Generation Of Narrow Bandwidth Nonclassical Photons And Its Properties

Sources for creating entangled photon pairs are essential parts of many quantum information protocols and quantum optical experiments.To date,the most widely used way of obtaining the entangled photon pairs is spontaneous parametric downconversion (SPDC) in a nonlinear crystal.An important step of quantum communication is quantum reperter which can store the photons.The atomic system is regard as a good candidate.To realize the efficient coupling between the atom and the photon is how to get a photon that has a comparable bandwidth with the natural bandwidth of the atom. But the SPDC photons can't interact with atoms efficiently because of its ultrawide bandwidht.So,its bandwidth should be compressed.Recently,there have been many primitive experiments in this field,in which an atomic system such as a hot atomic vapor cell or a cold atomic cloud of Rb or Cs is used as a quantum memory and a photonis used as a flying bit.The research aim of this subject is to prepare such photons.There are six parts in this thesis:the first three are the three steps to prepare narrow bandwidth nonclassical photon pairs;the next two are the investigations of some properties of mode locked photon pairs;the last one is the introduction of cavity locking and laser stabilization.1) Efficient cw violet-light generation in a ring cavity with a periodically poled KTP.The first requirement of this experiment is blue-UV laser source.Although the compact solid-state laser sources have been investigated thoroughly,the blue-UV spectral region laser diodes are still very difficult to make.The wavelength in our experiment is 390 nm,and its double frequency corresponds to the Rb atomic D2 line transition.So the best way to get the blue-UV source is through SHG.The theory of SHG is well established,and thanks to the development of cavity locking technique and production of new type nonlinear crystal,many realization of such sources have been reported.But according to our knowledge,no SHG experiment at 390 nm or below has been reported.As we known,the application of this spectral region is powerful,such as biomedical,high-resolution spectroscopy,atom cooling,quantum optics and so on. The crystal in our SHG experiment is PPKTP crystal,a new type crystal has larger optical nonlinearity and a good power-handing capability in the UV and visible spectra, and the conversion efficiency is about 9.5%with 73 mW pumping power.2) An ultra-bright two-photon source with a type-Ⅰbulk periodically poled potas- sium titanyl phosphate.SPDC as an easy manipulation,high signal-to-noise ratios and high efficiency way of preparing entangled photon pairs is widely used in quantum information protocols and quantum optics.The SPDC photons have been investigated for several decades.Many physical questions have been cleared through SPDC experiments, like Bell inequalities,teleportation.It's also applied to quantum cryptography, quantum computer and others.Here,we use type-ⅠPPKTP crystal to prepare biphotons. The purpose of this experiment is just to know the property of the crystal,like the width of crystal temperature,the efficiency of producing biphotons,which will be used in an OPO cavity in the next step.3) Observation of time correlation function of multimode two-photon pairs on a rubidium D2 line.Comparing with the normal SPDC photons,the narrow bandwidth SPDC photons have more advantages.Its bandwidth is about 6 orders of magnitudes less than the normal one.So it has very long coherence time and distance,and more importantly thanks to the narrow bandwidth,it could be expected to interact with atoms efficiently then it could be stored in atomic system.One of the disadvantages of the photon is that it always fly,so it is hardly to be located or to store.For this reason,the information inside the photons will be gone with the photons.But according to electromagnetically induced transparency(EIT) theory,if a narrow bandwidth photon with its frequency corresponding to a transition of a kind of atom,it can be stored in atomic system and its information could be preserved.So one could transfer the quantum information from the SPDC photons to atomic system and the converse works as well. This is the key point to realize quantum memory and quantum net.In our experiment, 21 MHz bandwidth biphotons are observed.4) Quantum interference of multimode two-photon pairs with a Michelson interferometer. Quantum interference is one of important and mystery things in quantum mechanics.The properties of a kind of photons can be obtained vis interferences.Commonly, there are two classes of it often being investigated,one is first-order interference and the other is second-order interference.One design many interferometers to realize the two classes interferences,like Michelson interferometer,Sagnac interferometer, and so on.Goto et.al investigate the mode locked photon pairs vis an unbalanced interferometer. We do a similar experiment in which a Michelson interferometer is used and we investigate the balance and unbalance cases.Even on the unbalance case,the ann difference is different from theirs and the interference figures are different too.We find the shape of interference fringes is sensitive to the arm difference and the phase.5) Experimentally measuring the coherence length of the single photon generated via a degenerated optical parametric oscillator far below threshold.At this part we measure the single photon coherence length of the mode locked photons directly from the OPO cavity.Though the bandwidth of the photons is determined by the linewidth of OPO cavity,the photon still have very short coherence length due to its abundant longitudinal modes.A similar experiment has been reported in Phys.Rev.Lett.90, 043602,but the photons in theirs is generated via a single-pass configuration in contrast with ours coming from an OPO of far below threshold.The basic idea of this work is when a photon is input a Michelson interferometer,if the path difference between two interfering beams is larger than the coherence length of the photon,there is no interference. If the path difference is less than the coherence length,then there is interference and the single count rate depends on the fine path difference of the order of the single photon wavelength.The result in our experiment is that the single photon coherence length is only about 90μm,corresponding to about 0.47 nm bandwidth,however the linewidth of OPO cavity is about 8 MHz.It indicates that if the further frequency filter absents,the mode locked photons couldn't interact with atoms effectively.6) For the laser stabilization and optical cavity locking are more and more important in modern optical experiments,understanding and knowing some skills about these is necessary.At this part,Ⅰtranslate some good articles about teaching basic knowledge of cavity and laser stabilization for beginners.These articles are all very good beginner's guides.I hope it will take some helps.