Quantum Computing Stack Exchange
Quantum Computing Stack Exchange

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1500 questions
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Which subatomic particle does each company use in quantum computing?

Probably each company (Google, Amazon, Intel, IBM, Microsoft, D-Wave and so on) uses a mix of subatomic particles and technologies. I would like to know which particles/technologies are used by each company. Are there specific reasons to choose a…
Felipe Rojo Amadeo
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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 there a simple, formulaic way to construct a modular exponentiation circuit?

I'm a newcomer to quantum computing and circuit construction, and I've been struggling to understand how to make a modular exponentiation circuit. From what I know, there are several papers on the matter (like Pavlidis, van Meter, Markov and Saeedi,…
Juan
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How to construct matrix of regular and "flipped" 2-qubit CNOT?

When constructing the matrices for the two CNOT based on the target and control qubit, I can use reasoning: "If $q_0$==$|0\rangle$, everything simply passes through", resulting in an Identity matrix style $\begin{bmatrix}1&0\\0&1\end{bmatrix}$ in…
Thomas Hubregtsen
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What exactly are Quantum XOR Games?

I have done some research & found a few different papers that discuss xor games (classic & quantum). I am curious if someone could give a concise introductory explanation as to what exactly xor games are & how they are or could be used/useful in…
user820789
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6 answers

How to construct a multi-qubit controlled-Z from elementary gates?

For the implementation of a certain quantum algorithm, I need to construct a multi-qubit (in this case, a three-qubit) controlled-Z gate from a set of elementary gates, as shown in the figure below. . The gates that I can use are the Pauli gates…
Dyon J Don Kiwi van Vreumingen
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How would a quantum computer be used to solve Partial Differential Equations?

Say you have a PDE you want to solve. What kind of quantum algorithms would you use to solve it? How do we input our problem on a quantum computer? What will be the output and in what form? I know that quantum algorithms for solving linear systems…
cnada
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Grover's algorithm: a real life example?

I'm fairly confused about how Grover's algorithm could be used in practice and I'd like to ask help on clarification through an example. Let's assume an $N=8$ element database that contains colors Red, Orange, Yellow, Green, Cyan, Blue, Indigo and…
01000001
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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 of $2$ qubits and H accepts just $1$ qubit, so…
PiMan
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Are there any estimates on how complexity of quantum engineering scales with size?

It seems to me that an extremely relevant question for the prospects of quantum computing would be how the engineering complexity of quantum systems scales with size. Meaning, it's easier to build $n$ $1$-qubit computers than one $n$-qubit computer.…
Keith Rush
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What is a stabilizer state?

I am reading through the paper "Direct Fidelity Estimation from Few Pauli Measurements" (arXiv:1104.4695) and it mentions 'stabilizer state'. "The number of repetitions depends on the desired state $\rho$. In the worst case, it is $O(d)$, but in…
Quantum Guy 123
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Building a quantum computer in simulation

If one wants to start building a quantum computer from scratch inside simulations (like how people get to build a classical computer from scratch in the Nand2Tetris course), is it possible? If yes, what would be some possible approaches? Also, what…
ak_nama
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What is the physical representation of a qubit?

In regular computers, bits may be physically represented using a wide variety of two-state devices, such as polarity of magnetization of a certain area of a ferromagnetic film or two levels of electric charge in a capacitor. But qubits have a…
user609
16
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2 answers

How to get started with the Alibaba Cloud Quantum Computing Service?

According to this press announcement from March 1st 2018, the Alibaba Cloud offers access to an 11 qubit quantum computer via their cloud services. Quote: Alibaba Cloud, [...] and Chinese Academy of Sciences (CAS) [...] have launched the…
blalasaadri
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What does "bipartite" mean?

This is a really easy question, but my mother language is not English and I get confused quite a lot reading Preskill notes. What does a bipartite system mean? Is this just that it "lives" in a tensor product of two Hilbert spaces? Does it mean that…
CFRedDemon
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