Experimental Research On Quantum Key Distribution Based On Photon Polarization And Orbital Angular Momentum

In the past two decades,with the development of information technology,the amount of information has grown exponentially.Currently,the widely used classic encryption technology provides a tolerable level of security for data in everyday life.However,these techniques are based on mathematical complexity that has never been proven to be safe.Significant advances in mathematics,such as finding backdoors,or major breakthroughs in newer computers,such as quantum computers,may make existing classic encryption techniques obsolete in a shorter period of time.The quantum key distribution(QKD)technology can share random numbers,that is,a security key,in authenticated users.This technique is based on physics and is theoretically proven to be safe.Based on this technology,coupled with the one-time technology that has been proven to be absolutely safe,unconditionally secure quantum communication can be realized.In addition,the technology can detect the presence of a side eavesdropper by means of a bit error rate.In 1984,Bennet and Brassard pro-posed the first quantum key distribution protocol,the BB84 protocol.After 35 years of development,quantum key distribution was realized from the first free space channel with only 32 cm,and the current 7,600 km intercontinental quantum key distribution between China and Austria was realized.In the Fibre Channel,the 2,000-kilometer-long Beijing-Shanghai trunk line from Beijing to Shanghai was completed.Nowadays,quantum secure communication technology with quantum key distribution technology as its core has moved from laboratory demonstration to practicalization and industrial-ization.Therefore,under the premise of ensuring security,it is also our goal to propose a more robust and low-cost quantum key distribution system.This paper summarizes the my work in the quantum key distribution experiments based on photon polarization and orbital angular momentum during doctoral study,mainly including the following aspects:First,polarization encoding is one of the most common ways to realize QKD en-coding.There are multiple light source schemes,a single light source based on an interferometer scheme,and a scheme of directly modulating a polarization controller.Aiming at the shortcomings of existing security schemes and low polarization extinctionratio,we propose a phase modulation polarization codec scheme based on Sagnac inter-ferometer.Not only does it make up for the safety loopholes of multiple light sources,its self-compensation in the polarization preparation and detection process also improves the robustness of the transmitting and receiving optical devices.Based on this Sagnac-interferometer,we realized the fiber based BB84 protocol and experimentally proved its better polarization extinction ratio and stability.Second,orbital angular momentum(OAM)has received increasing attention in re-cent years.When dealing with OAM beams,existing optical components change the OAM quantum number of incident light,such as an adjustable beam splitter.We pro-pose a beam splitter with polarization-independent OAM-independent adjustable beam splitting ratio,and experimentally verify its polarization independence and OAM inde-pendence.Using this tunable beam splitter,we built the Sagnac interferometer.The ex-cellent interference visibility in the experiment proves its potential application in quan-tum information processing or quantum key distribution based on OAM.Third,most of the existing QKD systems require a calibration reference system.In systems where the reference frame changes dramatically,such as a rotating satellite communicating with a ground receiving station,a large amount of resources are required to align the reference frame.A reference-frame-independent(RFI)QKD protocol can solve this problem.However,existing RFI-QKD protocols have the disadvantage of being loss sensitive and capable of only weak drifting to the reference frame.Based on the concept of rotating invariant photon state and the demonstration experiment,We de-signed and experimentlly implemented the BB84 system for combined polarization and orbital angular momentum using a fiber-optic Sagnac interferometer.The experimental results show the stability of our system and the effectiveness of the rotating reference frame.Fourth,in the process of achieving the goal of all-weather QKD,daylight QKD is a technical difficulty to break through.We use the methods of time filtering,frequency filtering and spatial filtering to implement the decoy BB84 protocol in the free space field channel.We continued our system for a week and the results proved the reliabil-ity.Due to the influence of atmospheric turbulence,the OAM beam will increase its inter-mode crosstalk when it propagates in free space,thus affecting the performance of optical communication based on OAM.We propose an adaptive optics scheme based on fast mirror and four-quadrant detector to attenuate the influence of atmospheric tur-bulence on the OAM beam.Based on this adaptive optics system,the free space field RFI-decoy-BB84 protocol is experimentally realized.The relatively low bit error rate and relatively high security key rate prove the application prospect of our system in free space field channel.