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1500 questions
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What is the longest time a qubit has survived with 0.9999 fidelity?
I am pretty intrigued by the record time that a qubit has survived.
Daniel Tordera
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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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How power-efficient are quantum computers?
Quantum algorithms scale faster than classical ones (at least for certain problem clases), meaning quantum computers would require a much smaller number of logical operations for inputs above a given size.
However, it is not so commonly discussed…
Alex Jone
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Is Google's 72 qubit device better than D-Wave's machines, which feature more than 2000 qubits?
Google recently announced the Bristlecone 72 qubit quantum computer.
However, D-Wave already announced quantum computers featuring more than $2000$ qubits.
Why is Google's new device newsworthy then? Is it better than D-Wave's machine in some…
Devansh Sharma
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Why do error correction protocols only work when the error rates are already significantly low to begin with?
Quantum error correction is a fundamental aspect of quantum computation, without which large-scale quantum computations are practically unfeasible.
One aspect of fault-tolerant quantum computing that is often mentioned is that each error-correction…
glS
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Given a decomposition for a unitary $U$, how do you decompose the corresponding controlled unitary gate $C(U)$?
Suppose we have a circuit decomposition of a unitary $U$ using some universal gate set (for example CNOT-gates and single qubit unitaries). Is there a direct way to write down the circuit of the corresponding controlled unitary $C_U$ using the same…
M. Stern
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Is HHL still BQP-complete when the matrix entries are only in {0,1}?
I'm studying BQP-completeness proofs of a number of interesting problems of Janzing and Wocjan, and Wocjan and Zhang. Janzing and Wocjan show that estimating entries of matrix powers $(A^m)_{ij}$ with $A_{ij}\in\{-1,0,1\}$ is (promise) BQP-complete.…
Mark Spinelli
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What is the probability $\Pr(\|U-I\|_{\rm op}<\varepsilon)$ of a Haar-random unitary being close to the identity?
If one generates an $n\times n$ Haar random unitary $U$, then clearly $\Pr(U=I)=0$. However, for every $\epsilon>0$, the probability
$$\Pr(\|U-I\|_{\rm op}<\varepsilon)$$
should be positive.
How can this quantity be computed?
Calvin Liu
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How should different quantum computing devices be compared?
In the last years, there has been a spur of demonstrations of devices able to perform proof of principle, small-scale, non-fault-tolerant quantum computation (or Noisy Intermediate-Scale Quantum technologies, how they have been referred to).
With…
glS
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What makes quantum computations different from randomized classical computations?
One of the many thing that confuse me in the field of QC is what makes the measurement of a qubit in a quantum computer any different than just choosing at random (in a classical computer) (that's not my actual question)
Suppose I have $n$ qubits,…
ItamarG3
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State of the art in quantum memory
Presently, how much information can a quantum computer store, in how many qubits? What restrictions are there and how does it vary across realizations (efficiency of data storage, ease of reading and writing, etc)?
Young
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Is the common Computer Science usage of 'ignoring constants' useful when comparing classical computing with quantum computing?
Daniel Sank mentioned in a comment, responding to (my) opinion that the constant speed-up of $10^8$ on a problem admitting a polynomial time algorithm is meager, that
Complexity theory is way too obsessed with infinite size scaling limits. What…
Discrete lizard
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Does quantum computing already possess the level of abstraction to be explicable even without knowledge of physics?
Currently, quantum computer science (in contrast to classical computer science) can mostly only be understood if one has a good inside knowledge of physics, or more precisely quantum physics. Only then one can really understand the explanations of…
Tetragrammaton
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Where does precisely the difficulty in exponentiating a Hamiltonian $H$ in the quantum simulation problem lay?
I've read in the Nielsen's, Chuang's "Quantum Computation and Quantum Information":
Classical simulation begins with the realization that in solving a simple differential equation such as $dy/dt = f(y)$, to first order, it is known that $y(t +…
brzepkowski
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Why do the IBM and Google processors both have 53 qubits?
As I understand from this IBM post both the IBM and Google teams have independently built 53-qubit processors. What is the significance of the number 53? It is purely coincidental, or is there a deeper reason why both team have the same number of…
Randomblue
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