Quantum Information Processing With Linear Optical Elements And Photons

Quantum information science is the combination of information science and quantum physics.Quantum information processing opens up completely new possibilities in communication,computation and other information processing tasks,which would be impossible with classical communication and classical computer.Although such benefits have been well understood in theory,it remains a formidable challenge for experimental implementations.Among the numerous quantum systems proposed for implementing quantum information processing,photon systems are prominent candidates,since they provide many advantages,such as long decoherence time,photons being easily transported, prepared and manipulated.Nonetheless,there are also some downsides,such as difficult storage of photons and interactions between different photons.At present interactions between different photons can be only probabilistically realized by using photon interference and detections.However,this method,together with quantum teleportation trick and quantum codes,has been demonstrated to be capable of creating scalable quantum computers.Since photon interference is key in making interactions between different photons,the interference quality would be an important factor in relation to the fidelity of quantum information processing,and hence,investigations on photon interference, particularly the multi-photon interference,are stimulated.On the other hand, researches on photon interference also promote the quantum information processing in photon systems.Although photon is potentially free from decoherence,in some cases coupling between different photon degrees would affect the state of the degrees considered. Therefore,it is also necessary to investigate the decoherence mechanism in photon systems and methods for dealing with the decoherence.Moreover,photon systems fit well for experimentally demonstrating different decoherence models and ways for beating the decoherence.This dissertation focuses on the above topics.We review the fundamentals and research progresses of photonic quantum information processing with linear optical elements,and give a detailed introduction to our research results.The main results are listed below:1.Multi-photon interference experiments and theoryPhoton interference results from indistinguishability.Distinguishability in multi- photon states is much more complicated than in two-photon states.Our group have done considerable experimental and theoretical investigations on multi-photon interference and distinguishability characterizations.The author,I,joined some of these researches and did some ancillary experimental manipulations or ancillary theoretical analyses.These researches include experimental multi-photon de Broglie wavelength demonstration,experimental high resolution quantum phase measurement,and experimentally characterizing distinguishability using three methods.2.A scheme for expanding a polarization-entangled W stateUsing photon interference and post-selection,numerous schemes have been proposed for preparing multi-photon polarization-entangled states and many of them have been realized in the experiments.We proposed a scheme for expanding a polarization-entangled W state.Our scheme is very simple and can be easily realized.In particular, it allows generating the three-photon polarization-entangled W state with the highest successful probability to date.Moreover,it also gives a new way to prepare W states using single photons and linear optical elements.3.Methods for implementing the quantum Fredkin gateWe presented several methods for implementing the quantum Fredkin gate.The heralded scheme needs four heralded controlled-NOT gates.It can be simplified to a post-selected one with two controlled-NOT gates and we gave a possible experimentally feasible way.We also proposed a scheme utilizing three-photon time entanglement but without ancillary photons.Although our schemes are still experimentally challenging, they for the first time open the door to experimentally realizing the quantum Fredkin gate.4.A proposal for a parity state re-encoderParity encoding is an efficient way to protect against the errors resulting from computational basis measurements.The key in the applications of parity states is a re-encoding process.We gave an experimental scheme for building a two-photon parity state re-coder and described the processes for realizing quantum computation and teleportation.Our scheme is feasible with current experimental technology.5.Generation of arbitrary four-photon polarization-entangled decoherence-free statesDecoherence-free subspace has been proved to be an efficient way to overcome decoherence problems.To protect an arbitrary logical qubit information from collective decoherence,at least a four-qubit decoherence-free subspace is requested.We proposed a new scheme for preparing arbitrary four-photon polarization-entangled decoherence- free states.This scheme can be easily realized,and also universal so that it could be applied to other quantum systems.Moreover,the key block in our scheme,a partial exchanging device,can be also applied to many other quantum information processing tasks.6.Dependence of the decoherence of polarization states due to coupling between photon frequency and polarization on the frequency spectrumWhen photons run in a birefringent medium with fixed optical axes,coupling between frequency and polarization will induce phase-damping decoherence to the polarization state.We investigated in detail the dependence of the decoherence process on the frequency spectrum,in particular the non-Markovian decoherence process in the case of color frequency spectrum.Although such decoherence model is very simple, it is the fundamental part of more complicated decoherence mechanisms.Moreover, our approaches provide possibilities for experimentally investigating the "memory effects" in the non-Markovian decoherence processes.In particular,our results on the decoherence of entangled states present a possible way for experimentally investigating entanglement dynamics.