Particle number operator
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In quantum mechanics, for systems where the total number of particles may not be preserved, the number operator is the observable that counts the number of particles.
The number operator acts on Fock space. Given a Fock state 
composed of single-particle basis states 
:
with creation and annihilation operators 
and a(φi) we define the number operator 
and we have:
where ni is the number of particles in state .
[edit] See also
[edit] References
- Bruus, Henrik, Flensberg, Karsten. (2004). Many-body Quantum Theory in Condensed Matter Physics: An Introduction. Oxford University Press. ISBN 0-19-856633-6.
- Second quantization notes by Fradkin
