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==See also== |
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*[[Harmonic oscillator]] |
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*[[Quantum harmonic oscillator]] |
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*[[Second quantization]] |
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*[[Quantum field theory]] |
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*[[Thermodynamics]] |
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*[[Thermodynamics]] |
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*[[Fermion number operator]] |
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*[[Fermion number operator]] |
Revision as of 17:23, 25 April 2015
In quantum mechanics, for systems where the total number of particles
composed of single-particle basis states
:
![{\displaystyle |\Psi \rangle _{\nu }=|\phi _{1},\phi _{2},\cdots ,\phi _{n}\rangle _{\nu }}](https://wikimedia.org/api/rest_v1/media/math/render/svg/6b0ff9891622072b0ce8bfd3e50d8f4b64247d22)
with creation and annihilation operators
and
we define the number operator
and we have:
![{\displaystyle {\hat {N_{i}}}|\Psi \rangle _{\nu }=N_{i}|\Psi \rangle _{\nu }}](https://wikimedia.org/api/rest_v1/media/math/render/svg/e5dacc7e23cd9d056e35b4dfebaa4b7b17af0204)
where
is the number of particles in state
. The above equality can be proven by noting that
![{\displaystyle {\begin{matrix}a(\phi _{i})|\phi _{1},\phi _{2},\cdots ,\phi _{i-1},\phi _{i},\phi _{i+1},\cdots ,\phi _{n}\rangle _{\nu }&=&{\sqrt {N_{i}}}|\phi _{1},\phi _{2},\cdots ,\phi _{i-1},\phi _{i+1},\cdots ,\phi _{n}\rangle _{\nu }\\a^{\dagger }(\phi _{i})|\phi _{1},\phi _{2},\cdots ,\phi _{i-1},\phi _{i+1},\cdots ,\phi _{n}\rangle _{\nu }&=&{\sqrt {N_{i}}}|\phi _{1},\phi _{2},\cdots ,\phi _{i-1},\phi _{i},\phi _{i+1},\cdots ,\phi _{n}\rangle _{\nu }\end{matrix}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/f0d384eb190f81a040dea5c10578ab02c35e38de)
then
![{\displaystyle {\begin{matrix}{\hat {N_{i}}}|\Psi \rangle _{\nu }=a^{\dagger }(\phi _{i})a(\phi _{i})|\phi _{1},\phi _{2},\cdots ,\phi _{i-1},\phi _{i},\phi _{i+1},\cdots ,\phi _{n}\rangle _{\nu }&=&{\sqrt {N_{i}}}a^{\dagger }(\phi _{i})|\phi _{1},\phi _{2},\cdots ,\phi _{i-1},\phi _{i+1},\cdots ,\phi _{n}\rangle _{\nu }\\&=&{\sqrt {N_{i}}}{\sqrt {N_{i}}}|\phi _{1},\phi _{2},\cdots ,\phi _{i-1},\phi _{i},\phi _{i+1},\cdots ,\phi _{n}\rangle _{\nu }\\&=&N_{i}|\Psi \rangle _{\nu }\\\end{matrix}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/c98310890f4659f69de8c7367a30cdba87a8ff5d)
See also
References