Photonics Devices For Chip-based Quantum Communication

Parallel computing has achieved great development,which brings challenges for classical public-key cryptosystem.In order to meet the challenge of the quantum computer,quantum communication has been attracted much attention in recent years.More over,quantum communication is the key ingredient of quantum information theory as well as basic of quantum computation and quantum teleportation.In the 80’s,the first quantum key distribution protocol(BB84 protocol)was proposed,the field has made great progress.In China,the research of the quantum communication has been in a leading position.For example,the achievement of China B ackbone Quantum Network from Beijing to Shanghai pointed out the direction for fiber quantum networking.It is also one of the most successful applications of decoy state QKD scheme.Satellite-to-ground QKD,which was implemented between Micius satellite and Graz ground station,is the most potential construction carrier of the space-based quantum communication in the future.However,the space-based and fiber-based quantum communication system have many limitations in practical application.First,the inherently unscalable system is introduced by the large-scale optical element bolted to large optical tables.The results came across huge challenge to build up robust system and commercial system;Secondly,the initial quantum intereference experiments were carried out by employing free-space devices,the operating system is incompatible with integrated circuit;Thirdly,Spaceborne QKD system is limited by the size and quality of devices.Miniaturized QKD system can reduce the pressure of the satellite.Although,quantum commication systems based on optical fiber devices are relatively easy to integrate,cooling technology is required to generate the entangled pair generate in fiber system.More over,high-dimensional QKD is necessary to achieve low and stable quantum bit error rate(QBER)well below both the coherent attack and individual attack.The scale of QKD system will become more and larger,and the expansion of spatial optical path and all-fiber optical path will become more and more difficult.How to realize high-dimensional quantum QKD system in a limited space becomes an urgent problem to be solved.In 1969,American engineer proposed the integrated optical circuit,integrated optical platform technology has made great progress.Photonics devices are particularly suitable for building larger scale optical circuit due to small footprint,rubust performance and compatibled with CMOS techonolgy.These provide a favorable research platform for construction of high dimensional quantum communication system.However,the bottleneck of chip-based quantum communication system need to breakthrough.1、Opto-Electronic hybrid integrated technology didn’t making breakthrough;2、Source and detector are difficult to integrated on the same chip;3、It is hard to choose suitable waveguide materials;4、There is not standard criteria to design the devices of the system;5、High-speed modulation in standard photonics platforms have been limited.Previous studies have used integrated heaters to modulate the thermal-dependent refractive index,yet it remains a challenge to achieve a high bit rate via the thermo-optic effect even in a freestanding structure,due to the slowly dissipated net heat flow.In this paper,we mainly focus on how to improve the chip utilization and solve the problem of a low bit rate of modulation.The main achievements are shown as follows:1、Waveguide taper as a key device in the chip-based QKD systems,the length is 400μm～500μm.By using a flat lens,we demonstrate a grating coupler with an ultrashort taper of 22.5-μm to connect a 10-μm-wide input waveguide and a 0.5-μm-wide output waveguide,achieving a transmission up to nearly 95.4%numerically in the communication band.To our best knowledge,this work is the first demonstration of an ultrashort taper based on flat lens,which significantly improves the integration of the photonics integrated circuits,and indicates an effective solution for potential applications in compactly integrated micro/nano optical devices.2.Due to high effective refraction index,photonic integrated devices based on SOI nanowires are usually severely polarization-sensitive.The random characteristics of photons are applied to surface various quantum protocols,such as E912 and BB843 where qubits can be encoded in polarization need using beam splitters and polarization beam splitters.In this paper,according to the polarization-sensitive,we introduce a novel structure which has double functions as a TM mode pass polarizer and polarization beam splitters.Such a structure holds a great potential in compact integration of the optical integrated circuit.We also design a polarization-insensitive high-visibility silicon-on-insulator quantum interferometer,numerically demonstrate interference at 1550 nm with visibilities of 99.50%and 93.99%for transverse-electric and transverse-magnetic polarization,respectively,revealing that the proposed interferometer structure is well capable of on-chip optical control especially in quantum optics regime.3、In order to solve the problem of a low bit rate of modulation,we based on standard silicon-on-insulator platforms,propose an interferometer structure operating at telecom wavelengths,which is driven by pump-induced nonlinear Kerr-effect.Special attention is given to the waveguide design in achieving the smallest full-vectorial effective mode area corresponding to the largest nonlinear phase shift.Rr exceeds 10 GHz in a 3-mm PW when Pp=4 W and T=10 ps,the modulation rate can reach 10GHZ.This method is of particular importance to quantum applications.In conclusion,we have made innovations and breakthroughs on the basis of existing works.According to actual requirements of quantum communication,we propose the optimization criteria for devices namely small footprint,low loss and inherent polarization independent of the systems.These work are the perspective study and basic of integrated optical circuit and quantum communication chips and have engineering feasibility and can be extended to other research fields such as micro/nano photonics,classical optical communication,sensing network,optical neural network and quantum computation.