The Research On Several Problems Of Entangled System Without Detection Loophole

The emergence of quantum mechanics has opened up new directions in physical researches,however,Albert Einstein and other scientists raised doubts about the completeness of quantum mechanics,and proposed the EPR paradox in 1935.It was not until nearly three decades later that physicist John Bell proposed the famous Bell inequality which denying the view of local realism.Many loopholes have been introduced into experiment because of the imperfect apparatus.Since then,scientists have made great efforts to close the locality loophole and detection loophole in the experiment,and deal with the freedom-of-choice loophole by means of the randomness in cosmic photons and human free will,and the persuasive loophole-free Bell test was finally achieved.Quantum has also emerged in many research fields because of its unique intrinsic properties.In this thesis,we mainly discuss three works based on the loopholefree Bell test-distributed quantum sensing,loophole-free Hardy’s paradox,and deviceindependent quantum randomness expansion:Quantum metrology can achieve better sensitivity compared to classical strategies.Its sensitivity can violate shot-noise limit and even saturate the Heisenberg limit by utilizing the quantum resources,such as entanglement,squeezing,etc.Distributed quantum metrology is widely used in many scenarios,for instance,quantum imaging and clock synchronization,which are based on the research model of distributed sensing networks.By utilizing entanglement among all sensors,the overall information of the system could be obtained.By constructing a high-efficiency system with two sensors on the basis of loophole-free Bell test apparatus,we realized the distributed quantum sensing in field for the first time.In discrete-variable quantum metrology,we achieved the unconditional violation of the shot-noise limit.This work is of great significance to the practical development of quantum sensing networks.The Bell test mainly perform statistics on measurement results with inequality,while Hardy’s paradox is an "all-versus-nothing" proof of quantum nonlocality.The original theoretical proposals of the paradox are based on the ideal quantum states.Due to the imperfect condition with errors and noises in the experiment,it is necessary to avoid them by statistical methods.Under the condition of original paradox,we have added the factor of system efficiency into the theoretical framework,so as to obtain the optimal state and measurement angles corresponding to the maximum successful probabilities with the different system efficiencies.The quantum nonlocality is guaranteed by Bell inequality while the paradox’s constraints are strictly satisfied.The feasibility of this experiment is proved by schematic design and theoretical simulation of the loophole-free Hardy-type paradox.Device-independent quantum random number is one of the most practical applications of quantum theory,that is,people do not need to trust the equipment they use and understand the internal operating principles,and the security of output could be guaranteed by a Bell test.If the consumed randomness is less than the generated output,than randomness expansion would be realized.We developed a high-efficiency system based on the Bell test,and achieved a net gain of 2.57×108 bits after about 19.2 hours of data collection,which successfully realized randomness expansion against quantum side information.This work promotes the development of practical application of quantum randomness,and is expected to be widely popularized and applied in the near future.The research of the Bell test proves quantum nonlocality while classical physics doesn’t possess this character.At the same time,as a rigorous measurement standard,loophole-free Bell test proves the existence of quantum nonlocality which are widely applied in many research scenarios,and promotes the development of quantum information field.