# Questions tagged [terminology]

For questions about the meaning and usage of vocabulary specific to quantum computing or quantum information.

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### Do the following circuits violate the principle of "no fast-forwarding"?

The no fast-forwarding principle states roughly that, given that we can simulate a Hamiltonian for time $t$ using $r$ gates, in order to perform the Hamiltonian simulation for time $2t$, we must in ...
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### Clarification defining/finding the relative phase of a qubit

Let the vector $|V\rangle = r_0 e^{i\theta_0} |0\rangle + r_1 e^{i\theta_1} |1\rangle$ correspond to the state of a qubit where $r_0,r_1,\theta_0,\theta_1 \in \mathbb{R}$. According to p. 22 of ...
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I was reading the paper Quantum computation, quantum state engineering, and quantum phase transitions driven by dissipation . The claim is that universal quantum computation can be achieved using the ...
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### What is fidelity in quantum computing?

As I studied quantum computing, I saw term 'Fidelity' in many papers that related to quantum algorithm. So, I really wonder about following two things. What is the real meaning of 'Fidelity' (As I ...
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### What is the difference between "maximally entangled" and "entangled" states?

when we talk about bell state we say that these states are maximally entangled. so just wanted to understand is there any difference between just entangled and maximally entangled ?
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### Why does the condition $\langle \varphi _k\left( t \right) |\dot{\varphi}_l\left( t \right) \rangle =0$ called parallel transport condition?

I'm reading this paper about nonadiabatic holonomic quantum computation and met the condition eq.(2) states that $\langle \varphi _k\left( t \right) |\dot{\varphi}_l\left( t \right) \rangle =0$ is the ...
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### What is dual space, and why is it required?

I recently started QC/QM and came across something called dual space? How do we explain concept of dual space to a layman. why do we need dual space? I tried to read and watch some resources on dual ...
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### Why do we use 'modes' in quantum optics?

In this material, it gives the definition of modes: The modes are basically defined by the properties of coherence and orthogonality: modes are orthogonal solutions of the wave equation. Since the ...
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### Where is entanglement present and absent in the one clean qubit model of computation?

Based on my limited and poor understanding, a quantum circuit in the "one clean qubit" model of quantum computation generally acts on a single pure (clean) qubit tensored with $n-1$ qubits ...
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### What is meant by "perfect state transfer"?

In discussions on many quantum algorithms especially related to quantum walks, I have seen the term "perfect state transfer" used to describe some property apparently related to the ...
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### What is the difference between "Shot-Noise-Limit" and "Standard Quantum Limit"?

It seems that in a lot of papers in the field of quantum metrology, there are two terms Shot-Noise-Limit and Standard Quantum Limit which are frequently referred to. What's the difference between them,...
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### How is a promise gap related to a spectral gap?

In linear algebra one often concerns oneself with the spectral gap of a given matrix, which may be defined as the difference between the smallest and second-smallest eigenvalue (or, depending on ...
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### Terminology: what do $|i\rangle$ and $|\mbox{-}i\rangle$ represent?

$|0⟩$ and $|1⟩$ are usually referred as the computational basis. $|+⟩$ and $|-⟩$, the polar basis. What about $|i\rangle$ and $|\mbox{-}i\rangle$? And collectively? Orthonormal states? References are ...
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### What is the pseudo threshold of a QECC using stabilizer formalism

Can someone explain what is the threshold and the pseudo threshold of a Quantum Error Correction Code , for instance the 9-qubit code, and how to calculate it using the stabilizer formalism simulation ...
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### What are boost and shift operators and why are they called so?

In some texts I see $X$ and $Z$ Pauli operators as being said as boost and shift operators respectively. But I came across some text that defines its own operators, namely:  X \vert j\rangle = \...
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### Why do we say 'classical computer' to mean 'digital computer'?

The term 'classical computer' is always used to mean standard digital computation (Turing model, Boolean circuits or just good old RAM). I have never seen it to mean other models of computation based ...
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### Terminology : Are basis states the same as basis vectors?

The question says it all. Explain any difference in terminology that can be encountered across Physics, Quantum Computing and Mathematics. Thanks!
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### What is meant with "shot" in quantum computation?

What is a "shot" in relation to quantum computing? I heard it is repeating time, but I don't understand it. Additionally, why is the default number of shots 1,024 in the IBM Q Experience? One more ...
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### What's the difference between T1 and T2?

I learned that T1 is relaxation time (time from $|1\rangle$ to $|0\rangle$) and T2 is coherence time. The relaxation is a specific case of decoherence. What's the difference between them and what's ...
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### What is a flying qubit?

DiVincenzo criteria for quantum communication mentions flying qubits: a. The ability to interconvert stationary and flying qubits b. The ability to faithfully transmit flying qubits between specified ...
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### Significance of the term "diffusion" in Grover's diffusion operator

In the context of Grover's algorithm, the diffusion operator is defined as $U_s = 2|s\rangle \langle s| - I$ with $|s\rangle\equiv |+\rangle^{\otimes n}$. What is the significance of the term "...
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### What is the difference between relaxation, dephasing, and decoherence?

Many sources seem to loosely use the terms "relaxation", "dephasing", and "decoherence" interchangeably, while others seem to treat certain of them as special cases of another terms, but I can't find ...
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### What does fidelity mean?

I am learning qiskit software and this term keeps popping up and I am unable to get a grasp on the technical definition given by wikipedia. For example, the functions state fidelity and process ...
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