Quantum Field Name Request

Help request.

What field?


H=|1st 0 0 0|
|0 2nd 0 0|
|0 0 3rd 0|
|0 0 0 4th|


A basic field is given that I remember from my modern physics course,
it assigns eigenvalues as the positional operators. I forget the
origin and common name. Does anybody know the name?

It is supposed to be common and recognized.

A positional operator was supposed to represent a set. Making all
momenta as a system obey the constraint. It is not the four momenta
matrix as the ?




.