# 量子位元

## 定義

${\displaystyle |\psi \rangle =\alpha |0\rangle +\beta |1\rangle ;\quad \alpha ,\beta \in \mathbb {C} }$

${\displaystyle |\alpha |^{2}+|\beta |^{2}=1\,}$

${\displaystyle 1=\langle \psi |\psi \rangle =(\alpha |0\rangle +\beta |1\rangle )^{\dagger }(\alpha |0\rangle +\beta |1\rangle )=(\alpha ^{*}\langle 0|+\beta ^{*}\langle 1|)(\alpha |0\rangle +\beta |1\rangle )}$
${\displaystyle =\alpha ^{*}\alpha \langle 0|0\rangle +\beta ^{*}\beta \langle 1|1\rangle }$
${\displaystyle =|\alpha |^{2}+|\beta |^{2}\,}$，即要求總機率要是1。

## 按方向所採的諸多表示法

### z方向

${\displaystyle z_{-}=|1\rangle \langle 1|={\begin{pmatrix}0\\1\end{pmatrix}}*{\begin{pmatrix}0&1\end{pmatrix}}={\begin{pmatrix}0&0\\0&1\end{pmatrix}}}$

### x方向

${\displaystyle x_{-}=|x_{-}\rangle \langle x_{-}|={\begin{pmatrix}{\frac {1}{\sqrt {2}}}\\-{\frac {1}{\sqrt {2}}}\end{pmatrix}}*{\begin{pmatrix}{\frac {1}{\sqrt {2}}}&-{\frac {1}{\sqrt {2}}}\end{pmatrix}}={\begin{pmatrix}{\frac {1}{2}}&-{\frac {1}{2}}\\-{\frac {1}{2}}&{\frac {1}{2}}\end{pmatrix}}}$

### y方向

${\displaystyle y_{-}=|y_{-}\rangle \langle y_{-}|={\begin{pmatrix}{\frac {1}{\sqrt {2}}}\\-{\frac {i}{\sqrt {2}}}\end{pmatrix}}*{\begin{pmatrix}{\frac {1}{\sqrt {2}}}&{\frac {i}{\sqrt {2}}}\end{pmatrix}}={\begin{pmatrix}{\frac {1}{2}}&{\frac {i}{2}}\\-{\frac {i}{2}}&{\frac {1}{2}}\end{pmatrix}}}$

## 註釋

1. ^ MA Nielsen, IL Chuang. Quantum Computation and Quantum Information, Cambridge University Press, Cambridge (2000).

## 參考文獻

• Michael A. Nielsen, Isaac L. Chuang: Quantum Computation and Quantum Information. Cambridge University Press, Cambridge 2000, ISBN 0-521-63503-9.
• Oliver Morsch: Quantum bits and quantum secrets - how quantum physics is revolutionizing codes and computers. Wiley-VCH, Weinheim 2008, ISBN 978-3-527-40710-1.
• Anthony J. Leggett: Quantum computing and quantum bits in mesoscopic systems. Kluwer Academic, New York 2004, ISBN 0-306-47904-4.