# Questions tagged [quantum-gate]

For questions regarding usage, performance, implementation, application or theory related to quantum gates.

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### Measuring vs gate - what is the relation?

I am reading some introductory quantum mechanics and I don't understand the connection between an observable and a gate. I thought a gate just applies a rotation to a state while a measurement gives ...
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1 vote
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### Mathematical expression of controlled Ry gate

"How can I describe the controlled Ry gate transformation on the ground state qubit using a mathematical formula similar to the one below, which is for the CNOT transformation?" The example ...
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### Why is the controlled-Z gate a symmetric gate?

I want to understand why this gate is symmetric. Consider a two qubits circuit (q1, q2) with a gate CZ controlled by the first q1, then if q1 is in the state $(a, b)^T$ and q2 is the state $(c, d)^T$, ...
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### unitary that transform $\sigma^x \pm \alpha \sigma^z$ into $\sigma^x$ and $\sigma^z$

Consider $\sigma^x \pm \sigma^z$, where $\sigma^x$ and $\sigma^z$ are Pauli $X$ and $Z$ matrices. Let unitary $U$ be a $\pi/4$ rotation matrix around $Y$-axis. Then, U(\sigma^x + \...
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### Is every code with a universal set of transversal gates trivial?

The quantum repetition code is an $[[n,1,1]]$ stabilizer code with stabilizer generators $Z_iZ_{i+1}$ for $i=1, \dots, n-1$. The Eastin-Knill theorem states that a $d >1$ code cannot have a ...
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