A _[squeezed](squeezed%20states.md)_ vacuum state is obtainable since a quantum vacuum isn't a vacuum in a literal sense. The squeezed vacuum may be represented as a [Quantum superposition](Quantum%20superposition.md) of [vacuum Fock states](The%20vacuum%20state.md#Fock%20state%20model) given as $|\xi\rangle_0=\sum_{n=0}^{\infty}a_n |0\rangle$ #QuantumMechanics/StationaryStateQuantumSystems #QuantumMechanics/QuantumOptics