Bell states can be used to quantify entanglement. Let m be the number of high-fidelity copies of a Bell state that can be produced using local operations and classical communication (LOCC). Given a large number of Bell states the amount of entanglement present in a pure state | ψ ψ ⟩ ⟩ can then be defined as the ratio of n / m , called the distillable entanglement of a particular state | ϕ ϕ ⟩ ⟩ , which gives a quantified measure of the amount of entanglement present in a given system. The process of entanglement distillation aims to saturate this limiting ratio. The number of copies of a pure state that may be converted into a maximally entangled state is equal to the von Neumann entropy S ( p ) of the state, which is an extension of the concept of classical entropy for quantum systems. Mathematically, for a given density matrix p , the von Neumann entropy S ( p ) is S ( p ) = − − T r ( p ln ⁡ ⁡ p ) . Entanglement can then be quantified as the entropy of entanglement, which is the von Neumann entropy of either p A or p B as: E = − − T r ( p A ln ⁡ ⁡ p A ) = − − T r ( p B ln ⁡ ⁡ p B ) ,