In quantum mechanics, a complete set of commuting observables (CSCO) is a set of commuting operators whose eigenvalues completely specify the state of a system.
Contents
- The Compatibility Theorem
- Discussion
- Examples of Compatible Observables
- Formal Definition of a CSCO Complete Set of Commuting Observables
- The Hydrogen Atom
- The Free Particle
- Addition of Angular Momenta
- References
Since each pair of observables in the set commutes, the observables are all compatible so that the measurement of one observable has no effect on the result of measuring another observable in the set. It is therefore not necessary to specify the order in which the different observables are measured. Measurement of the complete set of observables constitutes a complete measurement, in the sense that it projects the quantum state of the system onto a unique and known vector in the basis defined by the set of operators. That is, to prepare the completely specified state, we have to take any state arbitrarily, and then perform a succession of measurements corresponding to all the observables in the set, until it becomes a uniquely specified vector in the Hilbert space.
The Compatibility Theorem
Let us have two observables,
-
A andB are compatible observables. -
A ^ B ^ - The operators
A ^ B ^ [ A ^ , B ^ ] = 0 .
Discussion
We consider the two above observables
If the system happens to be in one of the eigenstates, say,
Examples of Compatible Observables
The Cartesian components of the position operator
Formal Definition of a CSCO (Complete Set of Commuting Observables)
A set of observables
- All the observables commute in pairs.
- If we specify the eigenvalues of all the operators in the CSCO, we identify a unique eigenvector in the Hilbert space of the system.
If we are given a CSCO, we can choose a basis for the space of states made of common eigenvectors of the corresponding operators. We can uniquely identify each eigenvector by the set of eigenvalues it corresponds to.
Discussion
Let us have an operator
However, if some of the eigenvalues of
It may so happen, nonetheless, that the degeneracy is not completely lifted. That is, there exists at least one pair
The same vector space may have distinct complete sets of commuting operators.
Suppose we are given a finite CSCO
where
If we measure
For a complete set of commuting operators, we can find a unique unitary transformation which will simultaneously diagonalize all of them. If there are more than one such unitary transformations, then we can say that the set is not yet complete.
The Hydrogen Atom
Two components of the angular momentum operator
So, any CSCO cannot involve more than one component of
Also, the Hamiltonian
Therefore a commuting set consists of
That is, the set of eigenvalues
The Free Particle
For a free particle, the Hamiltonian is
Again, let
The degeneracy in
So,
Addition of Angular Momenta
We consider the case of two systems, 1 and 2, with respective angular momentum operators
Then the basis states of the complete system are
Therefore, for the complete system, the set of eigenvalues