A Large-alphabet Quantum Key Distribution Protocol Using Orbital Angular Momentum Entanglement

We experimentally demonstrate a quantum key distribution protocol using entangled photon pairs in orbital angular momentum (OAM). Here Alice uses a fixed phase hologram to modulate her OAM state on one photon with a spatial light modulator (SLM), while Bob uses the designed N different phase holograms for his N-based keys on the other photon with his SLM. With coincidences, Alice can fully retrieve the keys sent by Bob without reconciliation. We report the experiment results with N=3 and OAM eigenmodes |?=±1>, and discuss the security from the light path and typical attacks.