求基态量子谐振子的⟨x⟩、⟨x2⟩、⟨px⟩和⟨px2⟩,然后确定位置和动量的测不准量。位置和动量的测不准量之积是否与海森堡测不准原理一致?
海森堡测不准原理
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基态量子谐振子的波函数为
利用此波函数,可以计算平均位置和位置平方的平均值。
平均位置
使用
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位置的平均平方
使用
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使用 以及
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位置的不确定性
动量的平均值
使用
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动量的平均平方
= ( α π ) 1 2 ( − 1 ) ℏ 2 ∫ − ∞ ∞ e − α x 2 2 ( ∂ ∂ x ) ( − α x e − α x 2 2 ) d x {\displaystyle \qquad \qquad \qquad \qquad \qquad \qquad \qquad ={\Bigl (}{\frac {\alpha }{\pi }}{\Bigr )}^{\frac {1}{2}}(-1)\hbar ^{2}\int _{-\infty }^{\infty }e^{\frac {-\alpha x^{2}}{2}}{\Biggl (}{\frac {\partial }{\partial x}}{\Biggr )}\ {\biggl (}-\alpha xe^{\frac {-\alpha x^{2}}{2}}{\biggl )}\ dx}
= α ( α π ) 1 2 ℏ 2 ∫ − ∞ ∞ e − α x 2 2 ( e − α x 2 2 + α x 2 e − 2 α x 2 2 ) d x {\displaystyle \qquad \qquad \qquad \qquad \qquad \qquad \qquad =\alpha {\Bigl (}{\frac {\alpha }{\pi }}{\Bigr )}^{\frac {1}{2}}\hbar ^{2}\int _{-\infty }^{\infty }e^{\frac {-\alpha x^{2}}{2}}{\biggl (}e^{\frac {-\alpha x^{2}}{2}}+\alpha x^{2}e^{\frac {-2\alpha x^{2}}{2}}{\biggl )}\ dx}
= α 3 2 π ℏ 2 ∫ − ∞ ∞ e − α x 2 d x ∫ − ∞ ∞ e − α x 2 2 α x 2 e − 2 α x 2 2 d x {\displaystyle \qquad \qquad \qquad \qquad \qquad \qquad \qquad ={\frac {\alpha ^{\frac {3}{2}}}{\sqrt {\pi }}}\ \hbar ^{2}\int _{-\infty }^{\infty }e^{-\alpha x^{2}}\ dx\int _{-\infty }^{\infty }e^{\frac {-\alpha x^{2}}{2}}\ \alpha x^{2}e^{\frac {-2\alpha x^{2}}{2}}\ dx}
使用 ∫ − ∞ ∞ e − α x 2 d x = π α {\displaystyle \int _{-\infty }^{\infty }e^{-\alpha x^{2}}\ dx={\sqrt {\frac {\pi }{\alpha }}}}
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= α 3 2 π ℏ 2 ( π α + α ∫ − ∞ ∞ x 2 e − α x 2 d x ) {\displaystyle \qquad \qquad \qquad \qquad \qquad \qquad \qquad ={\frac {\alpha ^{\frac {3}{2}}}{\sqrt {\pi }}}\ \hbar ^{2}{\biggl (}{\sqrt {\frac {\pi }{\alpha }}}+\alpha \int _{-\infty }^{\infty }x^{2}e^{-\alpha x^{2}}\ dx{\biggl )}}
使用 ∫ x 2 e − α x 2 d x = π erf ( α x ) 4 α 3 2 − x e − α x 2 2 α + C {\displaystyle \int x^{2}e^{-\alpha x^{2}}\ dx={\frac {{\sqrt {\pi }}{\text{erf}}({\sqrt {\alpha }}x)}{4\alpha ^{\frac {3}{2}}}}\ -\ {\frac {xe^{-\alpha x^{2}}}{2\alpha }}+C}
