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1500 questions
17
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Are all quantum algorithms hidden subgroup algorithms?
I am reading the paper "Quantum Hidden Subgroup Algorithms: An Algorithmic Toolkit" by Samuel Lomonaco and Louis Kauffman from the book, "Mathematics of Quantum Computation and Quantum Technology." See also, this arxiv link.
The authors suggest…
user2521987
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Is entanglement necessary for quantum computation?
Entanglement is often discussed as being one of the essential components that makes quantum different from classical. But is entanglement really necessary to achieve a speed-up in quantum computation?
DaftWullie
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17
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How can we keep Schrödinger's cat alive?
We know, Schrödinger's cat inside the box is in the equal superposition state of both alive and dead. We can express its state as $$|\text{cat}_\phi\rangle= \frac{|\text{alive}\rangle+e^{i\phi}|\text{dead}\rangle}{\sqrt{2}} \hspace{10mm} \text{where…
Syed Emad Uddin
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Are there results from quantum algorithms or complexity that lead to advances on the P vs NP problem?
On the surface, quantum algorithms have little to do with classical computing and P vs NP in particular: Solving problems from NP with quantum computers tells us nothing about the relations of these classical complexity classes1.
On the other hand,…
Discrete lizard
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Does a complete list of open quantum software projects exist?
An answer to
Is there any source which tabulates quantum computing algorithms for simulating physical systems? mentions the Quantum Algorithm Zoo, a list of quantum algorithms. Several answers to Programming quantum computers for non-physics majors…
agaitaarino
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17
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Obtaining gate $e^{-i\Delta t Z}$ from elementary gates
I am currently reading "Quantum Computation and Quantum Information" by Nielsen and Chuang. In the section about Quantum Simulation, they give an illustrative example (section 4.7.3), which I don't quite understand:
Suppose we have the Hamiltonian…
brzepkowski
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What does "measurement in a certain basis" mean?
In the Wikipedia article about Bell states it is written:
Independent measurements made on two qubits that are entangled in Bell states positively correlate perfectly, if each qubit is measured in the relevant basis.
What does it even mean to…
user72
17
votes
2 answers
What counts as an "ancilla" qubit?
I am getting confused about the meaning of the term "ancilla" qubit. It's use seems to vary a lot in different situations. I have read (in numerous places) that an ancilla is a constant input - but in nearly all of the algorithms I know (Simion's,…
Quantum spaghettification
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What do the off-diagonal elements of a density matrix physically represent?
For simplicity, let's take a density matrix for a single qubit, written in the $\{|0\rangle,|1\rangle\}$ basis:
$$ \rho = \begin{pmatrix} \rho_{00} & \rho_{01} \\ \rho_{10}^* & 1-\rho_{00} \end{pmatrix} $$
The diagonal elements give us the…
KnightShuffler
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How are gates implemented in a continuous-variable quantum computer?
I've mostly worked with superconducting quantum computers I am not really familiar with the experimental details of photonic quantum computers that use photons to create continuous-variable cluster states such as the one that the Canadian startup…
Mark Fingerhuth
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In a Quantum Turing Machine, how is the decision to move along the memory tape made?
Let, for a Quantum Turing machine (QTM), the state set be $Q$, and the alphabet of symbols be $\sum=\{0,1\}$, which appear at the tape head. Then, as per my understanding, at any given time while the QTM is calculating, the qubit that appears at its…
Prem
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How to measure in another basis
I am new to qiskit and I have to simulate a quantum circuit. I read this documentation https://qiskit.org/textbook/ch-states/single-qubit-gates.html where it is left as an exercise to the reader to write a function to measure in the $|+i\rangle$ and…
thedumbkid
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Why must quantum computers be kept near absolute zero?
Online descriptions of quantum computers often discuss how they must be kept near absolute zero $\left(0~\mathrm{K}~\text{or}~-273.15~{\left. {}^{\circ}\mathrm{C} \right.}\right)$.
Questions:
Why must quantum computers operate under such extreme…
Didix
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What is the opposite of measurement, in a quantum circuit?
My understanding is that at the level of quantum mechanics, almost all operations are reversible in time. Most gates in a quantum circuit clearly obey this rule; they can be reversed by applying some other gate. But a measurement gate seems to be…
lacker
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Is VQE a class of algorithms or a specific algorithm?
Is VQE a class of algorithms or a specific algorithm? For example, is QAOA a VQE or is VQE an algorithm distinct from QAOA that solves the same class of problems?
If VQE is a specific algorithm, what are its defining features that distinguish it…
Malcolm Regan
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