Experimental Study Of Silicon-based Integrated Quantum Photonic Chip

The generation and development of quantum information science shows us one completely different and nearly perfect picture,such as absolutely secure communi-cation,information teleportation and exponential growth's computation power.This beauty encourages scientific researchers to come and go in recent decades,elaborate and develop quantum technologies in different quantum systems.Photonic quantum system,with its perfect resist to environment noise and high fidelity single qubit mod-ulation,always plays an important role in quantum information transmission and pro-cessing.The realization of complex optical systems need to include tens of thousands fundamental elements,the waveguide technologies should be introduced when consid-ering the scalability,stability and miniaturization of the quantum device.The silicon is at the heart of microelectronic chips with its low-cost,easy to fabrication and dense in-tegrated capability.With mature fabrication technology,strong confinement to optical signals and high nonlinear parameter of silicon waveguides,the silicon-based photonic quantum chip has achieved rapid development in recent years.Besides functional quantum integrated optical chips,one well-equipped integrated optical system also needs the realization of chip-to-fiber coupling,on-chip quantum bit regulation and quantum signal measurement and processing.This thesis focuses on the design and experimental measurement of one such well-equipped system for silicon-based quantum photonic integrated circuits,mainly including two aspects,one is to realize the transmission and controlling of photonic quantum states,the other is the manipulation of novel quantum entangled photonic source with the nonlinear optical interactions in silicon waveguides.The contents of this thesis are listed as follows:(1)The property of waveguide system enable us to encode quantum information with transverse-modes.This method would decrease the footprint of the device,and could be extended to high-dimensional encoding,thus available to study on-chip high-dimensional quantum information.Via mode(de)multiplexer and polarization conver-tor,we realize the transmission and coherent conversion of quantum information be-tween the degrees of freedom of path,transverse-mode and polarization.High visibil-ity of single-photon and two-photon interference confirms the preservation of quantum coherence.The experimental results provide us with the ability to control and convert multiple degrees of freedom for photonic quantum information processing,and enhance the ability to deal with complex photonic quantum states.(2)Use transverse-mode to encode information as well,we realize on-chip Control-NOT(CNOT)gate through design and preparation new structures called as partial mode beam splitter and energy attenuator.CNOT gate is the most fundamental multi-qubit gate structure in quantum devices,and constitutes the universal quantum gate set with single-qubit gates.Two indistinguishable photons generated by PPKTP(Periodically Poled Potassium Titanyl Phosphate)crystal is input into circuits through grating cou-plers.We realize on-chip arbitrary single-qubit state preparation with path encoding and convert the encoding method to transverse-mode through mode multiplexers.After transverse-mode CNOT gate operation,we convert the encoding method to path again with mode demultiplexers,and realize arbitrary two-qubit state measurement with path encoding.The CNOT gate operation can entangle two separated quantum bits,and real-ize the preparation of arbitrary transverse-mode encoded Bell entangled state.The total size of gate structure is about 125×8?m2.(3)To show the large-scale integration power and high stability of silicon chips,we experimentally demonstrate on-chip multi-step quantum walk in finite regions.The sample consists of cascaded MZ(Mach-Zehnder)interferometers,and in the experi-ment,we use herald single-photon as the walker,which is achieved through two-photon source in free space.The results demonstrate different phenomenon from the quantum walk in unlimited regions.As one new quantum algorithm,quantum walk is especially suitable for realization and display in optical systems,and has a long-term application value.In above experiments,we show how to realize on-chip quantum state manipulation.The photon pair source we use is still obtained with spontaneous parametric down-conversion process(SPDC)in nonlinear crystals.In fact,we could prepare multiple quantum entangled source directly with the nonlinear interactions in silicon waveguides,which shows high third-order nonlinear parameter.(1)Through spontaneous four-wave mixing process(SFWM)in one 1cm long sil-icon waveguide,we study the situation of preparing multi-photon states with silicon waveguides,and obtain the polarization encoded four-photon state through building Sagnac loop in free space.At the same time,considering that in single pump con-figuration,the generated two photons are in different frequencies,and cannot be used in quantum interference,we introduce the dual-pump technique and adjust two pulsed lasers simultaneously to prepare frequency degenerate two-photon and four-photon en-tangled states.This technique could be used further in many applications where need multi-photon interference such as quantum algorithms.(2)To improve the integration level of quantum system,we need to integrate the modulation of pump laser and preparation of entangled photonic source directly on one single chip.We experimentally demonstrate the preparation of one on-chip transverse-mode entangled photon pair source.The pump laser is divided into two parts through one beam splitter after input into the circuit,we achieve the transverse-mode encoded two-photon entangled source via the nonlinear interactions in one 3mm long multimode waveguide.Large-scale integrated quantum photonic circuits need to cover various fundamen-tal components,such as quantum photonic source,on-chip filtering,quantum state ma-nipulation and so on.We have made a series of progress in design and measurement of the optical chip system,and have the basic ability to handle large-scale quantum pho-tonic circuits.With the update and upgrade of the system,lager size and more mean-ingful quantum function will be demonstrated and validated experimentally.