Experimental Realizations Of Quantum Information Processing On Single-Photon Few-Qubit System

Quantum mechanics was born in the beginning of the twentieth century,and there were different understandings and debates on this theory through the century. But with the arguments between the scientists,it is clear that the quantum mechanics is a powerful theory in many fields.With the combining of information science and quantum mechanics in ninety years of twentieth century,quantum information science attracts much attention.Quantum information science mainly includes quantum communication and quantum computation.The process of preparation,operation, transmission,storage and measurement quantum state is the fundamental question of quantum information.There are several systems can be used to study the quantum information process,such as linear optics,NMR system,cavity QED,quantum dot, atom system et al.Linear optical system is a good candidate for implementing quantum information process for two reasons.First,the modem communication is based on fiber net,and photons are natural flying bit.So the research of quantum communication is most in the optics system.Second,Knill,Laflamme,and Milbum show that probabilistic two-qubit operations implemented in linear optical circuits with ancilla photons can be used to build a scalable quantum computer.Their work has stimulated much attention on the experimental realization of linear optics quantum computation protocols.Single-photon few-qubit system is one of the basic linear optics system on quantum information research.The basic idea of single-photon few-qubit system is to encode two or more qubits on a photon by using different characters of photon. Usually,the quantum bit(qubit)can be encoded by polarization,space momentum and orbital angular momentum of photons.Especially,the space momentum and the orbital angular momentum(OAM)of photons can be used to encode high dimensional system(qudit).Single-photon few-qubit system can be used to study the superposition of quantum states,and can be easily extend to many-body or high dimensional systems.We experimentally implemented quantum random walk,quantum gambling machine and perfect discrimination of unitary operations by using single-photon few-qubit system.1.Orbital angular momentum of photons and quantum random walkOrbital angular momentum of photons has the relationship with the mode of light. As we know,light beams can carry OAM associated with helicity of their phase fronts, described by a phase term exp(il(?)),carries an OAM of lh per photon,where l can take any integer value.The OAM can make up an infinite dimensional Hilbert space,so it can be used to encode qubit or qudit.Orbital angular momentum of photons can be generated by computer generated holograms which are used in our experiment of quantum random walk.Quantum random walk is a kind of quantum algorithm,and it is quiet different form classical random walk.Numerous schemes of quantum random walk have been proposed using many kinds of system,including linear optics system.We encode space momentum of photon as a qubit and OAM of photon as a qudit,experimentally study the quantum random walk on the one-dimensional OAM space.We initialize the photons on a superposition of different routes,then change their OAM by using computer generated holograms due to different route.At last,we detect the probabilities of photons on each OAM state,which are also the results of quantum random walk.Because the photons "random walk" on the OAM space,the experimental setup is much simpler and easy to adjust.These contents will be discussed in the chapter two and chapter three.2.Quantum gameQuantum game is a new region combined with operational research and quantum theory.It is an important embranchment of quantum information.Quantum game contains three main questions:PQ penny flipover,prisoner's dilemma and quantum gambling.We experimentally realize the quantum gambling machine by using optical method,and discuss the best strategy of the participator to maximize their gains.We use two pieces of birefringent material calcites as polarizing beam splitters,which make the Mach-Zehnder interferometer inherently stable,and get the convictive results of the quantum gambling.We will introduce some quantum game theory and our experimental setup in the chapter four.3.Perfect discrimination of unitary operationsDistinguishable of quantum states is an fundamental question of quantum information,and a lot of works have been done on it.Similar to discrimination of quantum states,discrimination of quantum operation is also very important.But there are some differences between states and operation discrimination.It is shown that two unitary operations U and V can be perfectly discriminated by using parallel scheme or sequential scheme with finite number copies of unknown operation,no matter U and V are orthogonal or not.We will detailed introduce the two schemes of perfect discrimination of two unitary operation and our experiment based on these two schemes.The complexity and resource consumed are analyzed and compared with some other schemes of discrimination of unitary operations.