For questions about the usage of, origin of or reasons for specific notation used in the context of quantum computing.
Questions tagged [terminology-and-notation]
205 questions
53
votes
1 answer
What is meant by "Noisy Intermediate-Scale Quantum" (NISQ) technology?
Preskill introduced recently this term, see for example Quantum Computing in the NISQ era and beyond (arXiv). I think the term (and the concept behind it) is of sufficient importance that it deserves to be explained here in a pedagogical manner.…
agaitaarino
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48
votes
2 answers
What is the "surface code" in the context of quantum error correction?
I am studying Quantum Computing and Information, and have come across the term "surface code", but I can't find a brief explanation of what it is and how it works. Hopefully you guys can help me with this.
Ivanovitch
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47
votes
5 answers
How does bra-ket notation work?
Quantum algorithms frequently use bra-ket notation in their description. What do all of these brackets and vertical lines mean? For example: $|ψ⟩=α|0⟩+β|1⟩$
While this is arguably a question about mathematics, this type of notation appears to be…
Ella Rose
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39
votes
4 answers
What is meant by the term "computational basis"?
What is meant by the term "computational basis" in the context of quantum computing and quantum algorithms?
user1039
35
votes
2 answers
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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31
votes
2 answers
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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25
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2 answers
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 the exact meaning of coherence time T2?
peachnuts
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25
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2 answers
Significance of The Church of the Higher Hilbert space
The term "Church of the Higher Hilbert Space" is used in quantum information frequently when analysing quantum channels and quantum states.
What does this term mean (or, alternately, what does the term "Going to To the Church of the Higher Hilbert…
user3483902
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22
votes
6 answers
Quantum states are unit vectors... with respect to which norm?
The most general definition of a quantum state I found is (rephrasing the definition from Wikipedia)
Quantum states are represented by a ray in a finite- or infinite-dimensional Hilbert space over the complex numbers.
Moreover, we know that in…
Adrien Suau
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votes
3 answers
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
18
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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16
votes
1 answer
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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15
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3 answers
What is the difference between "code space", "code word" and "stabilizer code"?
I keep reading (e.g. Nielsen and Chuang, 2010; pg. 456 and 465) the following three phases; "code space", "code word" and "stabilizer code" - but am having a difficult time finding definitions of them and more importantly how they differ from one…
Quantum spaghettification
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15
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2 answers
What does quantum error correction code notation stand for?
I understand the notation for classical error correcting codes. E.g., "Hamming(7,4)" stands for a Hamming code that uses 7 bits to encode blocks of 4 bits.
What does the notation for quantum error correcting codes mean? E.g., there is a paper that…
Alexander Pozdneev
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14
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2 answers
What is the relation between POVMs and observables (as Hermitian operators)?
Let $\renewcommand{\calH}{{\mathcal{H}}}\calH$ be a finite-dimensional Hilbert space.
An observable $A$ is here a Hermitian operator, $A\in\mathrm{Herm}(\calH)$.
A POVM is here a collection of positive operators summing to the identity: $\{\mu(a):…
glS
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