##  [Fock Eigenstate](/fock-eigenstate-0) 

 Definition

A mathematical structure used to represent quantum states and the operations performed on them. It specifies how states are expressed, related, combined, and decomposed into components that support calculation. It does not by itself determine physical predictions without a mapping to observables and measurement rules. It enables precise computation of probabilities and expectation values from state and operator inputs. The concept is generally stable, though notation choices and computational methods evolve over time.



 

 

 

 

 

 





## Principle

Principle

Simultaneous eigenstates of commuting number operators provide a discrete orthonormal basis in Fock space; they are generated by applying creation operators to the vacuum in the appropriate symmetrized or antisymmetrized order.

 

 

 

 

 





## Demonstration

Demonstration

The single-mode photon number state |n> satisfies N|n> = n|n>, and multi-mode states |n1,n2> satisfy N1|n1,n2>=n1|n1,n2> etc.; in fermionic modes nk ∈ {0,1} due to the Pauli principle.

 

 

 

 

## Misapplication

Misapplication

Calling a state a Fock eigenstate when it is a superposition of different occupation numbers (for example, a coherent or squeezed state) or ignoring superselection rules that forbid certain superpositions.

 

 

 

 

 





## Consequence

Consequence

Fock eigenstates have well-defined integer particle counts per mode, are orthogonal and form a countable basis for expanding arbitrary states in the chosen Fock space, enabling number-resolved measurements and bookkeeping.

 

 

 

 

## Reversal

Reversal

A generic quantum state is a linear combination of Fock eigenstates and typically does not have definite occupation numbers; this inversion highlights complementary descriptions such as phase-coherent states.

 

 

 

 

 





## Boundary

Boundary

Defined relative to a specified mode decomposition and particle statistics; they do not capture states with indefinite particle number or excitations best described by non-Fock degrees of freedom (e.g., certain collective or topological excitations).

 

 

 

 

 





## Semantic Tension

Semantic Tension

Highlights the number–phase complementarity: Fock eigenstates minimize uncertainty in number at the cost of maximal phase indeterminacy, competing with representations that emphasize phase or coherence.

 

 

 

 

 





## Synthesis

Synthesis

A Fock eigenstate is the occupation-number basis element of a chosen mode decomposition, providing a discrete, statistically-constrained label set that is central for counting observables and many-body expansions.