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1500 questions
11
votes
1 answer
What is the relationship between Choi and Chi matrix in Qiskit?
I'm struggling with the framework for quantum process tomography on Qiskit.
The final step of such a framework is running fit method of ProcessTomographyFitter class. Documentation states that such function gives a Choi matrix as output.…
Daniele Cuomo
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11
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2 answers
How many classical bits are needed to represent a qubit?
I have two question concerning information content of qubit.
Question 1: How many classical bits are needed to represent a qubit:
A qubit can be represented by a vector $q = \begin{pmatrix}\alpha \\\beta \end{pmatrix}, ~~ \alpha, \beta \in…
Martin Vesely
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10
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Is there anything that can be programmed on a classical computer but not on a quantum computer?
Would we need to create new algorithms that only work on quantum computers or would be simply edit codes in languages such as C++ to involve the new primitives from quantum computing? Are there things that can be programmed on a classical computer…
user10151
10
votes
3 answers
Does the Quantum Fourier Transform (QFT) preserve entanglement?
It is well known that entanglement in a quantum state is not affected when you perform a combination of 1-qubit unitary transformations.
I have seen that the QFT can be decomposed into product of 1-qubit unitary transformations (for example in…
Hùng Phạm
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10
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1 answer
Global phase and single qubit gate: does it actually matter for two (or more) qubit gates?
Consider the $X$ gate. Mathematically, we have $X=i e^{-i\frac{\pi}{2} X}$
But as global phase of unitaries don't matter because they will simply act a global phase to the wavefunction, we can consider implementing $X$ by $e^{-i\frac{\pi}{2} X}$,…
Marco Fellous-Asiani
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10
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1 answer
What does it mean "less than identity" in the operator sum representation?
In Quantum Computation and Quantum Information by Nielsen and Chuang, Section 8.2.3, $\mathcal{E}=\sum_{k}E_k\rho E_k^{\dagger}$ gives the operator-sum representation. In general, it requires $\sum_k E_k E_k^{\dagger}\leq I$. But, what does it mean…
czwang
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10
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3 answers
Logical qubit initialization for the surface code
I am reading Fowler et al's paper on the surface code.. I do not understand how to initialize a logical qubit in an arbitrary state with the surface code. I do understand how to initialize the qubit in logical $|{0}\rangle$ and $|{1}\rangle$, but…
Peter-Jan
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10
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What authentication protocol to use for BB84 and other QKD protocols?
Just like other classical and quantum key distribution protocols, BB84 is vulnerable to "man"-in-the-middle attacks, where Eve pretends to be Bob to Alice, and Eve pretends to be Alice to Bob. The countermeasure against this potential…
Bruno Rijsman
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10
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1 answer
Why does representation theory often arise in the context of quantum algorithms for the hidden subgroup problem?
I noticed that approaches for finding quantum algorithms the hidden subgroup problem for both Abelian groups ($(\Bbb Z_n\times \Bbb Z_n, +)$, $(\Bbb R, +)$, etc.) and non-Abelian finite groups like the dihedral group and symmetric group often use…
Sanchayan Dutta
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10
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3 answers
How to efficiently calculate the inverse of a Kronecker product?
This is a follow-up question to a previous question I had, where the correct answer was to use the Kronecker product.
Given, for example, a vector representing two qubits $$\begin{bmatrix}0 \\ 1 \\ 0 \\ 0\end{bmatrix}$$ is there an algorithm to…
1ijk
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10
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4 answers
Does a quantum computer have a clock signal and if yes how big is it?
I think there can't be a computer running software without having a clock signal.
A fast classical computer has a clock rate between 4 to 5 GHz.
If quantum computers are so much faster they must have a clock rate which is a multiple of this.
Is this…
zomega
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10
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2 answers
Is there an algorithm for determining if a given vector is separable or entangled?
I'm trying to understand if there is some sort of formula or procedural way to determine if a vector is separable or entangled, i.e., whether or not a vector of size $mn$ could be represented by the tensor product of two vectors of size $m,n$,…
A Poor
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10
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0 answers
Entanglement-assisted hashing bound for asymmetric depolarizing channels
I reading the paper EXIT-Chart Aided Quantum Code Design
Improves the Normalised Throughput
of Realistic Quantum Devices, which proposes the use of QTCs in order to do quantum error correction for realistic quantum devices whose error model can be…
Josu Etxezarreta Martinez
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10
votes
2 answers
Number of qubits to achieve quantum supremacy?
Google's Sycamore paper describes achieving quantum supremacy on a $53$-qubit quantum computer. The layout of Sycamore is $n=6\times 9=54$ nearest neighbors, with one qubit nonfunctional. They apply $m=20$ total cycles in their experiment; each…
Mark Spinelli
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10
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1 answer
Ground state energy estimation - VQE vs. Ising vs. Trotter–Suzuki
Disclaimer: I am a software engineer who is curious about quantum computing. Although I understand some basic concepts, theory and math behind it, I am by no means experienced in this domain.
I am doing some preliminary research on the state of…
Anurag Bhandari
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