| Quantum entangled states are regarded as the source of many unique physicalphenomena, and are being investigated in many different domains in the hope that theymay be applied in quantum computation, quantum cryptography, quantum teleportation,and so on. Entangled states of light were the earliest type of entanglement studiedexperimentally, and their unique characteristics are gradually being understood. Thetwo-photon field produced by spontaneous parametric down-conversion (SPDC) in anonlinear optical crystal pumped by a laser is a good source of two-particle entanglement.Important theoretical and experimental research in quantum optics has beenaccomplished by taking advantage of their characteristics and many remarkableexperiments have been carried out.Meanwhile, with the continuous improvement of technology the photo-multipliertube used for single photon detection has now been largely replaced by the avalanchephotodiode (APD) operated in the Geiger mode. These APDs are being increasinglyapplied in various domains of science and technology including spectroscopy,fluorescence measurements, laser ranging, quantum optics, and wherever weak lightdetection is required because of their high quantum efficiency and high gain in the nearinfrared and also the visible spectral band. As a consequence, a precise knowledge of thequantum efficiency of single-photon detectors is required, but their calibration is by nomeans trivial. Traditional methods need a reference standard and there is muchuncertainty because either a standard light source or a separate reference detector isrequired. However, absolute calibration, as the ultimate aim in metrology, can overcomethe defects of these traditional methods.Absolute calibration of the quantum efficiency of single-photon detectors has beenachieved by using two-photon quantum entangled states and a reference detector, thoughthe measurement results are independent of the latter's parameters. By means of a noveldesign for the optical path and electronic circuit our group proposed a new scheme forthe absolute self-calibration of the quantum efficiency of single-photon detectors, whichis also based on entangled photon pairs but does not require any other detector,. Wehave demonstrated experimentally for the first time to our knowledge the absoluteself-calibration of the quantum efficiency of single-photon detectors with this schemeusing correlated photons from optical SPDC.In this thesis an overview is first presented of some basic features of quantumentangled states such as their origin, characteristics and applications. Next, schemes forthe production of two-photon quantum-entangled states by SPDC are introduced, withparticular emphasis on the generation by continuous wave laser pumping of a BBOcrystal satisfying type II phase matching conditions;the spatial distribution and therelevant nonlinear optics involved are discussed.After an outline of the principle, experimental schemes and development ofabsolute calibration based on entangled photon pairs, our scheme for the absoluteself-calibration of the quantum efficiency of a single-photon detector is described. Thetheoretical derivation, experimental scheme, principle of the electronic circuit,experimental results and other details are then presented.The single-photon detector calibrated in the experiment was a Si avalanchephotodiode in a Perkin Elmer SPCM-AQR-13 single-photon counting module. Ahelium-cadmium laser at 441.6nm was used to pump a BBO crystal to producedegenerate SPDC pairs of photons. Taking into consideration all the losses, the correctedquantum efficiency of the detector at 883.3 nm is 38.7±2.1%, which agrees well with thequantum efficiency value of 41.0±0.5% at this wavelength provided by the manufacturer.Finally, a summary is given and future prospects discussed. |
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