Preparation And Application Of Entangled Photons

Quantum information science, which is a combination of quantum mechanics andinformation science, attracts attentions of more and more physicists since its first ap-pearance at the end of last century. Quantum entanglement is one of the most importantfeatures of quantum information science differing from classical information science;it is an indispensable resource in quantum information processing and has played avery important role in quantum cryptography, quantum communication and quantumcomputation.Optical quantum information processing is widely used due to the advantage ofpreparing and controlling entangled photons. However, with the development of thetheory of quantum information, the object of concern developed from two qubits tomulti-qubits, and correspondingly, the field of optical quantum information processingdeveloped from two-photon to multi-photon. This dissertation starts with the prepara-tion of a polarization-entangled photon source pumped by a femtosecond pulse laser,which is suitable for multi-photon quantum information precoessing, focuses on thepreparation, measurement and interference of entangled photons. The main results areas follows:1. Preparation of high-brightness polarization-entangled photon source by usingspontaneous parametric down-conversion (SPDC) processEntangled photon pairs are important resource in optical quantum informationprocessing and high-brightness entangled photon pairs are essential in multi-photonquantum information processing. We prepared an ultra-bright beam-like source ofpolarization-entangled photon pairs. The brightness of this source is almost three timesas much as the photon pairs counting rate in other articles under the same condition.The high fidelity and brightness of this source make it suitable for multi-photon ex-periments in many quantum information processing tasks. It will greatly reduce thedifficulty of experiments involving three photon pairs, and experiments with four pho-ton pairs are within reach.2. A protocol for generating heralded multi-photon GHZ-type polarization entan-glement from PDC sourcesAt present, two-photon entanglement sources that can be experimentally preparedmostly have a random feature (like the PDC source). However, due to the random nature of PDC, i.e., there is no way to know whether the entanglement is generated withoutmeasuring, namely, destroying, the state. Consequently, it is instructive to constructreliable photon sources for heralded generation of polarization-entangled states fromPDC sources. We proposed a scheme to generate an N-photon GHZ-type polarization-entangled state from N + 1 PDC sources, by using only linear optical elements andprojective measurements. Successful preparation is heralded by detecting N + 2 pho-tons with ideal number-resolving photon detectors. This protocol can not avoid all thehigher-order terms of the PDC processes, however, it has superiority in the systemswith no higher-order terms since it requires less resources.3. Experimental measurement of lower and upper bounds of concurrence for mixedquantum statesThe characterization and quantification of entanglement have attracted much atten-tion and become a challenging problem to researchers. We experimentally measuredthe lower and upper bounds of concurrence for a set of two-qubit mixed quantum statesusing photonic systems, by performing symmetric and antisymmetric measurements ona two-fold copy of the states. The measured concurrence bounds are in agreement withthe results evaluated from the density matrices through quantum state tomography. Ifwe consider more general case of many photonic qubits and SPDC source, the numberof two-photon interferometers increases linearly with quibit numbers and data collec-tion times would not increase exponentially either. On the other hand, the exponentiallyincreasing experiment resources of tomography is inevitable. It should be mentionedthat, concurrence can not be calculated through quantum tomography if the scale of thestate is larger than 2 ? 2.4. Observation of a generalized bunching effect of six photonsPhoton interference which comes from indistinguishability plays an important rolein optical quantum information processing. When photons are indistinguishably in thesame temporal mode, their detection probability is greatly enhanced due to constructivemulti-photon interference, as compared to the case when they are distinguishable. Weobserved for the first time such a photon bunching effect for six photons. The observedenhancement factor in six-photon coincidence measurement is 17±2, which close tothe factor of 20 for an ideal case. Our result confirms that the six photons that weobtained have a high degree of indistinguishability.