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author | Prefetch | 2022-10-14 23:25:28 +0200 |
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committer | Prefetch | 2022-10-14 23:25:28 +0200 |
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diff --git a/source/know/concept/bell-state/index.md b/source/know/concept/bell-state/index.md new file mode 100644 index 0000000..15b916d --- /dev/null +++ b/source/know/concept/bell-state/index.md @@ -0,0 +1,93 @@ +--- +title: "Bell state" +date: 2021-03-09 +categories: +- Quantum mechanics +- Quantum information +layout: "concept" +--- + +In quantum information, the **Bell states** are a set of four two-qubit states +which are simple and useful examples of [quantum entanglement](/know/concept/quantum-entanglement/). +They are given by: + +$$\begin{aligned} + \boxed{ + \begin{aligned} + \ket{\Phi^{\pm}} + &= \frac{1}{\sqrt{2}} \Big( \Ket{0}_A \Ket{0}_B \pm \Ket{1}_A \Ket{1}_B \Big) + \\ + \ket{\Psi^{\pm}} + &= \frac{1}{\sqrt{2}} \Big( \Ket{0}_A \Ket{1}_B \pm \Ket{1}_A \Ket{0}_B \Big) + \end{aligned} + } +\end{aligned}$$ + +Where e.g. $\Ket{0}_A \Ket{1}_B = \Ket{0}_A \otimes \Ket{1}_B$ +is the tensor product of qubit $A$ in state $\Ket{0}$ and $B$ in $\Ket{1}$. +These states form an orthonormal basis for the two-qubit +[Hilbert space](/know/concept/hilbert-space/). + +More importantly, however, +is that the Bell states are maximally entangled, +which we prove here for $\ket{\Phi^{+}}$. +Consider the following pure [density operator](/know/concept/density-operator/): + +$$\begin{aligned} + \hat{\rho} + = \ket{\Phi^{+}} \bra{\Phi^{+}} + &= \frac{1}{2} \Big( \Ket{0}_A \Ket{0}_B + \Ket{1}_A \Ket{1}_B \Big) \Big( \Bra{0}_A \Bra{0}_B + \Bra{1}_A \Bra{1}_B \Big) +\end{aligned}$$ + +The reduced density operator $\hat{\rho}_A$ of qubit $A$ is then calculated as follows: + +$$\begin{aligned} + \hat{\rho}_A + &= \Tr_B(\hat{\rho}) + = \sum_{b = 0, 1} \Bra{b}_B \Big( \ket{\Phi^{+}} \bra{\Phi^{+}} \Big) \Ket{b}_B + \\ + &= \sum_{b = 0, 1} \Big( \Ket{0}_A \Inprod{b}{0}_B + \Ket{1}_A \Inprod{b}{1}_B \Big) + \Big( \Bra{0}_A \Inprod{0}{b}_B + \Bra{1}_A \Inprod{1}{b}_B \Big) + \\ + &= \frac{1}{2} \Big( \Ket{0}_A \Bra{0}_A + \Ket{1}_A \Bra{1}_A \Big) + = \frac{1}{2} \hat{I} +\end{aligned}$$ + +This result is maximally mixed, therefore $\ket{\Phi^{+}}$ is maximally entangled. +The same holds for the other three Bell states, +and is equally true for qubit $B$. + +This means that a measurement of qubit $A$ +has a 50-50 chance to yield $\Ket{0}$ or $\Ket{1}$. +However, due to the entanglement, +measuring $A$ also has consequences for qubit $B$: + +$$\begin{aligned} + \big| \Bra{0}_A \! \Bra{0}_B \cdot \ket{\Phi^{+}} \big|^2 + &= \frac{1}{2} \Big( \Inprod{0}{0}_A \Inprod{0}{0}_B + \Inprod{0}{1}_A \Inprod{0}{1}_B \Big)^2 + = \frac{1}{2} + \\ + \big| \Bra{0}_A \! \Bra{1}_B \cdot \ket{\Phi^{+}} \big|^2 + &= \frac{1}{2} \Big( \Inprod{0}{0}_A \Inprod{1}{0}_B + \Inprod{0}{1}_A \Inprod{1}{1}_B \Big)^2 + = 0 + \\ + \big| \Bra{1}_A \! \Bra{0}_B \cdot \ket{\Phi^{+}} \big|^2 + &= \frac{1}{2} \Big( \Inprod{1}{0}_A \Inprod{0}{0}_B + \Inprod{1}{1}_A \Inprod{0}{1}_B \Big)^2 + = 0 + \\ + \big| \Bra{1}_A \! \Bra{1}_B \cdot \ket{\Phi^{+}} \big|^2 + &= \frac{1}{2} \Big( \Inprod{1}{0}_A \Inprod{1}{0}_B + \Inprod{1}{1}_A \Inprod{1}{1}_B \Big)^2 + = \frac{1}{2} +\end{aligned}$$ + +As an example, if $A$ collapses into $\Ket{0}$ due to a measurement, +then $B$ instantly also collapses into $\Ket{0}$, never $\Ket{1}$, +even if it was not measured. +This was a specific example for $\ket{\Phi^{+}}$, +but analogous results can be found for the other Bell states. + + +## References +1. J.B. Brask, + *Quantum information: lecture notes*, + 2021, unpublished. |