We put forward the concept of quantum spiral bandwidth of the spatial mode function of the two-photon entangled state generated in spontaneous parametric down-conversion. We obtain the bandwidth using the eigenstates of the orbital angular momentum of the biphoton states, and reveal its dependence with the length of the down-converting crystals and waist of the pump beam. The connection between the quantum spiral bandwidth and the entropy of entanglement of the quantum state is discussed.
Juan P. Torres, Alexandrescu, A., Torner, L. Quantum spiral bandwidth of entangled two-photon states. "Physical review A", Novembre 2003, vol. 68, núm. 5, p. 50301-50304.