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= α 3 2 π ℏ 2 ( π α + α [ π erf ( α x ) 4 α 3 2 − x e − α x 2 2 α ] − ∞ ∞ ) {\displaystyle \qquad \qquad \qquad \qquad \qquad \qquad \qquad ={\frac {\alpha ^{\frac {3}{2}}}{\sqrt {\pi }}}\ \hbar ^{2}{\Biggl (}{\sqrt {\frac {\pi }{\alpha }}}+\alpha {\biggl [}{\frac {{\sqrt {\pi }}\ {\text{erf}}({\sqrt {\alpha }}\ x)}{4\alpha ^{\frac {3}{2}}}}-\ {\frac {xe^{-\alpha x^{2}}}{2\alpha }}{\biggl ]}_{-\infty }^{\infty }{\Biggl )}}
= α 3 2 π ℏ 2 ( π α + α [ π 4 α 3 2 erf ( α ∞ ) lim x → ∞ − x e − α x 2 2 α <
使用 lim x → ∞ erf ( x ) = 1 {\displaystyle \lim _{x\to \infty }{\text{erf}}(x)=1} 以及 lim x → − ∞ erf ( x ) = − 1 {\displaystyle \lim _{x\to -\infty }{\text{erf}}(x)=-1}
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= α 3 2 π ℏ 2 ( π α + α [ π 4 α 3 2 ( 1 ) − ( 0 ) ] − [ π 4 α 3 2 ( − 1 ) − ( 0 ) ] ) {\displaystyle \qquad \qquad \qquad \qquad \qquad \qquad \qquad ={\frac {\alpha ^{\frac {3}{2}}}{\sqrt {\pi }}}\ \hbar ^{2}{\Biggl (}{\sqrt {\frac {\pi }{\alpha }}}+\alpha {\Biggl [}{\frac {\sqrt {\pi }}{4\alpha ^{\frac {3}{2}}}}\ (1)-\ (0){\Biggl ]}\ -\ {\Biggl [}{\frac {\sqrt {\pi }}{4\alpha ^{\frac {3}{2}}}}\ (-1)-\ (0){\Biggl ]}{\Biggl )}}
= α 3 2 π ℏ 2 [ π α + α ( 2 π 4 α 3 2 ) ] {\displaystyle \qquad \qquad \qquad \qquad \qquad \qquad \qquad ={\frac {\alpha ^{\frac {3}{2}}}{\sqrt {\pi }}}\ \hbar ^{2}{\Biggl [}{\sqrt {\frac {\pi }{\alpha }}}+\alpha {\biggl (}{\frac {2{\sqrt {\pi }}}{4\alpha ^{\frac {3}{2}}}}\ {\biggl )}{\Biggl ]}}
= α π ℏ 2 [ π + 1 2 π ] {\displaystyle \qquad \qquad \qquad \qquad \qquad \qquad \qquad ={\frac {\alpha }{\sqrt {\pi }}}\ \hbar ^{2}{\Biggl [}{\sqrt {\pi }}+{\frac {1}{2}}{\sqrt {\pi }}{\Biggl ]}}
⟨ p x 2 ⟩ = 1 2 α ℏ 2 {\displaystyle \qquad \qquad \qquad \qquad \qquad \qquad \langle p_{x}^{2}\rangle ={\frac {1}{2}}\alpha \hbar ^{2}}
动量的误差
δ p x = ⟨ p x 2 ⟩ − ⟨ p x ⟩ 2 {\displaystyle \qquad \qquad \qquad \qquad \qquad \qquad \qquad \quad \delta p_{x}={\sqrt {\langle p_{x}^{2}\rangle -\langle p_{x}\rangle ^{2}}}}
= a ℏ 2 2 − 0 2 {\displaystyle \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad \ \ ={\sqrt {{\frac {a\hbar ^{2}}{2}}-0^{2}}}}
δ p x = a 2 2 ℏ {\displaystyle \qquad \qquad \qquad \qquad \qquad \qquad \qquad \quad \delta p_{x}={\sqrt {\frac {a^{2}}{2}}}\ \hbar }
位置误差和动量误差的乘积为
δ x ⋅ δ p x = 1 2 α ⋅ α 2 ℏ {\displaystyle \qquad \qquad \qquad \qquad \qquad \qquad \qquad \quad \delta _{x}\cdot \delta p_{x}={\frac {1}{\sqrt {2\alpha }}}\cdot {\sqrt {\frac {\alpha }{2}}}\ \hbar }
= 1 2 ℏ {\displaystyle \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad \ ={\frac {1}{2}}\ \hbar }
这等于 ℏ 2 {\displaystyle {\frac {\hbar }{2}}} ,因此,处于基态的量子谐振子与海森堡测不准原理一致。