared to what we use today, but quantum computers are much more powerful than today's computers. So if we manage to build quantum computers (hence use quantum cryptography),…
Archil Zhvania
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24
votes
3 answers
Is entanglement transitive?
Is entanglement transitive, in a mathematical sense?
More concretely, my question is this:
Consider 3 qubits $q_1, q_2$ and $q_3$. Assume that
$q_1$ and $q_2$ are entangled, and that
$q_2$ and $q_3$ are entangled
Then, are $q_1$ and $q_3$…
Peter
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