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35
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4 answers
Effects of quantum computing on parallel universes
I have heard a few times that one way of describing quantum computers is that they essentially use the computing power of their counterparts in alternate realities that they access through superposition. My first question is, of course,
is this…
Snowshard
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Are there problems in which quantum computers are known to provide an exponential advantage?
It is generally believed and claimed that quantum computers can outperform classical devices in at least some tasks.
One of the most commonly cited examples of a problem in which quantum computers would outperform classical devices is…
glS
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34
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What exactly is an oracle?
What exactly is an "oracle"? Wikipedia says that an oracle is a "blackbox", but I'm not sure what that means.
For example, in the Deutsch–Jozsa algorithm,$\hspace{85px}$,is the oracle just the box labeled $`` U_f " ,$ or is it everything between the…
Marco Fellous-Asiani
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How to derive the CNOT matrix for a 3-qubit system where the control & target qubits are not adjacent?
In a three-qubit system, it's easy to derive the CNOT operator when the control & target qubits are adjacent in significance - you just tensor the 2-bit CNOT operator with the identity matrix in the untouched qubit's position of…
ahelwer
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32
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Has there been any truly ground breaking advance in quantum algorithms since Grover and Shor?
(Sorry for a somewhat amateurish question)
I studied quantum computing from 2004 to 2007, but I've lost track of the field since then. At the time there was a lot of hype and talk of QC potentially solving all sorts of problems by outperforming…
Alex Kinman
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What is the difference between a relative phase and a global phase? In particular, what is a phase?
I know that $re^{i\theta} = x + iy$ for any complex number $x + iy$ by Euler's formula. How do you calculate relative and global phase?
LeWoody
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How do I build a gate from a matrix on Qiskit?
I'm creating a gate for a project and need to test if it has the same results as the original circuit in a simulator, how do I build this gate on Qiskit? It's a 3 qubit gate, 8x8 matrix:
$$
\frac{1}{2}
\begin{bmatrix}
1 & 0 & 1 & 0 & 0 & 1 & 0 & -1…
Nillmer
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When will we know that quantum supremacy has been reached?
The term "quantum supremacy" - to my understanding - means that one can create and run algorithms to solve problems on quantum computers that can't be solved in realistic times on binary computers. However, that is a rather vague definition - what…
blalasaadri
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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 to visualize.
What attempts have been made to…
James Wootton
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What could be the possible future applications for HHL algorithm?
Note on the vocabulary: the word "hamiltonian" is used in this question to speak about hermitian matrices.
The HHL algorithm seems to be an active subject of research in the field of quantum computing, mostly because it solve a very important…
Adrien Suau
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What's the point of Grover's algorithm if we have to search the list of elements to build the oracle?
Grover's algorithm is used, among other things, to search an item $\mathbf{y}$ in an unordered list of items $[\mathbf{x}_0, \mathbf{x}_1, ..., \mathbf{x}_{n-1}]$ of length $n$. Even though there are plenty of questions here regarding this topic, I…
incud
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Are there any organised resources available from where I can begin my quantum computing studies?
I am a computer science student and am currently searching for resources from where I can learn about quantum computers, quantum computing models, their working principles, their gates and some simple quantum algorithms.
Aman Gupta
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What precisely is quantum annealing?
Many people are interested in the subject of quantum annealing, as an application of quantum technologies, not least because of D-WAVE's work on the subject. The Wikipedia article on quantum annealing implies that if one performs the 'annealing'…
Niel de Beaudrap
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What's the difference between a pure and mixed quantum state?
As per my limited understanding, a pure state is the quantum state where we have exact information about the quantum system. And the mixed state is the combination of probabilities of the information about the quantum state of the quantum system.…
Koder101
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What is postselection in quantum computing?
A quantum computer can efficiently solve problems lying in the complexity class BQP. I have seen a claim that one can (potentially, because we don't know whether BQP is a proper subset or equal to PP) increase the efficiency of a quantum computer by…
Sir Cornflakes
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