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What does it mean for two qubits to be entangled?
Accepted answer
18 votes

For a simple example suppose you have two qubits in definite states $|0\rangle$ and $|0\rangle$. The combined state of the system is $|0\rangle\otimes |0\rangle$ or $|00\rangle$ in shorthand. Then if ...

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How does bra-ket notation work?
14 votes

This leads me to conclude that there is some difference/reason why bra-ket is especially handy for denoting quantum algorithms. There's already an accepted answer and an answer that explains 'ket', '...

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Purpose of using Fidelity in Randomised Benchmarking
6 votes

Nielsen and Chuang in their book "Quantum Computation and Quantum Information" have section (Chapter 9) on distance measures for quantum information. Surprisingly they say in Section 9.3 " How well ...

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If all quantum gates must be unitary, what about measurement?
4 votes

I'll add a small bit complementing the other answers, just about the idea of measurement. Measurement is usually taken as a postulate of quantum mechanics. There's usually some preceding postulates ...

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How to get the Bloch sphere angles given an arbitrary qubit?
Accepted answer
3 votes

As in if given $ |\psi\rangle=\alpha |0\rangle+\beta |1\rangle$, and you want it in the form $\cos(\theta/2) |0\rangle+e^{i\phi}\sin(\theta/2) |1\rangle$? Assume the state is normalised $|\alpha|^2+...

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