Monday, June 17, 2019

quantum field theory - Density operator in second quantization


I would want to understand why the density operator in second quantization takes the form:


$$\rho_\sigma(\mathbf{r})=\Psi_\sigma^\dagger(\mathbf{r})\Psi_\sigma(\mathbf{r})?$$


Is this a definition or can we derive it from some formula?




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