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狄拉克符号 .
狄拉克符号 或狄拉克标记 (英语:Dirac notation )是量子力学 中广泛应用于描述量子态 的一套标准符号系统。在这套系统中,每一个量子态 都被描述为希尔伯特空间 中的态矢量 ,定义为右矢 (ket ):
|
ψ
⟩
{\displaystyle |\psi \rangle }
;每一个右矢的共轭转置 定义为其左矢 (bra );换一种说法,右矢的厄米共轭 (即取转置 运算加上共轭复数 运算),就可以得到左矢。
此标记法为狄拉克 于1939年将“bracket”(括号)这个词拆开后所造的。[1] 在中国方面,一些旧有的教科书和文献中也将其译为“刁矢”和“刃矢”、或“彳矢”和“亍矢”,现已弃用。
右矢与左矢可分别用N×1阶和1×N阶矩阵 表示为:
|
ψ
⟩
=
(
ψ
1
ψ
2
ψ
3
ψ
4
⋮
ψ
N
)
{\displaystyle |\psi \rangle ={\begin{pmatrix}\psi _{1}\\\psi _{2}\\\psi _{3}\\\psi _{4}\\\vdots \\\psi _{N}\\\end{pmatrix))}
⟨
ψ
|
=
(
ψ
1
∗
,
ψ
2
∗
,
ψ
3
∗
,
ψ
4
∗
,
⋯
,
ψ
N
∗
)
{\displaystyle \langle \psi |={\begin{pmatrix}\psi _{1}^{*},&\psi _{2}^{*},&\psi _{3}^{*},&\psi _{4}^{*},&\cdots ,&\psi _{N}^{*}\end{pmatrix))}
不同的两个态矢量的内积 则由一个括号来表示:
⟨
ϕ
|
ψ
⟩
{\displaystyle \langle \phi |\psi \rangle }
,当狄拉克符号作用于两个基矢时,所得值为:
⟨
e
i
|
e
j
⟩
=
δ
i
j
{\displaystyle \langle e_{i}|e_{j}\rangle =\delta _{ij))
(
δ
i
j
{\displaystyle \delta _{ij))
为克罗内克函数 )
相同的态矢量内积为:
⟨
ψ
|
ψ
⟩
=
∑
i
|
ψ
i
|
2
{\displaystyle \langle \psi |\psi \rangle =\sum _{i}|\psi _{i}|^{2))
。
因为每个右矢是复 希尔伯特空间 中的一个矢量 ,而每个右矢-左矢关系是内积 ,而直接地可以得到如下的操作方式:
给定任何左矢
⟨
ϕ
|
{\displaystyle \langle \phi |}
、右矢
|
ψ
1
⟩
{\displaystyle |\psi _{1}\rangle }
以及
|
ψ
2
⟩
{\displaystyle |\psi _{2}\rangle }
,还有复数 c 1 及c 2 ,则既然左矢是线性泛函 ,根据线性泛函的加法与标量乘法的定义,
⟨
ϕ
|
(
c
1
|
ψ
1
⟩
+
c
2
|
ψ
2
⟩
)
=
c
1
⟨
ϕ
|
ψ
1
⟩
+
c
2
⟨
ϕ
|
ψ
2
⟩
{\displaystyle \langle \phi |\;{\bigg (}c_{1}|\psi _{1}\rangle +c_{2}|\psi _{2}\rangle {\bigg )}=c_{1}\langle \phi |\psi _{1}\rangle +c_{2}\langle \phi |\psi _{2}\rangle }
。给定任何右矢
|
ψ
⟩
{\displaystyle |\psi \rangle }
、左矢
⟨
ϕ
1
|
{\displaystyle \langle \phi _{1}|}
以及
⟨
ϕ
2
|
{\displaystyle \langle \phi _{2}|}
,还有复数c 1 及c 2 ,则既然右矢是线性泛函 ,
(
c
1
⟨
ϕ
1
|
+
c
2
⟨
ϕ
2
|
)
|
ψ
⟩
=
c
1
⟨
ϕ
1
|
ψ
⟩
+
c
2
⟨
ϕ
2
|
ψ
⟩
{\displaystyle {\bigg (}c_{1}\langle \phi _{1}|+c_{2}\langle \phi _{2}|{\bigg )}\;|\psi \rangle =c_{1}\langle \phi _{1}|\psi \rangle +c_{2}\langle \phi _{2}|\psi \rangle }
。给定任何右矢
|
ψ
1
⟩
{\displaystyle |\psi _{1}\rangle }
及
|
ψ
2
⟩
{\displaystyle |\psi _{2}\rangle }
,还有复数c 1 及c 2 ,根据内积的性质(其中c*代表c的复数共轭 ),
c
1
|
ψ
1
⟩
+
c
2
|
ψ
2
⟩
{\displaystyle c_{1}|\psi _{1}\rangle +c_{2}|\psi _{2}\rangle }
与
c
1
∗
⟨
ψ
1
|
+
c
2
∗
⟨
ψ
2
|
{\displaystyle c_{1}^{*}\langle \psi _{1}|+c_{2}^{*}\langle \psi _{2}|}
对偶。给定任何左矢
⟨
ϕ
|
{\displaystyle \langle \phi |}
及右矢
|
ψ
⟩
{\displaystyle |\psi \rangle }
,内积的一个公理 性质指出
⟨
ϕ
|
ψ
⟩
=
⟨
ψ
|
ϕ
⟩
∗
{\displaystyle \langle \phi |\psi \rangle =\langle \psi |\phi \rangle ^{*))
。给定任何算符
X
{\displaystyle X}
、左矢
⟨
ϕ
|
{\displaystyle \langle \phi |}
及右矢
|
ψ
⟩
{\displaystyle |\psi \rangle }
,它们之间的合法相乘满足乘法结合公理 ,例如,[2] :16-17
(
|
ω
⟩
⟨
ϕ
|
)
|
ψ
⟩
=
|
ω
⟩
(
⟨
ϕ
|
ψ
⟩
)
{\displaystyle (|\omega \rangle \langle \phi |)\ |\psi \rangle =|\omega \rangle \ (\langle \phi |\psi \rangle )}
、
⟨
ϕ
|
(
X
|
ψ
⟩
)
=
(
⟨
ϕ
|
X
)
|
ψ
⟩
{\displaystyle \langle \phi |\ (X|\psi \rangle )=(\langle \phi |X)\ |\psi \rangle }
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