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## Main Question or Discussion Point

Consider a qubit.

Given a state equation, how to calculate the probability of finding spin in +x direction?

Given a state equation, how to calculate the probability of finding spin in +x direction?

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- Thread starter srallaba
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Consider a qubit.

Given a state equation, how to calculate the probability of finding spin in +x direction?

Given a state equation, how to calculate the probability of finding spin in +x direction?

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$$P(\sigma_x)=\langle \sigma_x|\hat{\rho}|\sigma_x \rangle.$$

olgerm

Gold Member

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How to do it if I know spinor instead of ##\hat{\rho}##?

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$$P(\sigma_x)=\langle \sigma_x \psi \rangle \langle \psi|\sigma_x \rangle=\psi(\sigma_x) \psi^*(\sigma_x)=|\psi(\sigma_x)|^2.$$

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