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.