Purification Of Logic Bell States And Measurement Device-independent Quantum Secure Direct Communication

After the efforts of a large number of talented scientists in the early 20^th century,the basic framework of quantum mechanics has been completed and people have achieved very successful applications in the reality.In 1984,Bennet et al proposed the quantum key distribution?QKD?which can realize unconditionally secure distribution of key based on the basic principle of quantum mechanics.Based on the BB84 protocol,after more than two decades of development,quantum communication has gradually been accepted and has become a research hotspot.There are two very important branches of quantum information.One is the QKD for the purpose of distributing keys and the other is quantum secure direct communication?QSDC?for the purpose of directly transmitting information.However,both QKD and QSDC will face two problems in the practical application.One is the noise of channel in transmission and the other is the security of communication when the actual devices are defective.Therefore,entanglement purification and measurement device-independent?MDI?quantum communication have been proposed for these two problems.One the one hand,the quantum state will degrade from the pure state to the mixed state because of the quantum channel noise.The mixed state will not only affect the quality of quantum communication but also threaten the security of quantum communication.In 1996,Bennet proposed the model of entanglement purification to solve this problem.Later,people used the entangled states in the logic to decrease the noise because this quantum state itself has good resistance to noise after being logically encoded.The concatenated Greenberger-Horne-Zeilinger?C-GHZ?state is a very common entangled states in the logic.However,during the transmission of this logical quantum state,the bit-flip and phase-flip errors in the logical level can also occur.In this case,these errors cannot be solved by error correction.Therefore,the first work of this paper is to propose the entanglement purification protocol of logic quantum states with linear optics,which is feasible in the current condition.On the other hand,in order to defeat all the attacks in the detector's side in the practical application of QSDC,the MDI-QSDC protocol has been proposed by Zhou and Sheng in 2018.However,the original MDI-QSDC protocol encodes only in a single degree of freedom of the photon,so the channel capacity of this protocol is not too high.The second work of this paper is to achieve high-capacity MDI-QSDC through hyper-encoding with multiple degrees of freedom for a single photon,and its channel capacity can be doubled.Moreover,this paper also analyzes the security of two-way deterministic quantum key distribution?TD-QKD?under the faked-states attack.In this paper,we find that TD-QKD is secure under the faked-states attack,while the attack is efficient on traditional BB84 protocol.