High-capacity measurement-device-independent quantum secure direct communication

As an important branch of quantum communication, quantum secure direct communication (QSDC) can directly transmit secure information between two communication parties without keys. Measurement-device-independent quantum secure direct communication (MDI–QSDC) is immune to all the attacks at detector’s side and thus can eliminate the security loopholes associated with measurement devices. Existing MDI–QSDC protocols encode secure information in only one degree of freedom (DOF) of photons. In this paper, we propose a high-capacity MDI–QSDC protocol by encoding secure information in photons’ polarization and spatial-mode DOFs. With hyper-encoding in two DOFs, we can double photons’ channel capacity and effectively increase MDI–QSDC’s communication efficiency. Moreover, for each encoded qudit, the states in two DOFs are totally independent. If a detection failure or error occurs on one DOF, the state in the other DOF can still be used to transmit secure information. This hyper-encoding MDI–QSDC may have potential applications in future quantum communication.